Fundamentals of Post Photography (2): Color Space and Color Management

This paper is a series of digital photography post-processing courses, focusing on PS technology. ?

This chapter is the second chapter in the series, which mainly introduces various common color spaces and color management.

Now another concept: color space, color space and color gamut are basically the same. Color space and color mode are not the same thing at all, and cannot be confused. Color mode refers to the way of color expression. And color space refers to the range of colors. In fact, different color modes will correspond to different color spaces. Each device (especially the output device) has its own color space. The device's own color space is called device-dependent color space, otherwise it becomes device-independent color space.

Then why do you need the concept of color space? The main reason is that our existing equipment can only display some colors in CIE chromatography. The color range that a device can present or express is the color space of this device. Usually, in order to ensure the consistency between different devices, we also define a device-independent color space. The device-independent color spaces we often hear are sRGB, Adobe RGB and CYMK. Their specific scope is as follows:

? Add a knowledge point here. Why is the CIE standard chromatogram (CIE*x*y chromatogram to be exact) so horseshoe-shaped instead of the circular plane in the HSV color model above? That's because the colors of any two points in this horseshoe chromatogram are proportionally mixed on the connecting line of these two points, which means that this is a linear space. Only in this way can we ensure that the color gamut is closed and any two colors in the color gamut are mixed in proportion.

? We know that all colors are a mixture of red, green and blue, but different color gamut has different definitions of red, green and blue. The three vertices of a triangle are the definitions of red, green and blue in this color gamut. There are sizes of color gamut, but the number of colors in the color gamut is infinite. This is like two triangles with different areas, but both the big triangle and the small triangle contain infinite points.

Color gamut is defined by icc profile, and the usual extension is icm. It is a file developed by the International Color Alliance to describe the color gamut. It is called a configuration file in Chinese version of Photoshop and a description file in macOS. Usually, the display driver will install its own icc file when it is installed.

In the macOS display color setting interface, click "Open Description File" and use ColorSync to open the color gamut configuration file of the corresponding display, so we can easily find out which parameters are defined in the configuration. As shown in the figure below:

? In addition to the definition of three primary colors, we also need to know the white point of the display, which is what we usually know. At different color temperatures, the color tendency of the display is different. In addition, the color brightness is linearly and uniformly distributed in the vertical direction, but the human eye's perception of light intensity is not linear, but logarithmic, which is the same as the human ear's perception of sound intensity. Therefore, we also need to define a gray coefficient, also called Gamma value. Generally, the greater the gamma value, the greater the contrast will appear.

Finally, mathematically, the color gamut actually defines a 3×3 matrix, through which the colors in the color gamut can be converted into the reference color gamut (PCS). You can also invert the matrix and convert the color values in PCS into color gamut.

Second, Adolberg, sRGB and Simak.

The common color spaces in photography are sRGB and Adobe RGB. At present, our equipment can't represent all the colors of CIE, so neither Adobe RGB nor sRGB can represent all the colors of nature, including CYMK colors in the printing industry. The diagram of the four is 1.

Adobe RGB and sRGB belong to the color space of the display, although both color spaces are RGB color modes. However, as can be seen from Figure 1, their ranges are quite different.

Referring to figure 1, the person defined by sRGB color space thinks that the color of the vertex position above the orange triangle is green, but the adobe.

? People who define RGB think that green should be at the top of the red triangle. But the definitions of red and blue are the same. Triangles connecting three vertices form different color spaces. Therefore, different color spaces can only represent some colors of CIE standard chromatogram. Moreover, the sizes of different color spaces are different, and a large color space can represent more colors. For example, the color space of Adobe RGB is much larger than that of sRGB.

SRGB was developed by companies such as Microsoft. This color space standard is very suitable for transmission on the network, because most civilian devices can support it. Because the standard was formulated earlier, the equipment capacity was limited at that time, and the color range of sRGB was relatively small. Adobe established the Adobe RGB standard in 1998. Thanks to the popularity of Adobe's Photoshop and other software, Adobe RGB was quickly recognized by the industry. Because of the wide range of color space, it is more suitable for publishing, printing, design and other professional fields.

CYMK color space is subtractive color space, which is used for printing and printing. In the post-processing process of photos, we generally don't use CYMK color space directly. It should be noted that it does not completely overlap with sRGB, but Adobe RGB completely contains CYMK, so if your photos need to be developed or printed. You need to use Adobe RGB color space to post-process the photos, and finally convert them into CYMK mode for output to ensure sufficient color range.

? In addition to the commonly used Adobe RGB, sRGB, CYMK and other color spaces, there are some color spaces that we need to know. I'm just making a list here to illustrate.

Other color gamut you may encounter, such as Rec.2020, Rec. 2 100, suggestion. 709, basically do not need special attention. Interested readers can Baidu on their own.

? I don't know if readers have noticed, but I have never mentioned NTSC above. This is because this is an outdated color space. However, many display manufacturers often use it to mark the parameters of the panel, which is basically of little value. Although its color space is much larger than sRGB, it is not a covering relationship. 100% NTSC does not mean that it can overwrite sRGB with 100%.

? Many people must have come across the hard-earned films in Ps. After posting them online or sharing them with others, they found that the colors others saw were very different from their own. What's more, saving the photos of Ps on the computer and opening them with other software will lead to different colors from those seen in Ps. This is actually a problem that color space is not handled correctly.

Let's do an experiment about the specific reasons of color management errors. As shown below: Open edit in PS to open the dialog box for specifying configuration file.

? As can be seen from the figure 1, the range of Adobe RGB is larger than that of sRGB, but when storing pictures, we all store them with 8 bits per channel or 16 bits. For example, in 8-bit JPEG, each pixel of RGB (0,255,0) is used to represent the color of the top vertex of the gamut triangle, but this position is obviously not greener under sRGB color than under Adobe RGB color. So in Figure 3.2, it is obvious that the green color is obviously greener after the wrong Adobe RGB color space is specified. Because the blue and red vertices of the two color gamut triangles coincide, the purple and red of the flowers behind them do not change much.

? What I must emphasize here is that correct colors and beautiful colors are two different things. Some people think that the color of this device or software is good when they see that a photo is brighter on different monitors or software. If the sRGB picture file is displayed on a wide color gamut display without color management, it will look bright. But this is obviously wrong. Don't doubt the existence of this kind of software. I will talk to readers about this "famous" software later.

? If you go to Baidu to talk about color management, the content is actually very rich, and many people may not understand it. Simply put, the purpose of color management is to make the results seen in various links and different devices "basic", correct and consistent. At the same time, through reasonable color management scheme, more details can be retained in the later image processing process.

? Complete color management is a systematic science, but fortunately, we don't need to do it thoroughly. We only need to know the following knowledge: color space selection, color mode selection, color bit depth selection and display device selection. I will introduce these four contents in detail later. First, let's look at the software environment's support for color management.

? Color management under Mac: Apple has perfect support for color management, and most software can display correct colors. Comprises a picture viewer, a UI and a desktop, and color management can be complete.

? Color management under Windows: In fact, color management under Windows is really messy. In order to understand the color management under Windows, I even spent a week studying the Win32 WCS API based on UWP/WPF/D3D and the development interface of Win 10. The conclusion is: Except for a few softwares, most softwares completely ignore your monitor settings and regard you as sRGB. Its problems are mainly the following two.

? First of all, Windows is the "famous" software I mentioned earlier. Its interface can only work in sRGB mode, and the most terrible thing is that it will assume that your monitor also works in sRGB mode, which will cause the whole system to look oversaturated when you use Adobe RGB monitor.

? Secondly, when many softwares under Windows display non-sRGB pictures, there will be some very strange behaviors. Specifically, when viewing Adobe RGB pictures, the pictures will be oversaturated, mainly because the system will assume that your monitor works in sRGB mode, and the software will automatically convert Adobe RGB pictures into sRGB, resulting in an increase in saturation.

Next, we use the professional monitor SW270C of BenQ in my hand, and let the commonly used picture-related software under the two systems load an Adobe RGB picture, and compare it with the real camera shooting. BenQ's SW270C, as a professional monitor, supports 99% Adobe RGB. After hardware color calibration, the color accuracy is △E≤2, (△E is the color accuracy value, the smaller the color, the more accurate it is, △E.

? You can clearly see a picture of Adobe RGB color space from the above picture. The above three softwares are correct, and the following three softwares are obviously oversaturated.

? In addition, after testing, Edge and the new version of Chrome under Win 10 have been able to support correct color management. In the aspect of viewing software, ACD See (Pro 7, Pro 12, the 2020 ultimate version has been tested), HoneyView and IrfanView can correctly load the image embedded configuration file and the displayed icc file, but they must be set in the software settings, among which ACD See's quick viewing tool does not support it, and Irfanview needs to install plug-in support. Readers are advised to use these softwares to preview the saved pictures if they retouch them in Windows.

As can be seen from the above figure, the consistency of software under mac OS is very good, and basically all software can display colors correctly. Including GUI (desktop background) can correctly handle color space problems.

Whether it's Windows or Mac, you can automatically load the corresponding monitor configuration file after you correctly install the monitor driver, as shown in the following two figures. Unless you have calibrated the monitor or need to modify the settings here for other purposes. Otherwise, manual setting is not required. For iMac and MacBook Pro, the system can also automatically set the monitor by reading the EDID of the built-in screen, and the EDID data of the Apple monitor contains the information defined by Apple, so that the system can set it more accurately.

? When we use ACR to open the picture, click the following picture information text to pop up the workflow setting dialog box. In which you can select a color space and a color depth. Here I suggest that readers must set the color depth to 16bit, because the color depth of 8 bit is 24Bit per pixel (8bit per RGB channel). Although it can represent 224 colors, each three-color channel has only 256 brightness values. However, in the case of 16bit, 16,777,216 (24 times of 2) colors can be represented, but each channel has only 256 brightness values. However, in the case of 16 bit depth, each pixel can represent 28 1, 474,976, 7 10/0,656 (48 times of 2) colors, and each channel has 65536(2 16) brightness values.

? In the previous chapter, we introduced various color spaces, and there are many tutorials on the Internet that will advise readers to choose a larger color space, at least Adobe RGB or even ProPhoto RGB. But is this really the case?

We know that the area of Adobe RGB in chromatogram is larger than that of sRGB. Each pixel can express the same number of colors, and each expressible color is a point in the chromatogram. All these points are filled into the color gamut triangle, and obviously the points in the triangle with large area are more sparse. Therefore, under the same color depth, a smaller sRGB color gamut can make the color transition smoother.

In addition, unless your monitor can support Adobe RGB color gamut, even if you choose Adobe RGB color space, you will not see the extra colors on the monitor. For example, my BenQ SW270C supports 99% Adobe RGB color gamut, 97%P3 color gamut and 100% sRGB color gamut. Only on this kind of professional display, it makes sense to choose a higher color gamut than sRGB.

Photoshop colors are configured in the edit menu. Click the Color Settings item in the Edit menu to open the Configuration dialog box.

In this interface, the upper left workspace is the color space used when you create a new picture in Ps, and the color gamut in RGB/CYMK can be set separately. The "color management scheme" at the bottom left refers to the color gamut used when opening an existing picture. You can set the color management schemes for CYMK, RGB and grayscale images respectively. The specific rules for each option are as follows:

? 1, when the file contains a color configuration. The color scheme in the open file will be preserved.

? 2. When the file does not contain a color scheme. Use workspace to set the color gamut of RGB/CYMK.

? 1, when the file contains a color configuration. The color scheme in the open file will be preserved.

? 2. When the file does not contain a color scheme. Use workspace to set the color gamut of RGB/CYMK.

? In the color management scheme, there are two other options, namely profile mismatch and missing profile. Ask when opening, ask when pasting. Readers are advised to consult. This way, every time you open a file or paste a picture, if the color space of the file and the picture is inconsistent with the workspace, Photoshop will pop up a dialog box to remind you what to do. This can control the color space more freely.

? In this dialog box, users can choose to use the color scheme that comes with the file or change it to the color scheme of the workspace. The example in the figure is that the file is embedded in the sRGB color space, and Ps is set to Adobe RGB.

? In PS, the color mode can be selected from RGB mode, Lab mode and CYMK mode. Basically, we will process pictures in RGB mode (a few operations may need to be processed in Lab mode, which will be introduced in subsequent articles).

? Because CYMK is a subtraction model, the higher the value of each component, the more channels are absorbed. Therefore, in this mode, the results of many operations are contrary to those in RGB mode. For example, in CYMK mode, if we raise the curve of the channel, the brightness of the image will darken and look indirect. If we need to print and want to see the effect in CYMK mode, we can open the "Proofreading Settings" function in the view menu. You can preview the display effect of CYMK color mode in the window. The details are as follows

To sum up, the main tasks that need to be done in color management are as follows:

A: If you are just an amateur, you just want to share photos with friends online, and you don't have a professional or quasi-professional monitor. The recommended scheme 1 is as follows:

? 1.？ The color mode of ACR is set to sRGB mode. Set the color depth to 16 bits.

? 2.？ The workspace of Photoshop is set to sRGB mode.

? 3.？ Choose to switch to the workspace when opening an existing picture.

? 4.？ If you need to store intermediate procedures, please use a file format that supports 16bit, such as tif or psd.

B: If you have the ultimate pursuit of photo quality, or your photos have printing requirements, and you also have a professional or quasi-professional monitor. Option 2 is suggested as follows:

? 1 ACR is set to Adobe RGB mode. Color depth setting 16 bits.

? 2. The workspace of 2.Photoshop is set to Adobe RGB mode.

? 3. Keep the existing configuration file when opening the existing JPEG image, convert the color depth to 16bit, and then convert the color space to Adobe RGB.

? 4. If you need to store intermediate procedures, please use a file format that supports 16bit, such as tif or psd.

? 5. When it is used for sharing, it will be converted into sRGB output, and when it is used for printing, it will be converted into CYMK mode output after proofreading.

Neither of the above two schemes requires you to make any settings on the system. Try to choose 16bit color depth to improve the accuracy. The first scheme is the whole process sRGB. Generally speaking, sRGB is enough for printing. In the second scheme, it is very important that if you get a JPG image with sRGB and 8bit color depth, don't try to convert it to Adobe RGB, because doing so will not improve the image quality, but will reduce the image quality.

At the same time, I suggest that photographers who are not particularly good at computer foundation should try to use macOS to reduce the troubles caused by some strange-looking phenomena. If you must use Windows, and you happen to have a monitor that supports a wide color gamut, please use a picture browsing program that supports correct color management, such as HoneyView, ACDSee and other software, and set up the color management of the software.