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c#colorsbit-shiftmultiplication

How to correctly multiply two colors with byte components?

I would like to multiply two colors whose components are byte but I'm running into inconsistent results.

When components are float it's exact and easy:

float a = 1.0f;
float b = 1.0f;
float c = a * b;

When components are byte, I vaguely remember there's some bit-shifting, however my formula is clearly missing something:

byte a = 255;
byte b = 255;
byte c = (byte)((a * b) >> 8);
// 255 * 255 = 65025 then 65025 >> 8 = 254
// obviously, adding 1 to the result is just wrong

Note:

I can't use a Color type from popular frameworks such as Forms or WPF, as my project acts as an abstraction layer that is to be re-used from different frameworks.

Question:

What is the right algorithm for for multiplying byte colors using bit-shifts ?

Solution

  • Harold's comment worked for me:

    (byte)((a * b + 0xFF) >> 8)

    I am reposting his comment as an answer to make his solution easier to find.