Monitor gamma value FAQ
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March 20 08:06:48, 2024
1. What is a Gamma value? Gamma values ​​can be translated as "contrast coefficients." In the early screen display, there was a problem that the brightness of the cathode ray tube (CRT) was not directly proportional to the input voltage, but had a weight relationship, which was the gamma value. The relationship between voltage and output brightness. As shown in the figure: (gamma 1) This value will vary with the CRT but is generally close to 2.5. The gamma response of CRT is due to the electrostatic effect in the electron gun. Note: The gamma effect of the CRT will make the midtones darker relative to the dark and highlight areas. 2. What are the processing methods for the gamma value? Because most of the photosensitive device used in the video camera output voltage value is proportional to the light intensity. Therefore, a correction is made to the gamma value of the CRT so as to avoid the effect that the midtones of the image displayed on the video screen are too dark. The television standard defines correction of the gamma value on each television set by adding a correction circuit to the video camera. The correction circuit uses a gamma value of 0.45=1/2.2. This value is used to compensate for the CRT's 2.5 gamma and compensate for the apparent contrast reduction when the TV is placed in a dark home environment. Performing this correction in the studio is much more economical than correcting it on the video screen. 3. What is the computer's treatment of gamma? Many computer displays do not consider the monitor's gamma value problem. Each frame buffer value provided by the application software is linearly converted to a voltage value to drive the CRT transmission upon display. Thus, the value in the frame buffer is not linearly related to the final display brightness. As shown in the figure above, if the value of the frame buffer is 1/2, the resulting brightness will be less than 1/2. Some display systems include a hardware lookup table to correct screen gamma values. In these systems, the value of the frame buffer provided by the application performs gamma correction of the CRT through the lookup table in the display controller, and the value of the frame buffer is linearly related to the final display brightness, ie, the gamma value displayed by the system. Is 1.0. The Mac includes a lookup table in the display that can be used to correct CRT gamma values. However, the default value in the lookup table does not completely compensate for the CRT Gamma value. In contrast, the CRT gamma value must be partially compensated to make the display system's gamma value reach 1.8. The gamma value of 1.8 is very close to the effect of the laser printer, because the laser printer will produce a non-linear reaction effect due to dot gain and other reasons. . If a one-bit display is used and a grayscale image is generated by the dithering method in the difference between the two-pole values, a linearized luminance response can be generated regardless of the gamma response of the display. This fact is used in the following tests. The lack of standards in processing display gamma values ​​can cause problems for systems such as the Web because the system's images are displayed on different displays. A good-looking image on a brand monitor may not look good on another brand because of different gamma values. Without gamma correction, it has an effect on the change in image brightness, but it also affects the change in hue. For example, if a color has a red component of 50% and a green component of 25%, displayed on a CRT monitor with a gamma value of 2.5 instead of gamma correction, the resulting color intensity will be 18% for the red component; The green component amount is 3%. In addition to getting a darker effect, the color will appear reddish. Therefore, the CRT gamma correction is generally not performed, and the human skin color tends to appear dark red. 4. What is the gamma value of the display system? As mentioned above, simulating the change of gray from 0-100% by dithering the nearby points produces a linear luminance change independent of the gamma value of the display. This fact is used on the following image to demonstrate the effect of displaying gamma values. (Picture: gamma2) The image consists of 2 lines with 3 blocks per line, each block varying from 25% to 75%. The upper line uses the gray value, and the lower line simulates the gray value using the dithering method. It is displayed on a screen gamma-corrected display so that the brightness of the lower row of gray blocks corresponds to the above row. If the screen does not have a gamma correction, the top row of blocks will appear darker than the bottom row. The best viewing distance is six feet from the display. The following image can be used to directly estimate the gamma value of the display system. It is 6 feet away from the display and determines which column of the image has the same brightness as the top or bottom squares. The number below is the gamma value of the currently used display system. Figure: Gamma3 (gamma test image) 5. How is the color matching situation? As mentioned above, displaying the gamma value in a non-linear manner, when changing the relative brightness of the red, green, and blue channels, may have a great influence on the hue of the image. If there is no gamma correction, it may be the most important cause of color shift between different monitors. Other phenomena can also affect the hue produced by different monitors. The primary colors of red, green, and blue are different depending on the monitor. Colors can be tested using the device-independent CIE color mode, which is based on visual analysis of the human eye. Two color sources in the same CIE color coordinate system appear to be the same in human eyes. A source color defined in the CIE system has two axes x, y. Since x, y, and z have been normalized like x+y+z=1, the third axis z does not have to be explicitly represented. This normalized form is used when representing a color that is not related to brightness. A fairly complete color response feature can be described in a defined way: 1) The gamma value, or the response curve to the red, green, and blue channels. 2) The display of the CIE color on the screen device is generated by displaying white, red, green and blue colors. White is the white point of the monitor; red, green, and blue are the primary colors of the monitor. The TIFF file format already has a description of the above features. The PNG format that is considered to replace the GIF format also has descriptions of these parameters. If the image is used for Web display, it will have a description flag of the monitor chroma mode, so that when displayed on various monitors, the image may show a more stable effect. The web browser software matches the chromaticity information of the file with the chromaticity information of the local display system and uses gamma and color correction.