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/*
* The Python Imaging Library
* $Id$
*
* Alpha composite imSrc over imDst.
* https://en.wikipedia.org/wiki/Alpha_compositing
*
* See the README file for details on usage and redistribution.
*/
#include "Imaging.h"
#define PRECISION_BITS 7
typedef struct {
UINT8 r;
UINT8 g;
UINT8 b;
UINT8 a;
} rgba8;
Imaging
ImagingAlphaComposite(Imaging imDst, Imaging imSrc) {
Imaging imOut;
int x, y;
/* Check arguments */
if (!imDst || !imSrc || strcmp(imDst->mode, "RGBA") ||
imDst->type != IMAGING_TYPE_UINT8 || imDst->bands != 4) {
return ImagingError_ModeError();
}
if (strcmp(imDst->mode, imSrc->mode) || imDst->type != imSrc->type ||
imDst->bands != imSrc->bands || imDst->xsize != imSrc->xsize ||
imDst->ysize != imSrc->ysize) {
return ImagingError_Mismatch();
}
imOut = ImagingNewDirty(imDst->mode, imDst->xsize, imDst->ysize);
if (!imOut) {
return NULL;
}
for (y = 0; y < imDst->ysize; y++) {
rgba8 *dst = (rgba8 *)imDst->image[y];
rgba8 *src = (rgba8 *)imSrc->image[y];
rgba8 *out = (rgba8 *)imOut->image[y];
for (x = 0; x < imDst->xsize; x++) {
if (src->a == 0) {
// Copy 4 bytes at once.
*out = *dst;
} else {
// Integer implementation with increased precision.
// Each variable has extra meaningful bits.
// Divisions are rounded.
UINT32 tmpr, tmpg, tmpb;
UINT32 blend = dst->a * (255 - src->a);
UINT32 outa255 = src->a * 255 + blend;
// There we use 7 bits for precision.
// We could use more, but we go beyond 32 bits.
UINT32 coef1 = src->a * 255 * 255 * (1 << PRECISION_BITS) / outa255;
UINT32 coef2 = 255 * (1 << PRECISION_BITS) - coef1;
tmpr = src->r * coef1 + dst->r * coef2;
tmpg = src->g * coef1 + dst->g * coef2;
tmpb = src->b * coef1 + dst->b * coef2;
out->r =
SHIFTFORDIV255(tmpr + (0x80 << PRECISION_BITS)) >> PRECISION_BITS;
out->g =
SHIFTFORDIV255(tmpg + (0x80 << PRECISION_BITS)) >> PRECISION_BITS;
out->b =
SHIFTFORDIV255(tmpb + (0x80 << PRECISION_BITS)) >> PRECISION_BITS;
out->a = SHIFTFORDIV255(outa255 + 0x80);
}
dst++;
src++;
out++;
}
}
return imOut;
}
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