Saturday, June 3, 2023

What is the color space of monitors and TVs and what is it


The image produced by monitors is standardized in its most essential components: resolution, frame rate, color depth , gamma , color space.

To build a mathematical model of human color perception, two scientists – John Guild and David Wright, independently of each other, conducted experiments on people with normal vision.

As a result of these experiments, the CIE XYZ standard was adopted in 1931, which formed the basis of almost all other standards that mention color in one way or another. Of course, this model is not ideal.

For example, most of the colors of this space cannot be seen in reality. Areas magnified 10 times for clarity, within which colors are indistinguishable from each other to most people, are very uneven.

But this diagram is very convenient for describing the color gamuts of real devices. The straight line between two colors in the diagram shows the colors that can be obtained by mixing them in different proportions. It is enough to know the wavelength and the width of the peaks of the primary colors in order to find the coordinates of the point directly on the diagram without complicated calculations.

There are alternative spaces that display the full color gamut, with their own characteristics. For example, CIE Lab, where it is inconvenient to compare monitors due to non-linear transformations. But it’s convenient to compare printers, because colors are viewed relative to the white point, which for a printed image varies with lighting.

The most common color spaces will be discussed in this material.

analogue television standards. NTSC, SAMPT-C, PAL/SECAM, REC.601
NTSC became the color standard in 1953. In those days, televisions provided a very wide color gamut, but the phosphor used left long trails and did not give a bright enough picture, which led to the gradual abandonment of this standard by manufacturers.

As a result, the SAMPT-C standard appeared, taking into account the real color in TVs , which continued to be used in NTSC broadcasts.

This confusion (the use of the same name for both the color space standard and the broadcast system) is used by cunning manufacturers, taking another SAMPT-C (NTSC 1976) color gamut standard for calculating the coverage percentage relative to NTSC (NTSC 1976) on paper, the device looked “cooler” than in reality. In modern times, the NTSC color gamut standard (1953) is not used anywhere except for marketing.

A little later, other PAL / SECAM television standards were developed, which are described by a single standard REC.601. In the modern digital world, its only suitable application is the digitization of cassettes, with subsequent conversion to another, more suitable space.

But there is more. The h.264 decoders convert the encoded color information to the resulting RGB values ​​differently depending on the image size. Depending on image dimensions, REC.601 is sometimes incorrectly used instead of REC.709. This leads to colors being distorted into either reddish or yellowish areas.

sRGB, REC.709

sRGB and REC.709 appeared in much the same way as SAMPT-C – to bring order to the chaos that monitor manufacturers had created. And the fact that he switched so freely to LCD panels can be considered a miracle – the principle of obtaining the final picture is different (different phosphors, filters, and so on). An interesting feature of the standard is that it does not have a constant optoelectronic light characteristic (gamma).

Initially, the inverse gamma was used to compensate for the uneven luminosity of the phosphor from the signal level of the kinescope beam controlling the current, (it was easier for manufacturers) so that the final image looked as close as possible to the original. But modern monitors do not really need this – they can work with any gamma function.

Now gamma is needed for the optimal distribution of color information on a numerical sequence of bits. For example, in the HDTV broadcasting standard (REC.709), the numbers 0-15,236-255 are needed for frame synchronization, although only 0 and 255 are actually used for this purpose. To take into account the loss of this part of the range, the corresponding gamma function was selected. And what will happen to the image when a REC.709 signal is applied to an sRGB monitor can be seen if the HDMI setting in the video card driver is incorrect.

So, despite the fact that everywhere for sRGB gamma 2.2 is indicated, in fact the gamma varies from 1 to 2.4.

Blue is the local sRGB gamma value, dotted line is gamma 2.2, red is sRGB gamma.

This was done just for the optimal distribution of color by bits, taking into account the reflection of the lighting in the room on the monitor screen.

And everyone is used to the fact that the white point is indicated in kelvins (for example, 6500K), but this is also “not true”. By standard, the white color used in sRGB corresponds to daylight white in the midday sun, looks a little greener than the usual 6500K and is called D65.

So far, sRGB is the color standard for the web. It is in this space that image creators, designers, photographers who focus on digital publications should work. But the creators of video content should use another standard – REC.709, which, despite the same color gamut, has differences in the level of black and white points.

Another feature of sRGB is the attitude of monitor manufacturers to this standard. Even declaring factory calibration in sRGB, in fact, everything can differ from the standard, except for the primary colors, which complicates the work. Pay attention to reviews .

Adobe RGB is considered a standard in printing, due to the fact that the coordinates of the primary colors for are chosen in such a way as to exactly overlap swopCMYK – the color gamut standard for 4-ink printing. In the blue area, sRGB has very big problems. Even a cheap home inkjet printer produces richer blues than an expensive sRGB-only designer monitor.

The white point in Adobe RGB is not D65 but D50 as corresponding to white on high quality paper. Which can cause a lot of trouble even in amateur print because of the principle of its work. This is a substance that converts the ultraviolet part of the spectrum to blue, which makes yellowish bond paper appear bright and white, and prints on such paper change colors greatly depending on the light source.

An image destined for sRGB with color management turned off on such a monitor will be noticeably different from the original color, due to the fact that the green component is not only farther from the white point, but also slightly shifted away from the white point / white point line. green.”

Such a space is not suitable for content consumption, the colors are not only more saturated, but also change shades, which is most noticeable on faces, to the color of which the eye is more sensitive. For the same reason, for non-print content creators, this space will cause more problems than benefits – almost no one will see the image in its original form.

To use such a monitor properly, it will require a colorimeter-spectrophotometer to accurately calibrate both the monitor itself and the printer, D50 and D65 light sources to control prints, a windowless room painted with gray paint. And all this in order to exclude the influence of external lighting on the perception of color. Otherwise, it will just be a monitor with rich greens and blues.

Due to too wide coverage, the effect of posterization can be observed on 8-bit panels, and calibration through the LUT of a video card into more “narrow” spaces only enhances this effect. Therefore, in such monitors, a 14-bit LUT in the monitor itself and a 10-bit input are not a luxury, but a necessity.

But all these tricks are not enough when it comes to multi-color printers. Even a typical consumer 6-color printer can go beyond the capabilities of entry-level professional monitors, so going beyond the standard monitor coverage is highly desirable.

DCI-P3, Display-P3, P3-D65
Initially, DCI-P3 was the standard for cinemas.

The original standard has a white point brightness of only 45 nits (cd/m²) and a noticeable greenish tint, and the gamma used is 2.6. Most monitors, even if the brightness is turned down to the minimum, will still be noticeably brighter than the screen in the cinema is supposed to be.

Therefore, the standard has adaptations for consumer technology – Display-P3, P3-D65, which differ in the white point, and the gamma, which was taken as 2.2. What they have in common with the original standard is only the primary colors.

This standard is planned as a replacement for sRGB. In the near future, it will owe its arrival to the masses to quantum dots – a cheap phosphor that allows you to get almost any color without the use of rare earth metals.

There are more and more monitors that provide a sufficient level of coverage for the future standard, but now this causes some difficulties. Although browsers have learned how to convert colors, they need to know the scope of the monitor to do so. And Windows 10 does not know about this standard. And if you become the happy owner of a monitor with a color gamut other than sRGB, then if there are no settings, this can lead to color distortion.

Unlike Adobe RGB, the P3 family has expanded coverage not only in the area of ​​greens, but also in reds. This results in overly saturated, “acidic” colors. To avoid this, just download the appropriate profile and set it as the default for the monitor.

Unfortunately, manufacturers and reviewers do not often indulge in monitor profiles, and calibration costs money that you don’t want to spend. In this case, a standard profile will help, making browsing the Internet more enjoyable.

REC.2020 REC.2100

The latest format for digital television is REC.2020 REC.2100. Due to the fact that monochrome colors are used, even quantum dots cannot provide such coverage, which means that budget devices with 100% coverage are not expected in the foreseeable future. Most likely, this color space is destined for the fate of a container – a color space that does not correspond to any real device, but is used to store color information so that the device itself performs color conversions in accordance with its capabilities. This is already happening on YouTube. Where, in order to correctly display the color of the video in HDR format, it is recommended to convert it to the REC.2020 space before uploading.

First of all, when buying a monitor, you should remember that a deviation of more than 5% from the standard color gamut upwards leads to a significant color change, which can hardly be corrected without a calibrator . And there is nothing to fix the deviation to the smaller side.

Factory calibration does not guarantee that the monitor will be suitable for work.

Oddly enough, despite the obvious desire of manufacturers to make DCI-P3 the new “default” monitor standard, Windows 10 does not even know about the existence of this space. In order to fix this, you will need to manually assign the appropriate profile to the monitor.—priority-for-aspiring-students?t=1643994541880


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