Photoshop image modes. Introduction to color channels (RGB, CMYK, Spot, Lab, multi-channel and single-channel modes)
Why are different color models needed and why the same color can look different
Providing design services both in the field of web and in the field of printing, we often encounter the Client’s question: why are there the same corporate colors in the design layout of the site and in the design layout printing products look different? The answer to this question lies in the differences between color models: digital and printed.
The color of a computer screen varies from black (no color) to white (the maximum brightness of all components of color: red, green and blue). On paper, on the contrary, the absence of color corresponds to white, and the mixture maximum quantity colors - dark brown, which is perceived as black.
Therefore, when preparing for printing, the image must be converted from additive ("folding") flower models RGB into subtractive (“subtractive”) CMYK model. CMYK model uses the opposite colors of the original - the opposite of red is cyan, the opposite of green is purple and the opposite of blue is yellow.
Digital RGB color model
What is RGB?
The abbreviation RGB means the names of three colors used to display a color image on the screen: Red (red), Green (green), Blue (blue).
How is RGB color formed?
The color on the monitor screen is formed by combining rays of three primary colors - red, green and blue. If the intensity of each of them reaches 100%, then it turns out White color. The absence of all three colors produces black.
Thus, any color that we see on the screen can be described by three numbers indicating the brightness of the red, green and blue color components in the digital range from 0 to 255. Graphics programs allow you to combine the required RGB color from 256 shades of red, 256 shades of green and 256 shades of blue. The total is 256 x 256 x 256 = 16.7 million colors.
Where are RGB images used?
RGB images are used to display on a monitor screen. When creating colors for viewing in browsers, the same RGB color model is used as a basis.
Printing color model CMYK
What is CMYK?
The CMYK system is created and used for typographic printing. The abbreviation CMYK stands for the names of the primary inks used for four-color printing: cyan (Cyan), magenta (Magenta) and yellow (Yellow). The letter K stands for black ink (BlacK), which allows you to achieve a rich black color when printing. The last letter of the word is used, not the first, to avoid confusion between Black and Blue.
How is CMYK color formed?
Each of the numbers that define a color in CMYK represents the percentage of paint of that color that makes up the color combination. For example, to obtain a dark orange color, you would mix 30% cyan paint, 45% magenta paint, 80% yellow paint and 5% black paint. This can be expressed as follows: (30/45/80/5).
Where are CMYK images used?
The scope of application of the CMYK color model is full-color printing. It is this model that most printing devices work with. Due to color model mismatches, there is often a situation where the color you want to print cannot be reproduced using the CMYK model (for example, gold or silver).
In this case, Pantone inks are used (ready-made mixed inks of many colors and shades), they are also called spot inks (since these inks are not mixed during printing, but are opaque).
All files intended for printing must be converted to CMYK. This process is called color separation. RGB covers a larger color range than CMYK, and this must be taken into account when creating images that you later plan to print on a printer or printing house.
When viewing a CMYK image on a monitor screen, the same colors may appear slightly differently than when viewing an RGB image. The CMYK model cannot display the very bright colors of the RGB model; the RGB model, in turn, is not able to convey the dark, dense shades of the CMYK model, since the nature of the color is different.
The color display on your monitor screen changes frequently and depends on lighting conditions, monitor temperature, and the color of surrounding objects. In addition, many of the colors visible in real life, cannot be printed, not all colors displayed on the screen can be printed, and some printing colors are not visible on the monitor screen.
Thus, when preparing a company logo for publication on the website, we use the RGB model. When preparing the same logo for printing in a printing house (for example, on business cards or letterhead), we use a CMYK model, and the colors of this model on the screen may be visually slightly different from those we see in RGB. There is no need to be afraid of this: after all, on paper, the colors of the logo will closely match the colors that we see on the screen.
Understanding what you see in each channel gives you the knowledge to create complex highlights and fine-tune your images. In this article, you'll take a look inside the different color channels, starting with the most common image mode: RGB.
Let me make a reservation right away that the article does not cover. They are so important that they will be described in a separate article.
RGB channels
If you are preparing an image that will be sent to an inkjet printer, probably one you have at home (rather than a print shop), the mode RGB- what you need. After all, your monitor is RGB, just like your digital camera and scanner. Photoshop does not display individual channels in red, green and blue - they are shown in grayscale so you can easily see the most rich in color areas. Because colors in this mode are made up of light, white indicates areas where the color is at its fullest, black indicates areas where it is faint, and shades gray represent all the areas between them.
As you can see in the picture above, each channel contains different information:
Red. It is usually the lightest of all and shows the most difference color range. In the example given, it is very light, because there is a lot of red on the girl’s skin and hair. It can be very important when editing skin tone.
Green. You can think of it as the "contrast center" because it usually has the most contrast (this makes sense since digital cameras have twice as many green sensors as red or blue sensors). Keep this in mind when creating a layer mask to sharpen an image or when working with displacement maps.
Blue. Typically the darkest of the group, it can be useful when you need to create a complex selection to isolate an object. This is where you will encounter problems such as noise and grain.
CMYK channels
While you probably spend most of your time working with RGB images, you may also need to work with images in CMYK. Its name refers to cyan, magenta, yellow and black inks used by commercial printers to print newspapers, magazines, product packaging and so on. This mode also has a composite channel.
If you plan to print an image using a regular laser or inkjet printer, you won't need it. Plus, this mode robs you of several precious filters and adjustment layers. Professional letterpress printing, on the other hand, divides the CMYK of your image into individual color separations. Each division is a perfect copy of the color channel you see in Photoshop, printed in the appropriate color (cyan, magenta, yellow or black). When a printing press layers these four colors on top of each other, they form a full-color image (this technique is known as four-color printing).
Because they represent colors rather than light, grayscale information has the opposite meaning than RGB. In this mode, black indicates full strength and white indicates the weakest expression of the color.
Spot channels
In the CMYK printing environment, there is a special type of finished ink called spot color, which requires a special kind of channel. If you're a graphic designer working in pre-press, product design, or an advertising agency, you'll need to know how to work with spot colors.
Channels Lab
Lab mode Separates brightness values (how bright or dark an image is) from color information. This color mode is not used for image output like the RGB and CMYK modes, but instead is useful when you want to change only the brightness values of an image (while sharpening or brightening it), without shifting the colors.
In a similar way, you can adjust just the color information (say, to get rid of a hue) without changing the brightness value. And if you look at the palette, you'll see X-ray-like images.
The following channels are available in Lab mode:
- Brightness. It contains desaturated parts of the image, it looks like a really nice black and white version. Some people swear that by separating it into a new document and then doing a little editing, you can create a black-and-white image worthy of Ansel Adams.
- A. It contains half of the color information: a mixture of magenta (understand as "red") and green.
- b. the other half: a mixture of yellow and blue.
Multi-channel mode
You will not need this mode unless you are preparing images for printing in a printing house. However, you may end up in this mode by accident. When you remove one of the document's color channels in RGB, CMYK, or Lab mode, Photoshop will convert the document to this mode without warning. If this happens, use the History palette to go back a step or press Ctrl+Z to undo your action.
There is no composite channel in this mode. This mode is designed exclusively for two- or three-color print jobs, so when you switch to it, the program will convert any existing color channels to spot ones.
When you convert an image to this mode, Photoshop immediately performs one of the following operations (depending on where you were previously):
- Converts RGB to cyan, magenta and yellow spot channels;
- converts CMYK to cyan, magenta, yellow and black spot;
- converts Lab into alpha channels named Alpha 1, Alpha 2 and Alpha 3;
- transforms Grayscale(Grayscale) to black spot.
These changes cause drastic color shifts, but you can edit them individually, both the content and the spot color, to create the image you want.
Once you're done editing, save the image as a PSD or as a DCS 2.0 file if you need to transfer it to your prepress software.
Single channel modes
The other picture modes aren't very interesting since they only have one channel. These modes include Bitmap, Grayscale, Duotone, and Indexed Color.
If you notice an error in the text, select it and press Ctrl + Enter. Thank you!
Did you know that Photoshop is colorblind?
When I say colorblind, I don't mean that I have slight problems seeing shades of green and purple. I mean he's completely color blind. All Photoshop sees is black and white. Black, white and many shades of gray in between. The most powerful graphics editor in the world, industry standard Among photographers, designers and virtually all creative professionals, capable of producing millions and even billions of colors, have no idea what color is.
You may be looking at a photo of a crystal blue ocean you took on your last vacation, but Photoshop sees it as a gray ocean. Have you ever captured a rainbow crossing the sky after an evening summer storm? Photoshop sees her as beautiful set shades of gray. What about the famous pot of gold? For Photoshop it's just a big pot of something gray.
Don't sympathize with Photoshop. He is absolutely happy in his colorless world.
In fact, the only reason Photoshop shows us images in color is because people themselves expect to see them in color. We wouldn't know what to think if everything was shown in black and white. But not Photoshop. There is nothing more precious to him than black, white and gray colors.
So Photoshop has no idea what color is in front of it and all it knows and sees is black, white and gray, how does it show us images in color? I mean this image, which is open in Photoshop:
Photo open in a Photoshop document window.
Obviously, this boy (or girl) is colored. Well, really, I don't think there are more colorful birds than this one. But it's not just birds here. The leaves on the background are colored. The piece of wood on which the bird is sitting is also in color. Everything in the image is in color! And this image is open in Photoshop, so how can this be if Photoshop is color blind? And if it really doesn't see color, how does Photoshop do such a great job of showing us something it can't see?
To answer this question we need to consider two things. First - color modes (colormode) and the second - color channels (channels). Both are very interconnected, if you understand color modes (colormode), That color channels (colorchannels) will also become clearer to you.
We know, or at least agree, that Photoshop doesn't see color. All he sees is black, white and gray. So how does it take these blacks, whites and grays and turn them into the colors we see on our screen? The answer is addiction. Dependence on what, you ask? It's an addiction color mode (colormode), which uses Photoshop.
There are quite a few color modes in Photoshop, but the two main ones are RGB and CMYK. A couple of others you might have heard while working with Photoshop are: Grayscale(Grayscale) and Lab (pronounced “el-ey-bee,” but not “Lab”). These are all examples of color modes, and they define how Photoshop translates its black and white information into color, with the exception of color mode Grayscale(Grayscale), which does not use colors. This is a strictly black and white mode, and is quite often used for fast conversion color image to black and white.
Of all the four modes that I mentioned, the only one we will consider is RGB. The CMYK mode is suitable for printing and publications, we will return to it some other time. Mode Grayscale(Grayscale) as I said, it is used strictly for black and white images, and the Lab mode is not understood by most people living on this planet, as well as those living on other planets, although it is often used for professional editing images, but even the people who use it don't have a complete understanding of how it works. Which leaves us with only RGB.
By far the most widely used color mode in computers and technology in the world is color RGB mode. Photoshop uses it, other programs on your computer use it, your monitor, digital camera and scanner, your TV, and even small screen your mobile or iPod use this mode, as do portable gaming systems like PSP Sony or Nintendo DS. If it's a device that somehow displays or creates images, or image editing software like Photoshop, it uses the RGB color mode. Sounds quite loud, doesn't it? And of course it is. For everything it has quite a wide meaning and importance, RGB is an abbreviation of three colors − red(red), green(green) and blue(blue).
RGBand color channels: the color world of red, green and blue.
What is so unusual about these three colors - red, green and blue? Yes, in general, only that they are primary colors. What does it mean? This means that every color you and I see is created from some combination of red, green and blue. How do we get yellow? By mixing red and green. How do we get purple? By mixing red and blue. What about orange? 100% red and 50% green. And these are just simple examples. Every single color we see is created by a combination of these three colors. I know it sounds almost unreal, but it's true.
When you mix the most bright options these colors together, you get a pure white color. When you completely remove all three of these colors, you get pure black. And when you mix an equal amount in percentage from 0 to 100%, you will get shades of gray.
Let's look at our bird image again:
A very colorful image indeed, but where do all these colors come from? To explain for beginners, let's look at the information that is told to us at the top of the Photoshop document window:
Information at the top of the document window.
In what I've marked with a red circle, Photoshop is explaining to us that the image is used in the RGB color mode, which means that every color we see in the photo is created from some combination of red, green and blue. If you want to verify this, all you need to do is hover your mouse over any part of the image and look at Information panel (Info) in Photoshop.
I'll hover my mouse over the tip of the beak near the bright red area.
Hover your mouse over the tip of the bird's beak.
Let's go to the panel Information(Info) in Photoshop to see what it tells us about this point in the image:
Photoshop Information Panel.
The part that interests us in the panel Information(Info) Photoshop, located at the top left, it shows us the RGB values. The only thing you have to understand is that Photoshop does not display RGB colors as percentages, meaning we won't see values like "10%" red(red), 40% green(green) and 50% blue(blue)". Instead, RGB has values from 0 to 255, where 0 means complete absence specified color in the image, and 255 indicates that full color is used.
So if we look at the area that I highlighted, we can see that the point contains the values 216 for red(red) (very great importance), 59 green(green) (quite small value) and 1 blue(blue) (could be 0), which means that at this point there is practically no blue color, and a very small amount of green. The vast majority of the color comes from red, which is generally natural since the bird's beak is definitely red.
Let's look at another point. I hover my mouse over the area around the bird's back:
Hover the mouse cursor over a point in the bird's back area.
This area looks quite green to me, and if we look at what the panel tells us Information(Info):
Photoshop's Info panel, showing us the RGB values for an area of the image.
Then we will make sure that green(green) is the dominant color with a value of 180. Red(red) has a value of only 20, which is a very small value, and blue(blue) even less than 16.
Let's do this again. I'll hover my mouse cursor somewhere around the bird's head:
Hover the mouse cursor over a point near the bird's head.
This time blue should have more high value in the panel Information(Info):
Photoshop's Info panel showing RGB information for a point selected on the bird's head.
And again we were convinced that this time blue(blue) color has a predominant value of 208 and is the dominant color. Of course, the bird's head is not pure blue. It's more purple-blue, which explains why green(green) has a great value of 100, and even red(red) has a fairly large value of 90. All three colors mixed together on the screen to create the purple-blue color we see.
I could keep hovering over any point in the photo (I don't want to, but I could) and we could watch the values change red(red), green(green) and blue(blue) in the panel Information(Info), since each individual color in the image consists of a certain combination of these three colors.
This is how RGB mode works. Let's repeat, RGB means nothing more than Red (R ed), Green (G reen) and Blue (B lue), and because this image is in RGB mode, Photoshop represents each color using combinations of red, green, and blue.
The next thing we will look at in the second part of the image is - color channels(color channel).
On this moment We found out that Photoshop doesn't see color. Everything in the world of Photoshop is created from black, white and some shades of gray. We also learned that Photoshop uses the RGB color mode to display colors on screen by mixing different combinations red(red), green(green) and blue(blue). But how does Photoshop know how much red, green, and blue to mix to make a single color on screen when it doesn't know what color it should be? I mean, it's great that Photoshop can render pure yellow by mixing full color red(red) with a value of 255, and also green(green) with the same value, but how does it know that yellow should be displayed?
The answer is no. How, no way?
And like this. Photoshop doesn't know that you expect to see yellow in a certain part of the image. It only knows that it is displayed when red(red) with a value of 255 and green(green) with a value of 255, and excludes blue(blue). If this combination creates the exact color you and I call “yellow,” then great, but Photoshop isn't left out. All he knows is to “display red(red) with a value of 255, green(green) – 255 and blue(blue) 0 at a specific pixel." When adding different colors to images, Photoshop is a paint-by-numbers artist.
So, since Photoshop adds a certain amount red(red), green(green) and blue(blue). How does he know how much of each color to add when all he understands is black, white and gray? Two words… Color channels(Color Channels).
Let's look at the bird image again:
So we see this image with you. This is how Photoshop sees it:
But, wait. He also sees it like this:
But how does he see it in two different black and white versions? Good question. The answer is no. He sees it in three different black and white versions. Here's a third one:
Everything we see in one color image, Photoshop sees in three separate ones. black and white images X. Each of these images represents a color channel. The first represents the red channel, the second represents the green, and the third represents the blue. Three separate channels for three different colors combined together will create a full color image.
Think of color channels as color filters. While Photoshop displays a color image on the screen, it knows which colors to display due to the brightness of the light passing through the filters. First it illuminates through a red filter (red channel). If no light passes through the filter, Photoshop knows to display red with a value of 0. If all light passes through the filter, then Photoshop displays full red with a value of 255. If the amount of light passing through the filters is slightly less, Photoshop displays red color with a value between 0 and 255, depending on how much light passes through the filter. It then does the same with the green filter (green channel), setting it to 0 if no light passes through the filter, 255 if all the light passes through the filter, and some value between 0 and 255 if some light passes through. Then he does the same with the blue filter (blue channel). It then knows what value to set red, green and blue to and combines them to create the color we see. It does all this for every pixel in your image, so if your image contains millions of pixels, like most photos taken with a digital camera these days, Photoshop does this operation a million times just to display the image you see. on the screen. See how much Photoshop loves you? So, a second ago I said that Photoshop is not left behind. Let's move on.
Photoshop's "filters" use those three separate black and white images we saw. Red:
So how does Photoshop use this black and white image as a red filter? Remember how I said that Photoshop assigns red values from 0-255 based on how much light passes through the filter? So, how much light passes through the filter depends on how bright the black and white portions of the image are. Any area of pure black will not allow any light to penetrate, this means that in these areas of the image the red value will be 0. Any areas of pure white that allow all light to penetrate, in these areas the red value will be equal to 255. And in areas with the various shades of gray, which are the majority in the image, some amount of light passes through depending on how light or dark the gray area is represented.
In the image above we can see that the brightest areas of the image are in the bird's beak and chest, which confirms what I just said: these areas contain large quantity red in full color. Likewise, the areas of the back, wings and belly are very dark, so there should not be much or no red in these areas.
Let's look again at the full color version of the image:
We said that the beak and chest should contain a lot of red, and as you can see, it does! We also said that the back, wings and belly shouldn't have much or no red in them, and I really don't see any red on them.
Let's look again at the black and white image that Photoshop uses for the green channel:
This black and white image contains a lot of highlights, which means there should be a lot of green in the photo. What's also strange is that one of the brightest areas in the image is near the bird's chest, but I don't remember it being there green color. Let's check this by looking at the full color image again:
There is, of course, a lot of green in the image, which explains the many bright shades of gray in black and white image. If I look at the side of the bird's chest that had the brightest area in the black and white image, it won't be green. In fact it is very yellow! How is this possible? Just. Red and green combined make yellow, so to represent yellow, Photoshop mixed red and green together.
Let's look at another black and white image that Photoshop uses as its blue channel:
There are a lot of very dark areas in this image, especially on the bird itself, with the exception of the head, which is very light. This should mean that only one part of the bird will appear blue - its head. Although her belly should also have a noticeable amount of blue, so should her legs and the piece of wood she's sitting on. Let's get a look:
We were convinced that the bird's head was very blue, we also saw that its belly, as well as its legs and a piece of wood, were also blue. The rest of the bird does not have noticeable blue areas, which is why dark areas appeared in these places in the black and white image.
We've figured out everything about how the RGB color mode and color channels work in Photoshop, everything except one thing. We still haven't seen where you can access these color channels. You'll find them in the appropriately named panel Channels(Channels), which are grouped together with the palette layers(Layers).
Channels panel (Channels) Photoshop.
Palette Channels(Channels) looks about the same as the palette layers(Layers), only it shows information about color channels(color channels) instead of layers. Here you see one Red(Red), one Green(Green) and one Blue(Blue) channel, and each of them contains its own version of a black and white image, exactly like the one I showed in this tutorial. The topmost "RGB" channel is not really a channel. This is simply a combination of three channels, giving us a full-color photo. You can click individually on each channel in the palette Channels(Channels) to display its black and white image in the document window.
That's all. We now know that Photoshop sees everything through the lens of black, white and gray colors. We know that using RGB mode (set by default anyway) mixes different quantities red, green and blue to produce the full color image we see on our screens. And we also know that depending on how much red, green and blue there is, the black and white version of the image for each of the three channels will be different, that all these operations are performed for each individual pixel in the image. And thus, you and I can see a full-color version of the image, while Photoshop is content with black and white.
Now we know how much Photoshop loves us. This concludes this lesson.
Various color modes:
- RGB mode (millions of colors)
- CMYK mode (four-color printing colors)
- Indexed Color Mode (256 colors)
- Grayscale mode (256 shades of gray)
- Bit mode (2 colors)
A color mode, or picture mode, determines how colors are combined based on the number of channels in the color model. Different color modes give different levels color detail and file size. For example, use the CMYK color mode for images in a full-color printed brochure, and the RGB color mode for images intended for the web or Email to reduce file size while maintaining true colors.
RGB color mode
RGB mode in Photoshop uses the RGB model, assigning an intensity value to each pixel. In 8-bit-per-channel images, intensity values range from 0 (black) to 255 (white) for each of the RGB color components (red, green, blue). For example, the color bright red has a value of R=246, G=20 and B=50. If the values of all three components are the same, the result is a neutral gray shading. If the values of all components are equal to 255, then the result is pure white, and if 0, then pure black.
To reproduce colors on screen, RGB images use three colors, or channel. In 8-bit-per-channel images, each pixel contains 24 bits (3 x 8-bit channels) of color information. In 24-bit images, three channels produce up to 16.7 million colors per pixel. In 48-bit (16 bits per channel) and 96-bit (32 bits per channel) images, each pixel can produce even more colors. In addition to being the default mode for new images created in Photoshop, the RGB model is also used to display colors computer monitors. This means that when working in color modes other than RGB (such as CMYK), Photoshop converts the image to RGB for display on the screen.
Although RGB is the standard color model, exact range Colors displayed may vary depending on the application and output device. Photoshop's RGB mode changes depending on the workspace settings you make in the dialog box "Adjusting Colors".
CMYK mode
In CMYK mode, the pixel for each process ink is assigned a percentage value. The lightest colors (highlight colors) are assigned a lower value, and the darker colors (shadow colors) are assigned a higher value. For example, a bright red color might be made up of 2% cyan, 93% magenta, 90% yellow and 0% black. In CMYK images, if all four components are 0%, the color produced is pure white.
The CMYK mode is designed to prepare an image for printing using process colors. The result of converting an RGB image to CMYK is color separation. If the original image was RGB, it is best to edit it in RGB mode and only convert it to CMYK at the very end of the edit. In RGB command mode "Proof Settings" allow you to simulate the effects of CMYK conversion without changing the data itself. CMYK mode also allows you to work directly with CMYK images taken from a scanner or imported from professional systems.
Although CMYK is the standard color model, the exact range of colors reproduced may vary depending on the press and printing conditions. Photoshop's CMYK mode changes depending on the workspace settings you make in the dialog box "Adjusting Colors".
Lab color mode
Color model L*a*b* (Lab) of the International Illuminating Commission is based on the perception of color by the human eye. In Lab mode, numerical values describe all the colors that a person with normal vision sees. Because Lab values describe what a color looks like, rather than how much of a particular ink a device (such as a monitor, desktop printer, or digital camera) requires to reproduce colors, the Lab model is considered hardware independent color model. Color management systems use Lab as a color reference to produce predictable results when converting color from one color space to another.
In Lab mode there is a brightness component (L) that can range from 0 to 100. In the palette Adobe colors and in the Color panel components a(green-red axis) and b(blue-yellow axis) can have values ranging from +127 to –128.
Lab images can be saved in the following formats: Photoshop, Photoshop EPS, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, TIFF, Photoshop DCS 1.0 and Photoshop DCS 2.0. 48-bit (16 bpc) Lab images can be saved in Photoshop, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, and TIFF formats.
Note.
DCS 1.0 and DCS 2.0 files are converted to CMYK upon opening.
Grayscale mode
Grayscale mode uses different shades of gray in images. Up to 256 shades of gray are allowed in 8-bit images. Each pixel in a grayscale image contains a brightness value ranging from 0 (black) to 255 (white). 16- and 32-bit images have significantly more shades of gray.
Grayscale values can also be expressed as a percentage of total black paint coverage (a value of 0% is equivalent to white and 100% to black).
Grayscale mode uses the range determined by the workspace settings specified in the dialog box "Adjusting Colors".
Bit mode
Bit mode represents each pixel in an image as one of two values (black or white). Images in this mode are called bitmap (1-bit) images because there is exactly one bit per pixel.
Duplex mode
Duplex mode creates monotone, duplex (two-color), triotone (three-color), and tetratone (four-color) grayscale images using one to four custom inks.
Indexed Colors Mode
Indexed Colors mode produces 8-bit images with a maximum of 256 colors. When converted to indexed color mode, Photoshop builds image color table (CLUT), which stores and indexes the colors used in the image. If the color of the source image is not in this table, the program selects the closest available color or performs dithering to simulate the missing color.
Although this mode has a limited color palette, it can reduce the file size of an image while maintaining the image quality needed for multimedia presentations, web pages, etc. Editing capabilities in this mode are limited. If you need to do a lot of editing, you should temporarily switch to RGB mode. In Indexed Color mode, files can be saved in the following formats: Photoshop, BMP, DICOM (Digital Imaging and Communications Format), GIF, Photoshop EPS, large documents(PSB), PCX, Photoshop PDF, Photoshop Raw, Photoshop 2.0, PICT, PNG, Targa® and TIFF.
Multi-channel mode
Multichannel images contain 256 gray levels for each channel and can be useful for specialized printing. These images can be saved in the following formats: Photoshop, Large Document Format (PSB), Photoshop 2.0, Photoshop Raw, and Photoshop DCS 2.0.
The following information may be helpful when converting images to multichannel.
Layers are not supported and are therefore flattened.
The color channels of the original image become spot color channels.
Converting a CMYK image to multichannel mode creates cyan, magenta, yellow, and black spot color channels.
Converting an RGB image to multi-channel mode creates cyan, magenta, and yellow spot color channels.
Removing a channel from an RGB, CMYK, or Lab image automatically converts the image to multichannel by flattening the layers.
To export a multi-channel image, you must save it in Photoshop DCS 2.0 format.
Note.
Images with indexed and 32-bit colors cannot be converted to Multichannel mode.
- Translation
I'm going to take a tour of the history of the science of human perception that led to the creation of modern video standards. I will also try to explain commonly used terminology. I'll also briefly discuss why the typical game creation process will, over time, become more and more similar to the process used in the film industry.
Pioneers of color perception research
Today we know that the retina of the human eye contains three different types of photoreceptor cells called cones. Each of the three types of cones contains a protein from the opsin family of proteins that absorbs light in different parts of the spectrum:Light absorption by opsins
Cones correspond to the red, green and blue parts of the spectrum and are often called long (L), medium (M) and short (S) according to the wavelengths to which they are most sensitive.
One of the first scientific works on the interaction of light and the retina was the treatise “Hypothesis Concerning Light and Colors” by Isaac Newton, written between 1670-1675. Newton had a theory that light of different wavelengths caused the retina to resonate at the same frequencies; these vibrations were then transmitted through the optic nerve to the "sensorium".
“Rays of light falling on the bottom of the eye excite vibrations in the retina, which propagate along the fibers of the optic nerves to the brain, creating the sense of vision. Different types rays create vibrations of different strengths, which, according to their strength, excite sensations of different colors ... "
More than a hundred years later, Thomas Young came to the conclusion that since resonance frequency is a system-dependent property, in order to absorb light of all frequencies, there must be an infinite number of different resonance systems in the retina. Jung considered this unlikely, and reasoned that the quantity was limited to one system for red, yellow and blue. These colors have traditionally been used in subtractive paint mixing. In his own words:
Since, for reasons given by Newton, it is possible that the movement of the retina is of an oscillatory rather than a wave nature, the frequency of the oscillations must depend on the structure of its substance. Since it is almost impossible to believe that each sensitive point of the retina contains an infinite number of particles, each of which is capable of vibrating in perfect harmony with any possible wave, it becomes necessary to assume that the number is limited, for example, to the three primary colors: red, yellow and blue...Young's assumption about the retina was wrong, but he concluded correctly: there are a finite number of cell types in the eye.
In 1850, Hermann Helmholtz was the first to obtain experimental proof of Young's theory. Helmholtz asked a subject to match the colors of different patterns of light sources by adjusting the brightness of several monochrome light sources. He came to the conclusion that to compare all samples, three light sources are necessary and sufficient: in the red, green and blue parts of the spectrum.
The Birth of Modern Colorimetry
Fast forward to the early 1930s. By that time, the scientific community had enough good show O internal work eyes. (Although it took another 20 years for George Wald to experimentally confirm the presence and function of rhodopsins in retinal cones. This discovery led him to the Nobel Prize in Medicine in 1967.) Commission Internationale de L'Eclairage (International Commission on Illumination), CIE, set out to create a comprehensive quantitative assessment of human color perception. Quantification was based on experimental data collected by William David Wright and John Guild under parameters similar to those first chosen by Hermann Helmholtz. The base settings were 435.8 nm for blue, 546.1 nm for green and 700 nm for red.
John Guild's experimental setup, three knobs adjusting primary colors
Due to the significant overlap in M and L cone sensitivities, it was not possible to match some wavelengths to the blue-green portion of the spectrum. To “match” these colors, I needed to add a little base red as a reference point:
If we imagine for a moment that all primary colors contribute negatively, then the equation can be rewritten as:
The result of the experiments was a table of RGB triads for each wavelength, which was displayed on the graph as follows:
CIE 1931 RGB color matching functions
Of course, colors with a negative red component cannot be displayed using the CIE primaries.
We can now find the trichrome coefficients for the light spectral intensity distribution S as the following inner product:
It may seem obvious that sensitivity to different wavelengths can be integrated in this way, but in fact it depends on the physical sensitivity of the eye, which is linear with respect to wavelength sensitivity. This was empirically confirmed in 1853 by Hermann Grassmann, and the integrals presented above in their modern formulation are known to us as Grassmann's law.
The term “color space” arose because the primary colors (red, green and blue) can be considered the basis of a vector space. In this space various colors, perceived by a person, are represented by rays emanating from a source. The modern definition of vector space was introduced in 1888 by Giuseppe Peano, but more than 30 years earlier James Clerk Maxwell was already using the nascent theories of what later became linear algebra to formally describe the trichromatic color system.
CIE decided that, to simplify calculations, it would be more convenient to work with a color space in which the coefficients of the primary colors are always positive. The three new primary colors were expressed in RGB color space coordinates as follows:
This new set Primary colors cannot be realized in the physical world. It's simply a mathematical tool that makes working with color space easier. In addition, to ensure that the coefficients of the primary colors are always positive, the new space is arranged in such a way that the color coefficient Y corresponds to the perceived brightness. This component is known as CIE brightness(You can read more about this in Charles Poynton's excellent Color FAQ article).
To make it easier to visualize the resulting color space, we'll perform one last transformation. Dividing each component by the sum of the components we get dimensionless quantity color, independent of its brightness:
The x and y coordinates are known as chromaticity coordinates, and together with the CIE luminance Y they make up the CIE xyY color space. If we plot the chromaticity coordinates of all colors with a given brightness on a graph, we get the following diagram, which is probably familiar to you:
XyY diagram CIE 1931
The last thing you need to know is what is considered white in the color space. In such a display system, white is the x and y coordinates of the color, which are obtained when all the coefficients of the RGB primary colors are equal to each other.
Over the years, several new color spaces have emerged that improve upon the CIE 1931 spaces in various ways. Despite this, the CIE xyY system remains the most popular color space for describing the properties of display devices.
Transfer functions
Before looking at video standards, two more concepts need to be introduced and explained.Optoelectronic transfer function
The optical-electronic transfer function (OETF) determines how linear light, captured by the device (camera) must be encoded in the signal, i.e. this is the function of the form:
V used to be analog signal, but now, of course, it is digitally encoded. Typically, game developers rarely encounter OETF. One example in which the feature will be important is the need to combine video footage with computer graphics in a game. In this case, it is necessary to know which OETF the video was recorded with in order to recover the linear light and mix it correctly with the computer image.
Electro-optical transfer function
The electronic-optical transfer function (EOTF) performs the opposite task of OETF, i.e. it determines how the signal will be converted into linear light:
This feature is more important for game developers because it determines how the content they create will be displayed on users' TV screens and monitors.
Relationship between EOTF and OETF
The concepts of EOTF and OETF, although interrelated, serve for different purposes. OETF is needed to represent the captured scene from which we can then reconstruct the original linear lighting (this representation is conceptually an HDR (High Dynamic Range) frame buffer) regular game). What happens during the production stages of a regular film:- Capture scene data
- Inverting OETF to restore linear lighting values
- Color correction
- Mastering for various target formats (DCI-P3, Rec. 709, HDR10, Dolby Vision, etc.):
- Reducing the dynamic range of a material to match the dynamic range of the target format (tone mapping)
- Convert to target format color space
- Invert EOTF for the material (when using EOTF in the display device, the image is restored as desired).
Until now, the standard technical process of the game looked like this:
- Rendering
- HDR Frame Buffer
- Tonal correction
- Invert EOTF for the intended display device (usually sRGB)
- Color correction
Standard SDR color grading workflow (image credit: Jonathan Blow)
After the introduction of HDR, most games began to move towards a process similar to that used in film production. Even in the absence of HDR, a cinematic-like process allowed for optimized performance. Doing color grading in HDR means you have a whole dynamic range scenes. In addition, some effects that were previously unavailable become possible.
We are now ready to look at the various standards currently used to describe television formats.
Video standards
Rec. 709
Most standards related to video broadcasting are issued by the International Telecommunication Union (ITU), a UN body primarily concerned with information technology.ITU-R Recommendation BT.709, more commonly referred to as Rec. 709 is a standard that describes the properties of HDTV. The first version of the standard was released in 1990, the latest in June 2015. The standard describes parameters such as aspect ratios, resolutions, and frame rates. Most people are familiar with these specifications, so I will skip them and focus on the color and brightness sections of the standard.
The standard describes in detail chromaticity, limited to the xyY CIE color space. The red, green and blue illuminants of a display standard must be selected such that their individual chromaticity coordinates are as follows:
Their relative intensity must be adjusted so that the white point has chromaticity
(This white point is also known as CIE Standard Illuminant D65 and is similar to capturing the chromaticity coordinates of the spectral intensity distribution of normal daylight.)
Color properties can be visually represented as follows:
Coverage Rec. 709
The area of the chromaticity scheme bounded by the triangle created by the primary colors given system display is called coverage.
Now we move on to the brightness portion of the standard, and this is where things get a little more complicated. The standard states that "General optical-electronic transfer characteristic in the source" is equal to:
There are two problems here:
- There is no specification on what physical brightness corresponds to L=1
- Although it is a video broadcast standard, it does not specify EOTF
Where L = 1 corresponds to a luminance of approximately 100 cd/m² (the unit of cd/m² is called a "nit" in the industry). This is confirmed by the ITU in the latest versions of the standard with the following comment:
In standard production practice, the encoding function of the image sources is adjusted so that the final image has the desired appearance as seen on the reference monitor. The decoding function from Recommendation ITU-R BT.1886 is taken as a reference. The reference viewing environment is specified in ITU-R Recommendation BT.2035.Rec. 1886 is the result of work to document the characteristics of CRT monitors (the standard was published in 2011), i.e. is a formalization of existing practice.
Elephant Graveyard CRT
The nonlinearity of brightness as a function of applied voltage has led to the way CRT monitors are physically designed. By pure chance, this nonlinearity is (very) approximately the inverted nonlinearity of human brightness perception. When we moved to digital representation signals, this led to the fortunate effect of uniformly distributing the sampling error across the entire brightness range.
Rec. 709 is designed to use 8-bit or 10-bit encoding. Most content uses 8-bit encoding. For it, the standard states that the distribution of the signal brightness range should be distributed in codes 16-235.
HDR10
When it comes to HDR video, there are two main contenders: Dolby Vision and HDR10. In this article I will focus on HDR10 because it is an open standard that has become popular faster. This standard is chosen for Xbox One S and PS4.We'll start again by looking at the chrominance portion of the color space used in HDR10, as defined in the ITU-R BT.2020 (UHDTV) Recommendation. It contains the following chromaticity coordinates of primary colors:
Once again, D65 is used as the white point. When visualized on an xy Rec. 2020 looks like this:
Coverage Rec. 2020
It is clearly noticeable that the coverage of this color space is significantly greater than that of Rec. 709.
Now we move on to the brightness section of the standard, and this is where things get interesting again. In his 1999 Ph.D. thesis “Contrast sensitivity of the human eye and its effect on image quality”(“Contrast sensitivity of the human eye and its influence on image quality”) Peter Barten presented a slightly scary equation:
(Many of the variables in this equation are themselves complex equations; for example, brightness is hidden inside the equations that calculate E and M).
The equation determines how sensitive the eye is to changes in contrast at different brightnesses, and various parameters determine viewing conditions and certain properties of the observer. "Minimum distinguishable difference"(Just Noticeable Difference, JND) is the inverse of Barten's equation, so for EOTF sampling to get rid of viewing conditions, the following must be true:
The Society of Motion Picture and Television Engineers (SMPTE) decided that Barten's equation would be a good basis for a new EOTF. The result was what we now call SMPTE ST 2084 or Perceptual Quantizer (PQ).
PQ was created by choosing conservative values for the parameters of the Barten equation, i.e. expected typical consumer viewing conditions. PQ was later defined as the sampling that, for a given luminance range and number of samples, most closely matches Barten's equation with the chosen parameters.
The discretized EOTF values can be found using the following recurrent formula for finding k< 1 . Last value sampling will be the required maximum brightness:
For maximum brightness At 10,000 nits using 12-bit sampling (which is used in Dolby Vision) the result looks like this:
EOTF PQ
As you can see, sampling does not cover the entire brightness range.
The HDR10 standard also uses EOTF PQ, but with 10-bit sampling. This is not enough to stay below the Barten threshold in the 10,000 nit brightness range, but the standard allows metadata to be embedded into the signal to dynamically adjust peak brightness. Here's what 10-bit PQ sampling looks like for different brightness ranges:
Various EOTF HDR10
But even so, the brightness is slightly above the Barten threshold. However, the situation is not as bad as it might seem from the graph, because:
- The curve is logarithmic, so the relative error is actually not that great
- Do not forget that the parameters taken to create the Barten threshold were chosen conservatively.
Here's an example of what 8-bit Rec sampling looks like. 709 with 100 nits peak brightness:
EOTF Rec. 709 (16-235)
As you can see, we're well above Barten's threshold, and importantly, even the most indiscriminate buyers will tune their TVs to well above 100 nits peak brightness (usually 250-400 nits), which will raise the Rec curve. 709 is even higher.
Finally
One of the biggest differences between Rec. 709 and HDR in that the brightness of the latter is indicated in absolute values. In theory, this means that content designed for HDR will look the same on all compatible TVs. At least until their peak brightness.There is a popular misconception that HDR content will be brighter overall, but general case this is wrong. HDR films will most often be produced in such a way that average level Image brightness was the same as for Rec. 709, but so that the brightest parts of the image are brighter and more detailed, which means the midtones and shadows will be darker. In combination with absolute values HDR brightness means that for optimal viewing, HDR requires good conditions: In bright light, the pupil constricts, which means that details in dark areas of the image will be more difficult to see.
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