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January 2004
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2004.01.14 17:24 "Re: COLORMAP and byte padding", by Phillip Crews
At 11:43 AM 1/14/2004 , Joris wrote: > > For the perfectionists who are wondering: multiplying by 257 works > > perfectly. > > Agreed on the multiplying by 257. I dissagree on the 'for the > perfectionists' part, though, because multiplying by 257 is simply the only > correct thing to do and that's all there is to it. > > The general rule if 0 maps to 0: divide by the max in the scale you're > leaving, and multiply by the max in the scale you're entering. Thus, going > from 8bit to 16bit becomes: > > uint16val = uint8val/255*65535 > = uint8val*257 > = uint8val*256+uint8val*1 > = ((uint8val << 8) | uint8val) I agree here. Converting 4-bit to 8-bit is a multiply by 17 as well, since x^2 -1 = (x-1) * (x+1), so 255 = 15*17. > Going from 16bit to 8bit becomes: > uint8val = uint16val/65535*255 > = uint16val/257 I disagree. Using integer division, the only 16-bit pixel value that will result in white (255) is 0xffff, and 0x0100 will incorrectly be black. I've considered (uint16val+128)/257, but prefer uintval/256 because it is significantly faster and evenly spills the range of valid 16-bit values into 8-bit buckets. - Phillip