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Speed up `Image.blend()` by akx · Pull Request #9649 · python-pillow/Pillow · GitHub

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Speed up Image.blend() #9649

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Merged
mergify merged 3 commits into from
Jun 29, 2026
+18 −11
Conversation 13 Commits 3 Checks 55 Files changed 1
Merged

Speed up Image.blend() #9649

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9787733
Blend: shortcut to copy when same image pointer is passed in twice
akx Jun 2, 2026
92a4479
Blend: hoist static image size variables out of loops for optimization
akx Jun 2, 2026
7a0534a
Blend: note imIn1/imIn2/imOut can't alias each other
akx Jun 2, 2026
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29 changes: 18 additions & 11 deletions src/libImaging/Blend.c
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Original file line number Diff line number Diff line change
Expand Up @@ -35,7 +35,7 @@ ImagingBlend(Imaging imIn1, Imaging imIn2, float alpha) {
}

/* Shortcuts */
if (alpha == 0.0) {
if (imIn1 == imIn2 || alpha == 0.0) {
return ImagingCopy(imIn1);
} else if (alpha == 1.0) {
return ImagingCopy(imIn2);
Expand All @@ -46,23 +46,30 @@ ImagingBlend(Imaging imIn1, Imaging imIn2, float alpha) {
return NULL;
}

// We know these won't change in the loops below,
// so the compiler can optimize better if we pull them out.
int ysize = imIn1->ysize;
int linesize = imIn1->linesize;

if (alpha >= 0 && alpha <= 1.0) {
/* Interpolate between bands */
for (y = 0; y < imIn1->ysize; y++) {
UINT8 *in1 = (UINT8 *)imIn1->image[y];
UINT8 *in2 = (UINT8 *)imIn2->image[y];
UINT8 *out = (UINT8 *)imOut->image[y];
for (x = 0; x < imIn1->linesize; x++) {
for (y = 0; y < ysize; y++) {
// restrict safe: imIn1 and imIn2 are read-only; imOut is a new allocation
UINT8 *restrict in1 = (UINT8 *)imIn1->image[y];
UINT8 *restrict in2 = (UINT8 *)imIn2->image[y];
UINT8 *restrict out = (UINT8 *)imOut->image[y];
for (x = 0; x < linesize; x++) {
out[x] = (UINT8)((int)in1[x] + alpha * ((int)in2[x] - (int)in1[x]));
}
}
} else {
/* Extrapolation; must make sure to clip resulting values */
for (y = 0; y < imIn1->ysize; y++) {
UINT8 *in1 = (UINT8 *)imIn1->image[y];
UINT8 *in2 = (UINT8 *)imIn2->image[y];
UINT8 *out = (UINT8 *)imOut->image[y];
for (x = 0; x < imIn1->linesize; x++) {
for (y = 0; y < ysize; y++) {
// restrict safe: imIn1 and imIn2 are read-only; imOut is a new allocation
UINT8 *restrict in1 = (UINT8 *)imIn1->image[y];
UINT8 *restrict in2 = (UINT8 *)imIn2->image[y];
UINT8 *restrict out = (UINT8 *)imOut->image[y];
for (x = 0; x < linesize; x++) {
float temp = (float)((int)in1[x] + alpha * ((int)in2[x] - (int)in1[x]));
if (temp <= 0.0) {
out[x] = 0;
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