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January 2004
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2004.01.15 17:24 "Re: COLORMAP and byte padding", by Marti Maria
Hi,
>
> Your code will produce false results. Although I'd divide by 256, I would
> not multiple by 256 (which is where your errors come in). You'd still need
> to multiple by 257 so as to re-map the range back correctly, so a16_by256
> should use the same calculation as a16_by257.
Ok, I have replaced the multiplication by
a16_by256 = ((unsigned short) (a8_by256 << 8) | a8_by256 );
And the results are still:
Max error: by 256 = 255, by 257 = 128
Mean error: by 256 = 85.3333, by 257 = 64.249
I was proposing the macro:
#define RGB_16_TO_8(rgb) (BYTE) ((((rgb)*65281+8388608)>>24)&0xFF)
Because this *does* produce same results as floor(rgb / 257.0 + 0.5)
Max error: by macro = 128, by double = 128
Mean error: by macro = 64.249, by double = 64.249
Check it by yourself :-)
Regards,
Marti.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define RGB_16_TO_8(rgb) (unsigned char) ((((rgb)*65281+8388608)>>24)&0xFF)
#define RGB_8_TO_16(rgb) (unsigned short)((((unsigned short)(rgb))<<8)|rgb))
int main()
{
int i;
unsigned char byMacro_8, byDouble_8;
unsigned short byMacro_16, byDouble_16;
int errorMacro, errorDouble;
int maxMacro = 0, maxDouble = 0;
double sumMacro=0., sumDouble=0.;
for (i=0; i < 0xFFFF; i++) {
byMacro_8 = (unsigned char) RGB_16_TO_8(i);
byDouble_8 = (unsigned char) (double) floor(i/257.+.5);
byMacro_16 = (unsigned short) RGB_8_TO_16(byMacro_8);
byDouble_16 = (unsigned short) (double) floor(byDouble_8*257.0+0.5);
errorMacro = abs(byMacro_16 - i);
errorDouble = abs(byDouble_16 - i);
if (errorMacro > maxMacro) maxMacro = errorMacro;
if (errorDouble > maxDouble) maxDouble = errorDouble;
sumMacro += errorMacro;
sumDouble += errorDouble;
}
printf("Max error: by macro = %d, by double = %d\n", maxMacro,
maxDouble);
printf("Mean error: by macro = %g, by double = %g\n", sumMacro / 65535.,
sumDouble / 65535.);
return 0;
}