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Stop decoding BC1 punchthrough alpha in BC2&3 #4144

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merged 3 commits into from
Apr 15, 2021
Merged

Stop decoding BC1 punchthrough alpha in BC2&3 #4144

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097f7d0
Stop decoding BC1 punchthrough alpha in BC2&3
jansol Oct 15, 2019
ddd3a2b
Add tests for issue #4142
jansol Jan 9, 2020
b08c514
BcnDecode: add comment about BC2&3 color blocks
jansol Jan 9, 2020
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Binary file added Tests/images/dxt5-colorblock-alpha-issue-4142.dds
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15 changes: 15 additions & 0 deletions Tests/test_file_dds.py
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Original file line number Diff line number Diff line change
Expand Up @@ -190,6 +190,21 @@ def short_file():
short_file()


def test_dxt5_colorblock_alpha_issue_4142():
""" Check that colorblocks are decoded correctly in DXT5"""

with Image.open("Tests/images/dxt5-colorblock-alpha-issue-4142.dds") as im:
px = im.getpixel((0, 0))
assert px[0] != 0
assert px[1] != 0
assert px[2] != 0

px = im.getpixel((1, 0))
assert px[0] != 0
assert px[1] != 0
assert px[2] != 0


def test_unimplemented_pixel_format():
with pytest.raises(NotImplementedError):
Image.open("Tests/images/unimplemented_pixel_format.dds")
13 changes: 8 additions & 5 deletions src/libImaging/BcnDecode.c
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,7 @@ decode_565(UINT16 x) {
}

static void
decode_bc1_color(rgba *dst, const UINT8 *src) {
decode_bc1_color(rgba *dst, const UINT8 *src, int separate_alpha) {
bc1_color col;
rgba p[4];
int n, cw;
Expand All @@ -84,7 +84,10 @@ decode_bc1_color(rgba *dst, const UINT8 *src) {
r1 = p[1].r;
g1 = p[1].g;
b1 = p[1].b;
if (col.c0 > col.c1) {


/* NOTE: BC2 and BC3 reuse BC1 color blocks but always act like c0 > c1 */
if (col.c0 > col.c1 || separate_alpha) {
p[2].r = (2 * r0 + 1 * r1) / 3;
p[2].g = (2 * g0 + 1 * g1) / 3;
p[2].b = (2 * b0 + 1 * b1) / 3;
Expand Down Expand Up @@ -150,13 +153,13 @@ decode_bc3_alpha(char *dst, const UINT8 *src, int stride, int o) {

static void
decode_bc1_block(rgba *col, const UINT8 *src) {
decode_bc1_color(col, src);
decode_bc1_color(col, src, 0);
}

static void
decode_bc2_block(rgba *col, const UINT8 *src) {
int n, bitI, byI, av;
decode_bc1_color(col, src + 8);
decode_bc1_color(col, src + 8, 1);
for (n = 0; n < 16; n++) {
bitI = n * 4;
byI = bitI >> 3;
Expand All @@ -168,7 +171,7 @@ decode_bc2_block(rgba *col, const UINT8 *src) {

static void
decode_bc3_block(rgba *col, const UINT8 *src) {
decode_bc1_color(col, src + 8);
decode_bc1_color(col, src + 8, 1);
decode_bc3_alpha((char *)col, src, sizeof(col[0]), 3);
}

Expand Down