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If height is scaled down significantly more than width, resize vertically first #9549

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3cab8da
If height is being scaled down, but not width, resize vertically first
radarhere Apr 7, 2026
2df5bbc
Merge branch 'main' into resize
radarhere Aug 16, 2026
c47dfdf
Delete intermediate image on error
radarhere Sep 7, 2026
c7835cd
Merge branch 'main' into resize
radarhere Sep 7, 2026
1446b77
Update order condition
radarhere Sep 8, 2026
eb69fd6
Do not shift bounds by zero
radarhere Sep 8, 2026
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9 changes: 1 addition & 8 deletions src/PIL/Image.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2442,14 +2442,7 @@ def resize(
(box[3] - reduce_box[1]) / factor_y,
)

if self.size[1] > self.size[0] * 100 and size[1] < self.size[1]:
im = self.im.resize(
(self.size[0], size[1]), resample, (0, box[1], self.size[0], box[3])
)
im = im.resize(size, resample, (box[0], 0, box[2], size[1]))
else:
im = self.im.resize(size, resample, box)
return self._new(im)
return self._new(self.im.resize(size, resample, box))

def reduce(
self,
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140 changes: 89 additions & 51 deletions src/libImaging/Resample.c
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Original file line number Diff line number Diff line change
Expand Up @@ -828,82 +828,120 @@ ImagingResampleInner(
Imaging imTemp = NULL;
Imaging imOut = NULL;

int i, need_horizontal, need_vertical;
int i, second_pass, need_horizontal, need_vertical, error = 0;
int ybox_first, ybox_last;
int ksize_horiz, ksize_vert;
int ksize_horiz = 0, ksize_vert = 0;
int *bounds_horiz, *bounds_vert;
double *kk_horiz, *kk_vert;

need_horizontal = xsize != imIn->xsize || box[0] || box[2] != xsize;
need_vertical = ysize != imIn->ysize || box[1] || box[3] != ysize;

ksize_vert = precompute_coeffs(
imIn->ysize, box[1], box[3], ysize, filterp, &bounds_vert, &kk_vert
);
if (!ksize_vert) {
return NULL;
}
// If height is being scaled down more than twice the amount the width is,
// run the vertical pass first, to make the horizontal pass faster
int horizontal_first =
!((imIn->ysize - ysize) > 0 &&
(imIn->ysize - ysize) > (imIn->xsize - xsize) * 2);

// First used row in the source image
ybox_first = bounds_vert[0];
// Last used row in the source image
ybox_last = bounds_vert[ysize * 2 - 2] + bounds_vert[ysize * 2 - 1];

/* two-pass resize, horizontal pass */
if ((need_horizontal && horizontal_first) || need_vertical) {
ksize_vert = precompute_coeffs(
imIn->ysize, box[1], box[3], ysize, filterp, &bounds_vert, &kk_vert
);
if (!ksize_vert) {
return NULL;
}
}
if (need_horizontal) {
if (horizontal_first) {
// First used row in the source image
ybox_first = bounds_vert[0];
// Last used row in the source image
ybox_last = bounds_vert[ysize * 2 - 2] + bounds_vert[ysize * 2 - 1];

// Shift bounds for vertical pass
if (ybox_first != 0) {
for (i = 0; i < ysize; i++) {
bounds_vert[i * 2] -= ybox_first;
}
}
}

ksize_horiz = precompute_coeffs(
imIn->xsize, box[0], box[2], xsize, filterp, &bounds_horiz, &kk_horiz
);
if (!ksize_horiz) {
free(bounds_vert);
free(kk_vert);
return NULL;
error = 1;
goto end;
}
}

// Shift bounds for vertical pass
for (i = 0; i < ysize; i++) {
bounds_vert[i * 2] -= ybox_first;
#define PASS(function, w, h, offset, ksize, bounds, kk) \
second_pass = imTemp != NULL; \
imTemp = ImagingNewDirty(imIn->mode, w, h); \

@hugovk hugovk Sep 7, 2026 •
edited
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Possible memory leak here: if the second pass's ImagingNewDirty fails, imTemp is overwritten with NULL before we goto end, so the intermediate image from the first pass (now only referenced by imIn) is never deleted. We had an ImagingDelete(imTemp) call before.

@radarhere radarhere Sep 7, 2026

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Good catch. I've pushed a commit.

if (!imTemp) { \
if (second_pass) { \
ImagingDelete(imIn); \
} \
error = 1; \
goto end; \
} \
function(imTemp, imIn, offset, ksize, bounds, kk); \
if (second_pass) { \
ImagingDelete(imIn); \
} \
imIn = imTemp;

if (horizontal_first) {
if (need_horizontal) {
PASS(
ResampleHorizontal,
xsize,
ybox_last - ybox_first,
ybox_first,
ksize_horiz,
bounds_horiz,
kk_horiz
);
}

imTemp = ImagingNewDirty(imIn->mode, xsize, ybox_last - ybox_first);
if (imTemp) {
ResampleHorizontal(
imTemp, imIn, ybox_first, ksize_horiz, bounds_horiz, kk_horiz
if (need_vertical) {
PASS(ResampleVertical, xsize, ysize, 0, ksize_vert, bounds_vert, kk_vert);
}
} else {
if (need_vertical) {
PASS(
ResampleVertical,
imIn->xsize,
ysize,
0,
ksize_vert,
bounds_vert,
kk_vert
);
}
free(bounds_horiz);
free(kk_horiz);
if (!imTemp) {
free(bounds_vert);
free(kk_vert);
return NULL;
if (need_horizontal) {
PASS(
ResampleHorizontal, xsize, ysize, 0, ksize_horiz, bounds_horiz, kk_horiz
);
}
imOut = imIn = imTemp;
}

/* vertical pass */
if (need_vertical) {
imOut = ImagingNewDirty(imIn->mode, imIn->xsize, ysize);
if (imOut) {
/* imIn can be the original image or horizontally resampled one */
ResampleVertical(imOut, imIn, 0, ksize_vert, bounds_vert, kk_vert);
}
/* it's safe to call ImagingDelete with empty value
if previous step was not performed. */
ImagingDelete(imTemp);
free(bounds_vert);
free(kk_vert);
if (!imOut) {
return NULL;
}
} else {
// Free in any case
end:
if (ksize_horiz) {
free(bounds_horiz);
free(kk_horiz);
}
if (ksize_vert) {
free(bounds_vert);
free(kk_vert);
}
if (error) {
return NULL;
}

/* none of the previous steps are performed, copying */
if (!imOut) {
if (imTemp) {
imOut = imTemp;
} else {
// none of the previous steps are performed, copying
imOut = ImagingCopy(imIn);
}

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