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98 lines
2.6 KiB
C
98 lines
2.6 KiB
C
/*
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* The Python Imaging Library
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* $Id$
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*
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* Alpha composite imSrc over imDst.
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* http://en.wikipedia.org/wiki/Alpha_compositing
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*
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* See the README file for details on usage and redistribution.
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*/
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#include "Imaging.h"
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typedef struct
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{
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UINT8 r;
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UINT8 g;
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UINT8 b;
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UINT8 a;
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} rgba8;
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Imaging
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ImagingAlphaComposite(Imaging imDst, Imaging imSrc)
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{
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Imaging imOut;
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int x, y;
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/* Check arguments */
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if (!imDst || !imSrc ||
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strcmp(imDst->mode, "RGBA") ||
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imDst->type != IMAGING_TYPE_UINT8 ||
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imDst->bands != 4)
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return ImagingError_ModeError();
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if (strcmp(imDst->mode, imSrc->mode) ||
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imDst->type != imSrc->type ||
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imDst->bands != imSrc->bands ||
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imDst->xsize != imSrc->xsize ||
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imDst->ysize != imSrc->ysize)
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return ImagingError_Mismatch();
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imOut = ImagingNew(imDst->mode, imDst->xsize, imDst->ysize);
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if (!imOut)
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return NULL;
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ImagingCopyInfo(imOut, imDst);
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for (y = 0; y < imDst->ysize; y++) {
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rgba8* dst = (rgba8*) imDst->image[y];
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rgba8* src = (rgba8*) imSrc->image[y];
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rgba8* out = (rgba8*) imOut->image[y];
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for (x = 0; x < imDst->xsize; x ++) {
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if (src->a == 0) {
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// Copy 4 bytes at once.
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*out = *dst;
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} else {
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// Integer implementation with increased precision.
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// Each variable has extra meaningful bits.
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// Divisions are rounded.
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// This code uses trick from Paste.c:
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// (a + (2 << (n-1)) - 1) / ((2 << n)-1)
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// almost equivalent to:
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// tmp = a + (2 << (n-1)), ((tmp >> n) + tmp) >> n
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UINT16 blend = dst->a * (255 - src->a);
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UINT16 outa255 = src->a * 255 + blend;
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// There we use 7 bits for precision.
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// We could use more, but we go beyond 32 bits.
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UINT16 coef1 = src->a * 255 * 255 * 128 / outa255;
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UINT16 coef2 = blend * 255 * 128 / outa255;
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#define SHIFTFORDIV255(a)\
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((a >> 8) + a >> 8)
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UINT32 tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
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out->r = SHIFTFORDIV255(tmpr) >> 7;
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UINT32 tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
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out->g = SHIFTFORDIV255(tmpg) >> 7;
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UINT32 tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
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out->b = SHIFTFORDIV255(tmpb) >> 7;
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out->a = SHIFTFORDIV255(outa255 + 0x80);
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}
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dst++; src++; out++;
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}
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}
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return imOut;
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}
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