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Fix coefficients calculation (#2162)

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Fix coefficients calculation

* test for regression

* detailed comments what is going on
prevent setting the `k[-1]` item

* more readable
This commit is contained in:
Alexander Karpinsky 2016-10-17 11:24:40 +03:00 committed by wiredfool
parent 2e1a3c29aa
commit b096fcc20f
2 changed files with 19 additions and 3 deletions
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11
Tests/test_image_resample.py
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@ -336,6 +336,17 @@ class CoreResampleCoefficientsTest(PillowTestCase):
self.assertEqual(test_color // 2, px[2, 0])
# print '\r>', size, test_color // 2, px[2, 0]
def test_nonzero_coefficients(self):
# regression test for the wrong coefficients calculation
# due to bug https://github.com/python-pillow/Pillow/issues/2161
im = Image.new('RGBA', (1280, 1280), (0x20, 0x40, 0x60, 0xff))
histogram = im.resize((256, 256), Image.BICUBIC).histogram()
self.assertEqual(histogram[0x100 * 0 + 0x20], 0x10000) # first channel
self.assertEqual(histogram[0x100 * 1 + 0x40], 0x10000) # second channel
self.assertEqual(histogram[0x100 * 2 + 0x60], 0x10000) # third channel
self.assertEqual(histogram[0x100 * 3 + 0xff], 0x10000) # fourth channel
if __name__ == '__main__':
unittest.main()

11
libImaging/Resample.c
View File

@ -176,18 +176,23 @@ precompute_coeffs(int inSize, int outSize, struct filter *filterp,
k = &kk[xx * kmax];
for (x = 0; x < xmax; x++) {
double w = filterp->filter((x + xmin - center + 0.5) * ss);
k[x] = w;
ww += w;
// We can skip extreme coefficients if they are zeroes.
if (w == 0) {
// Skip from the start.
if (x == 0) {
// At next loop `x` will be 0.
x -= 1;
// But `w` will not be 0, because it based on `xmin`.
xmin += 1;
xmax -= 1;
} else if (x == xmax - 1) {
// Truncate the last coefficient for current `xx`.
xmax -= 1;
}
continue;
}
k[x] = w;
ww += w;
}
for (x = 0; x < xmax; x++) {
if (ww != 0.0)