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From Wikipedia:

The other form is where the 8 bits directly describe red, green, and blue values, typically with three bits for red, three bits for green and two bits for blue. This second form is often called 8-bit truecolor

What is the reason only 2 bits are used for blue as opposed to red or green? Does it have to do with human perception of color or was it just an arbitrary decision?

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    RGB16 has the same preference and assigns an extra bit to green, for the same reasons. – tofro Sep 5 at 14:35
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    Digressive question, not in any way a comment on this question: when Wikipedia asserts "[t]his second form is often called 8-bit truecolor", is anybody else tempted to throw in a 'citation needed'? I can't figure out whether I've just been out of the loop on terminology. – Tommy Sep 5 at 15:56
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    @Tommy: I'd call it "8-bit direct color" (en.wikipedia.org/wiki/Color_depth#Direct_color). "True color" usually means RGB888. – fadden Sep 5 at 18:32
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    @Tommy With 8 bits only, "true" is a very relative term. The standard term I know for 16-bit color is "high", and "true" starts at 24-bits color resolution. – tofro Sep 6 at 7:30
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    @Tommy Wikipedia should have another term "I don't believe it". "Citation needed" should be used for things that I might believe but that isn't common knowledge so I'd like to see it written down somewhere. "8 bit truecolor" is a ridiculous term. – gnasher729 Sep 7 at 21:30
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Because the human eye is less sensitive to blue colour.

It's also more senstitive to green than red, so depending on the number of bits available modulo 3 :

  • 0 : The same number of bits is used for each colour (example: 24-bit)
  • 1 : The extra bit is accorded to the green colour, which is the one the human eye is most sensitive (example: 16-bit)
  • 2 : The extra bits are accorded to green and red, as the human eye is less sensitive to blue (example: 8-bit, 32-bit)
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    Also 1: the extra bit is assigned to all three channels. This is rare, but used for example in the SAM Coupé's 7-bit palette. – user3570736 Sep 5 at 7:20
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    @user3570736 Interesting catch. I guess it makes sense to get "pure white" or "pure gray", which is complicated on systems whose number of bits is not the same among channels. – Bregalad Sep 5 at 8:46
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    @user3570736 not that rare actually, lots of 8-bit computers have 4-bit RGBI colour (where I is the "intensity" which basically increases all three channels). – Muzer Sep 5 at 15:24
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    @Muzer RGBI still works in the Microsoft color command for cmd.exe. – Monty Harder Sep 5 at 15:43
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    RGB555 is a thing because RGB565 has the impure grey level issue. The high bit in RGB555 is simply ignored. Some systems will use ARGB1555/RGBA5551 to get basic transparency for something like 3D textures, but 16-bit texture support isn't too popular. – fadden Sep 5 at 16:03
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What is the reason only 2 bits are used for blue as opposed to red or green? Does it have to do with human perception of color or was it just an arbitrary decision?

It's a combination of being practical constraint by 8 bit words (*1) and adapting this to human physiology. The human eye is most sensitive to green and red (with a little tilt toward green), while least sensitive to blue (*2,3). See also this answer regarding why green is quite common.

For computing the most simple solution is to assign the colours in a colour word to an independent bit, leaving the misery that 8 cannot be divided by 3 without rest (*4). So next best is assigning a different amount to each. To do so the assignment with the least loss is to be selected, and that's where human physiology comes in again and tells that a reduction of resolution for blue is the least offensive. So the bit gets dropped there.

Bregalads Answer offers a nice and easy to use cookbook to decide what colours should get which bit length assigned when using dedicated bit fields.


Now, looking at the way we perceive light, It may be way more appropriate to use a palette instead of bitfields. Here colours (including brightness) can be specified in a way more natural way - just, handling them isn't as easy and cheap (*5).


*1 - This is where 'octal' word sizes would give a more 'natural' representation. Considering that computing started out with the majority of designs using multiple of 3 word sizes (see here as well), today's ubiquitous use of colour graphics seems like staircase wit.

*2 - Colour is a continuum, and the human eye has got certain spots of maximum sensitivity, but they are not hard focused, but ranges with a main peak. These ranges even overlap quite a lot between red and green while overlap between green and blue is small. Also the distances between are not the same.

To make it even more complex, Human vision also has a fourth component we usually assume with light and dark (B&W). While it has a quite wide sensitivity, it also has a peak just at the border between green and blue, partly filling that gap - one of the reason why we experience bluish light to be brighter than any other.

As result, we do see brightness on its own (with peak at 500 nm) plus three colours, but they are as well in different grades of brightness and all of them overlapping - perfect for neuronal networks, isn't it?

Remember that white/black/blue/gold dress?

*3 - See also this answer regarding why green (and amber) screens were most common

*4 - One of the disadvantages of being a Human. (Many) Birds feature four colour sensors, the additional being in near-UV. In addition, some have filters within their colour regions 'sharpening' colours. Something we seem to have lost. So they see colour not only more differentiated, but as well separated from brightness.

*5 - While light itself is defined on a linear scale and presented of a mixture of discrete values, human vision is not. Just have a look at the various colour scales that have been invented to describe human vision and you'll see that there's not an easy way to describe it in 2 or 3 dimensions at all. It's been a topic since ancient times and may go on forever.

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    Some (very few) humans have four distinct cones, too! Even amongst those with "normal" colour vision, there's variation in relative sensitivity across the spectrum; our colour representations are something of a compromise to fit an approximately average person. – Toby Speight Sep 5 at 17:42
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    I'm sure that the Acorn Archimedes' hybrid approach — 8bpp colour in which 4 bits specify brightness and 4 bits are a lookup into a colour palette — is definitely recognition of the peculiarities of human vision as noted here, and not just a way to save some transistors. – Tommy Sep 5 at 18:16
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    @Tommy It for sure is a different way to capture more dynamic with limited resources.Then again, it won't replace a full 256 entry palette that could be made using the same screen data. – Raffzahn Sep 5 at 19:04
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    @Tommy: I'm not familiar with the Archimedes, but a common approach for machines that produce a composite video output is to generate a composite video signal by delaying a square wave by a certain amount controlled by some bits, filtering it, and adding a DC bias.controlled by some other bits. A true-color composite signal should allow the amplitude of the chroma signal to be adjusted, but 1970s-1980s computers typically only offer on/off control. The Atari 2600 is the first machine I know of to use this approach, and it was adopted not because it would allow 128 beautiful colors... – supercat Sep 7 at 17:55
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    ...but because the designers wanted to have enough gray levels to be usable on black and white sets, and have a few "good" colors available for various purposes. The fact that the palette offers nice chroma gradients rainbow effects turned out to be an unexpected bonus which wasn't exploited until the system had been out for years. – supercat Sep 7 at 17:57
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One way to avoid this imbalance is to divide the total palette by something other than a power of two when assigning colour channels. This is done most famously by the Web Safe Palette, which is a uniform 6x6x6 RGB cube totalling 216 colours. For comparison the RGB332 palette is an 8x8x4 RGB cuboid.

Because there is no integer cube-root of 256, there are a few palette entries left over from this process. These can be used to cover spot-colours which would otherwise need to be dithered to be displayed accurately with the base palette.

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    What you're describing isn't truecolor/direct color, it's a specific type of indexed color. – Mark Sep 5 at 20:56
  • @mark Yes, it's not (strictly), thus not an direct answer, but nonetheless valuable information added which goes beyond what a commend is good for. – Raffzahn Sep 5 at 22:32
  • @Mark: Generally the 6x6x6 color cube is implemented using a 256x18 or 256x24 lookup table, but that's an implementation detail. I'd consider it "direct" color in a drawing system that makes no effort to handle or exploit any color outside the 6x6x6 cube. I don't know of any machines that use a hard-wired mapping between 8-bit values and RGB values in the range 0..5, but it wouldn't be overly difficult. Use the top two bits to identify cases which of (rgb) have values over 3. 00=none; 01=red and either green or not blue; 10=green but not red; 11=blue but not green. – supercat Sep 7 at 17:49
  • Use two bits each for rgb if 0..3; for the 4..5 case, use one bit to select between 4-5 and the other to indicate whether the "next" color is also 4..5. Using a 32x6 ROM would be cheaper than using a bunch of AND and OR gates, but what's important is that a hardware 6x6x6 color cube mapping would be feasilble. – supercat Sep 7 at 17:51

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