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I was not able to find an answer via web search. AFAIK LCD displays can control brightness via changing brightness of backlight, in turn via electric current value (up/down). I don't recall reading about changing properties of screen layers or else by a user. What is the point of having two sliders: brightness / contrast?

On laptops there is usually only brightness but on most TVs and standalone displays I've used both brightness and contrast. What each usually does/change? Why laptops have one slider and standalones two? TIA

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In video processing, even before you think about a picture shown on any specific type of screen, you have the concepts of brightness and contrast.

You change brightness of a video signal by adding (or subtracting) an offset. You change the contrast by multiplying the signal with a coefficient.

That is what is done inside a TV video processing path, whether the signal is analog like it was on CRT TVs or digital like with modern TVs.

But the brightness "knob" on a modern TV does not directly control the backlight brightness. That's because of many things like being able to have a wide contrast range by dynamically adjusting both the backlight and the pixel data on screen, or have multiple areas with different range, sometimes called local dimming.

Now, computer screens are not TVs. Some still have both brightness and contrast settings. For a laptop, you generally just want to be able to change how bright the screen is. Changing how bright is does not mean you are actually controlling only the backlight or only altering the video pixel data. The one brightness knob may control both the backlight and pixel data in any way necessary to be able to provide the best picture at different settings.

Also computer screen use standards such as sRGB how the colors and contrast should look like, so there is little motivation to alter the contrast. In video player programs you can change the contrast and brightness for the decoded data. For TVs, they also use standards, but with TVs, consumers are expected to have access to legacy features to adjust their settings to their likings, including contrast, brightness, saturation, hue, and even sharpness and whatnot.

If you simply talk about LCDs such as the ones with HD44780 or similar controller, the contrast is just the LCD glass drive voltage basically.

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  • \$\begingroup\$ Fascinating. I had no idea displays' hardware makes black pixels look more whitish. Maybe because for a long time ads for OLEDs emphasize perfect black as apposed to LCD; so I have not thought LCDs make black pixels less black on purpose (and as you write even on one knob displays). Now I wonder how OLED TVs increase "brightness" (I own only smartphone OLED, no TVs). - do they abandon perfect black and change offset? \$\endgroup\$ Dec 19, 2023 at 20:57
  • \$\begingroup\$ @Martian2020 No I don't know how OLEDs work but if I had to guess off is still off and the range of current is just higher. It may be set with some reference or just using higher digital numbers. \$\endgroup\$
    – Justme
    Dec 19, 2023 at 21:10
  • \$\begingroup\$ From a practical point of view I now try to find out at which brightness setting my LCD TV has maximum dynamic range. 0 now: seems not good image, next guess is 50% (no offset?). On your comment for OLED, "using higher digital numbers": black is 0x0x0 RGB, higher will be not black. \$\endgroup\$ Dec 19, 2023 at 21:16
  • \$\begingroup\$ But maybe dynamic range is same for any brightness...if LCD can change by many more steps than 256 steps of 8 (24 total) bits RGB data. Do you know how many steps typical LCD display can do on each pixel/crystal? maybe worth separate question here. \$\endgroup\$ Dec 19, 2023 at 21:23
  • \$\begingroup\$ @Martian2020 Depends on LCD. Maybe from 6 to 12 bits. And don't forget video is video until converted to drive the pixels. It might never be RGB in form you know it. For example 8-bit video is from 16 to 235 so there's not even full 256 steps in use but 220. Even your computer may not send 0..255 range to a TV. \$\endgroup\$
    – Justme
    Dec 19, 2023 at 21:29

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