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How do DirectDraw Surfaces store 16-bit images?

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My question is this, how does a DirectDraw surface store the pixel values for a 16-bit image. It must have to store each pixel''s colour value in 2 bytes. The bitmaps I am loading into my game are 24 bit, so they must have 3 bytes to store the RGB values for each pixel. So how does DirectDraw convert the 3 bytes into 2? Please help!

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It stores them in 555 mode (5 bits for red, 5 for...) if you have a 555 graphic card and 565 if you have one of those cards. Yes, that makes 15 or 16-bits, 16-bits equals 2 bytes. In case of 555 the high bit is always ignored ie. 0RRRRRGGGGGBBBBB.

The easiet (but not always the best, but its sufficient) way to convert a 24-bit pixel to 16-bit is to just downshift each RGB-component 3 (or 2) bits and then mask them together again. Like:

RRRRRRRRGGGGGGGGBBBBBBBB (this is our origional 24-bit pixel)

then do this for every component (this is for 565)
RRRRRRRRGGGGGGGGBBBBBBBB & 111110000000000000000000
RRRRR0000000000000000000 << (24 - 16)

and then for green...
RRRRRRRRGGGGGGGGBBBBBBBB & 000000001111110000000000
00000000GGGGGG0000000000 << (24 - (16 - 5))

and then the same for blue...

"This album was written, recorded and edited at Gröndal, Stockholm in the year of 2000. At this point in time money still ruled the world. Capitalistic thoughts were wide spread. From the sky filled with the fumes of a billionarie''s cigar to the deepest abyss drenched in nuclear waste. A rich kid was a happy kid, oh..dirty, filthy times. Let this be a reminder."
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