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Ensure TIFF RowsPerStrip is multiple of 8 for JPEG compression #5588

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merged 2 commits into from Jul 18, 2021
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Ensure TIFF RowsPerStrip is multiple of 8 for JPEG compression #5588

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Ensure TIFF RowsPerStrip is multiple of 8 for JPEG compression
kmilos Jul 7, 2021
abb192c
Added test
radarhere Jul 7, 2021
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5 changes: 3 additions & 2 deletions Tests/test_file_libtiff.py
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Expand Up @@ -968,10 +968,11 @@ def test_realloc_overflow(self):
assert str(e.value) == "-9"
TiffImagePlugin.READ_LIBTIFF = False

def test_save_multistrip(self, tmp_path):
@pytest.mark.parametrize("compression", ("tiff_adobe_deflate", "jpeg"))
def test_save_multistrip(self, compression, tmp_path):
im = hopper("RGB").resize((256, 256))
out = str(tmp_path / "temp.tif")
im.save(out, compression="tiff_adobe_deflate")
im.save(out, compression=compression)

with Image.open(out) as im:
# Assert that there are multiple strips
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3 changes: 3 additions & 0 deletions src/PIL/TiffImagePlugin.py
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Expand Up @@ -1577,6 +1577,9 @@ def _save(im, fp, filename):
# aim for 64 KB strips when using libtiff writer
if libtiff:
rows_per_strip = min((2 ** 16 + stride - 1) // stride, im.size[1])
# JPEG encoder expects multiple of 8 rows
if compression == "jpeg":
rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, im.size[1])
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Suggested change
rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, im.size[1])
rows_per_strip = ((rows_per_strip + 7) // 8) * 8

Because rows_per_strip has already been set to be im.size[1] at minimum, and the calculation here only ever rounds up, the min part is unnecessary.

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I believe it is needed, precisely because it rounds up. This is to support the rare case of small images where im.size[1]<8 and you end up as a single strip. Libtiff/jpeg codec have no problem handling the single/last strip that is not factor of 8, as it is padded internally.

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Ok, thanks

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@radarhere radarhere Jul 10, 2021 •
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Suggested change
rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, im.size[1])
rows_per_strip = min(((rows_per_strip + 7) // 8) * 8, rows_per_strip)

What about this?

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That won't work for the corner case where you have u multi-strip w/ e.g. rows_per_strip=2 and im.size[1]=6. For JPEG, you want to end up as single strip of 6 rows.

else:
rows_per_strip = im.size[1]
strip_byte_counts = stride * rows_per_strip
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