# What's the point of 48 bit colour?

I've read that the human eye can only really decipher around 10 million different colours, so what's the point in technology that uses 48-bit colour, which allows for $2.81\times 10^{14}$ colours. Is this not like giant, super inefficient overkill?

• I'm not sure that this is the correct forum for this interesting question. May 2 '16 at 22:07
• @YuvalFilmus I think it's just about on-topic. Graphics and human-computer interaction are both CS and this question seems to be in those worlds. May 2 '16 at 22:17

Some reasons for using more bits per color channel:

1. Tetrachromats. There are rare humans with four types of cone receptors in their retinas instead of the usual three. These people can distinguish an order of magnitude more color shades than the rest of us.

2. Color gamut. Your question assumes that the 16 million colors available in 8-bit per channel RGB exactly covers the colors that normal humans can see. But if that were so then we would only need one RGB color space. We have more.. More bits per channel allows a wider gamut of colors to be represented without posterization, because the increased bit depth means smaller gaps between the individual shades.

3. Image manipulation. Every image manipulation, from switching color models for printing, to sharpening, to resizing, to tone curve adjustment, loses information. The more information you start with the more you have at the end of post-processing. The more information you start with the easier it is to avoid the introduction of visible artifacts like posterization.

• Your third point I think is the answer I was looking for thanks! How much information is lost exactly when performing certain manipulations though?
– Eric
May 3 '16 at 13:50

The 10 million you gave - something about that, but we do not know which one exactly because it differs per person.

So there is the case that some range of blue is not fully seen by some person, but the orange range is, and another one with different color spectrum looks at the same e.g. screen and also has plausible view without discontinuities.
Another point is using graphic effects, manipulating the image - with more discrete values the roundoffs are smaller.

The gamut, which represents what we see and how screens are displaying it we are far behind really showing all colors. Also please remember that every person has dense and sparse regions in different places, depending fully on individual rods and cones distribution in eye.

Let me introduce also some analogy to this "overkill":
Please think of frames per second in movies / games. For the most people 24fps is motion enough, 30fps for the majority, 60 fps - beyond human perception, but still we increase the counter and the feeling of motion being natural increases, which is also hard to explain because we cannot really "see" events lasting below 40ms, but we can tell the difference in reception, which with higher fps feels more natural.

Think about what we hear - 22kHz is the upper bound, but we preserve 44kHz and it makes sound less mechanical and the illusion of being at concert hall increases with higher sampling rate.
The same happens with screens (printers etc.).
There are hues that the most of the people can arrange colors in increasing order - for example people are very sensitive to green hue and see far more then screen can show.

• I can see flicker on those old-crt screens at 60hz. Only on 80Hz does it finally go away.... So 60 Hz is hardly beyond human perception. The 48 color depth is also very handy if you want to edit the picture and enhance some colors. Having more depth to begin with helps prevent posterization when messing around with colors. May 3 '16 at 0:48
• The part of manipulation I think is included. About flicker that was precisely the point, some people see it some do not. Anyway there are more possibilities - CRT was not keeping up (is off track or heavily used), you had in mind half frames, and I was about full frames or you have this rare high-frequency sight and read blazingly fast. (Sorry, just like to cover other possibilities).
– Evil
May 3 '16 at 1:05
• yes the rare high-frequency thing and no, no magic speed reading powers :-< May 3 '16 at 1:30
• Oh my, so you are the one with high context switching. You do not get fulled by color shift tricks... Try speed reading, you can master it faster than the most people ;)
– Evil
May 3 '16 at 1:38
• That's a good point I hadn't thought of. The density of colours we can see over the entire optical range might be small, but in certain ranges it might be very large. Thanks for the answer!
– Eric
May 3 '16 at 13:52

Regarding color gamut... it is almost impossible for ordinary 24 bit RGB to represent the color of many metals with any degree of accuracy. Increasing the bit depth should help improve that situation.