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rgb332.md

@@ -1,8 +1,8 @@
 # RGB332
 
-RGB332 is a general 256 color [palette](palette.md) that encodes one color with 1 byte (i.e. 8 bits): 3 bits for red, 3 bits for green and 2 bits for blue (human eye is least sensitive to blue). RGB332 is an implicit palette -- it doesn't have to be stored in memory because the color index itself determines the color and vice versa. Compared to the classic 24 bit RGB (which assigns 8 bits to each of the RGB components), RGB332 is very "[KISS](kiss.md)/[suckless](suckless.md)" and often [good enough](good_enough.md) (especially with [dithering](dithering.md)) as it saves memory, avoids headaches with [endianness](byte_sex.md) and represents each color with just a single number (as opposed to 3), so it is often used in simple and limited computers such as [embedded](embedded.md). It is also in the [public domain](public_domain.md), unlike some other palettes, so it's additionally a legally safe choice. RGB332 also has a "sister palette" called [RGB565](rgb565.md) which uses two bytes instead of one and so offers many more colors.
+RGB332 is a general 256 color [palette](palette.md) that encodes one color with 1 [byte](byte.md) (i.e. 8 [bits](bit.md)): 3 bits (highest) for red, 3 bits for green and 2 bits (lowest) for blue (as human eye is least sensitive to blue). RGB332 is an implicit palette -- it doesn't have to be stored in memory because the color index itself determines the color and vice versa. Compared to the classic 24 bit RGB (which assigns 8 bits to each of the RGB components), RGB332 is very "[KISS](kiss.md)/[suckless](suckless.md)" and often [good enough](good_enough.md) (especially with [dithering](dithering.md)) as it saves memory, avoids headaches with [endianness](byte_sex.md) and represents each color with just a single number (as opposed to 3), so it is often used in simple and limited computers such as [embedded](embedded.md). It is also in the [public domain](public_domain.md), unlike some other palettes, so it's additionally a legally safe choice. RGB332 also has a "sister palette" called [RGB565](rgb565.md) which uses two bytes instead of one and so offers many more colors.
 
-A disadvantage of plain 332 palette is the linearity of each component's intensity, i.e. lack of [gamma correction](gamma_correction.md), so there are too many almost indistinguishable bright colors while too few darker ones { TODO: does a gamma corrected 332 exist? make it? ~drummyfish }. Another disadvantage is the non-alignment of the blue component with red and green components, i.e. while R/G components have 8 levels of intensity and so step from 0 to 255 by 36.4, the B component only has 4 levels and steps by exactly 85, which makes it impossible to create exact shades of grey (which has to have all R, G and B components equal).
+A disadvantage of plain 332 palette lies in the linearity of each component's intensity, i.e. lack of [gamma correction](gamma_correction.md), so there are too many almost indistinguishable bright colors while too few darker ones { TODO: does a gamma corrected 332 exist? make it? ~drummyfish }. Another disadvantage is the non-alignment of the blue component with red and green components, i.e. while R/G components have 8 levels of intensity and so step from 0 to 255 by 36.4, the B component only has 4 levels and steps by exactly 85, which makes it impossible to create exact shades of grey (which of course have to have all R, G and B components equal).
 
 The RGB values of the 332 palette are following:
 
@@ -41,15 +41,52 @@ The RGB values of the 332 palette are following:
 #ffda00 #ffda55 #ffdaaa #ffdaff #ffff00 #ffff55 #ffffaa #ffffff
 ```
 
-With RGB332 addition/subtraction of colors can be [approximated](approximation.md) in a very fast way using the [OR](or.md)/[AND](and.md) operation -- however this only works sometimes (check visually). For example if you need to quickly brighten/darken all pixels in a 332 image, you can just OR/AND each pixel with these values:
+## Operations
+
+Here are [C](c.md) functions for converting RGB332 to RGB24 and back:
 
 ```
-brighten by 3:   | 0x6d  (011 011 01)
-brighten by 2:   | 0x49  (010 010 01)
-brighten by 1:   | 0x24  (001 001 00)
-darken by 1:     & 0xdb  (110 110 11)
-darken by 2:     & 0xb6  (101 101 10)
-darken by 3:     & 0x92  (100 100 10)
+unsigned char rgbTo332(unsigned char red, unsigned char green, unsigned char blue)
+{
+  return ((red / 32) << 5) | ((green / 32) << 2) | (blue / 64);
+}
+
+void rgbFrom332(unsigned char colorIndex, unsigned char *red, unsigned char *green, unsigned char *blue)
+{
+  unsigned char value = (colorIndex >> 5) & 0x07;
+  *red = value != 7 ? value * 36 : 255;
+
+  value = (colorIndex >> 2) & 0x07;
+  *green = value != 7 ? value * 36 : 255;
+  
+  value = colorIndex & 0x03;
+  *blue = (value != 3) ? value * 72 : 255;
+}
 ```
 
-TODO: conversion to/from RGB24
+Addition/subtraction of two RGB332 colors can be performed by simply adding/subtracting the two color values as long as no over/underflow occurs in either component -- by adding the values we basically perform a parallel addition/subtraction of all three components with only one operation. Unfortunately checking for when exactly such overflow occurs is not easy to do quickly { Or is it? ~drummyfish }, but to rule out e.g. an overflow with addition we may for example check whether the highest bit of each component in both colors to be added is 0 (i.e. `if (((color1 & 0x92) | (color2 & 0x92)) == 0) newColor = color1 + color2;`). { Code untested. ~drummyfish }
+
+Addition/subtraction of colors can also be [approximated](approximation.md) in a very fast way using the [OR](or.md)/[AND](and.md) operation instead of arithmetic addition/subtraction -- however this only works sometimes (check visually). For example if you need to quickly brighten/darken all pixels in a 332 image, you can just OR/AND each pixel with these values:
+
+```
+brighten by more:  doesn't really work anymore
+brighten by 3:     | 0x6d  (011 011 01)
+brighten by 2:     | 0x49  (010 010 01)
+brighten by 1:     | 0x24  (001 001 00)
+darken by 1:       & 0xdb  (110 110 11)
+darken by 2:       & 0xb6  (101 101 10)
+darken by 3:       & 0x92  (100 100 10)
+darken by more:    doesn't really work anymore
+```
+
+{ TODO: Would it be possible to accurately add two 332 colors by adding all components in parallel using bitwise operators somehow? I briefly tried but the result seemed too complex to be worth it though. ~drummyfish }
+
+Inverting a 332 color is done simply by inverting all bits in the color value.
+
+TODO: blending?
+
+## See Also
+
+- [RGB565](rgb565.md)
+- [palette](palette.md)
+- [grayscale](grayscale.md)