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Accept grayscale JPEGs with non-1x1 sampling factors #99

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Accept grayscale JPEGs with non-1x1 sampling factors #99

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a255555
crn_jpgd: accept grayscale JPEGs with non-1×1 sampling factors
illwieckz Aug 5, 2026
aa8b02b
test: add JPEG samples with grayscale component using 1×1 and 2×2 sam…
illwieckz Aug 5, 2026
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8 changes: 6 additions & 2 deletions crnlib/crn_jpgd.cpp
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Expand Up @@ -2330,8 +2330,12 @@ void jpeg_decoder::init_frame() {
int i;

if (m_comps_in_frame == 1) {
if ((m_comp_h_samp[0] != 1) || (m_comp_v_samp[0] != 1))
stop_decoding(JPGD_UNSUPPORTED_SAMP_FACTORS);
if ((m_comp_h_samp[0] != 1) || (m_comp_v_samp[0] != 1)) {
// Some grayscale JPEGs contain non-1×1 sampling factors.
// There is no chroma plane to subsample, so normalize to the expected layout.
m_comp_h_samp[0] = 1;
m_comp_v_samp[0] = 1;
}

m_scan_type = JPGD_GRAYSCALE;
m_max_blocks_per_mcu = 1;
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14 changes: 14 additions & 0 deletions test/checksums.tsv
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Expand Up @@ -180,6 +180,20 @@ build/test/crunch-format-jpg-to-all/sample-black-64x64.crn 19fb840c8dbb7da96400a
build/test/crunch-format-jpg-to-all/sample-black-64x64.dds cadaf30985bb62b98cd601793d6440c70fa32e24459b780e043576d70817e2d4131f8902c703aa48cb7014b6e07f729e33ed9d989b0db0535261a2c6808805c2
build/test/crunch-format-jpg-to-all/sample-black-64x64.ktx 00e261c23642594e12152e7a0f8ad56a3ed87526bb18ee2e5e0f65f1ce21bfc288bd4e6043807a7fa642f7130645025d3e621e8731bc4304e665c46a975f1c6a
build/test/crunch-format-jpg-to-all/sample-black-64x64.jpg 4e080976a21bf93553425038f152934dbb292937e8fefaa7f9bd876b551e03d4778b54c41e64f470acabea03ff8e7ad3325daf790df1b7b9042930f48a00591a
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.tga c0c0863c373eae0936868e86e4ff9898714a78d5e1f96b972bffdce4f2e2cb3b0065477ce43f3118f256fe74e461e91c29db58e03c3bd32cac238e6e478f9300
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.bmp d6247ea2517362b1f3c98d1542cacd1c0cb429d00a7635169adf24ac253f37217446e6eec6e06820afc99c35534a907c9539c41a5b3dc0de51bc161ebe11e860
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.png 59533b55e3302cee1faa5583fe831435110933b0bc8030cf8e70380982f0704cb83abbae3a5fcdec3c20157a793f8a7908c9393373ad213559b0e0358d5471ea
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.crn 869a12414daecba28f7d4bb1f54467f21c01f356daa59b8e4ffc78ec643fb0f7a5db536c8fa8e62325c7625a718f153c2327f6ce95d6fc05f3fb26a9bac46ceb
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.dds e9ea76b8a7eaa5a227659d86db3c64703ffe09788e49b9ccf9833c392d39d8ef8626865bebb1a977f5970ba21f3156d1f7741cb5d635804e8f852f2af8b83328
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.ktx 44e1df3fa8d784d7e490d8440eba4e049b721af932b7898c317d03474290669531afe85916d3452bc1d0966a9a6d36cf53964e98eb2dfdc1cabf6d7c02afab61
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling1x1.jpg fe96bbc6e9c37ba66a981d40bd46d05ea0a385499c7feed7839915da665c376cc1257cd82721ee8d454dc2f425c1bfc16490ae4607465b3b9545fc4741ce0a2f
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.tga c0c0863c373eae0936868e86e4ff9898714a78d5e1f96b972bffdce4f2e2cb3b0065477ce43f3118f256fe74e461e91c29db58e03c3bd32cac238e6e478f9300
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.bmp d6247ea2517362b1f3c98d1542cacd1c0cb429d00a7635169adf24ac253f37217446e6eec6e06820afc99c35534a907c9539c41a5b3dc0de51bc161ebe11e860
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.png 59533b55e3302cee1faa5583fe831435110933b0bc8030cf8e70380982f0704cb83abbae3a5fcdec3c20157a793f8a7908c9393373ad213559b0e0358d5471ea
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.crn 869a12414daecba28f7d4bb1f54467f21c01f356daa59b8e4ffc78ec643fb0f7a5db536c8fa8e62325c7625a718f153c2327f6ce95d6fc05f3fb26a9bac46ceb
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.dds e9ea76b8a7eaa5a227659d86db3c64703ffe09788e49b9ccf9833c392d39d8ef8626865bebb1a977f5970ba21f3156d1f7741cb5d635804e8f852f2af8b83328
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.ktx 44e1df3fa8d784d7e490d8440eba4e049b721af932b7898c317d03474290669531afe85916d3452bc1d0966a9a6d36cf53964e98eb2dfdc1cabf6d7c02afab61
build/test/crunch-format-jpg-to-all/sample-grayscale-sampling2x2.jpg fe96bbc6e9c37ba66a981d40bd46d05ea0a385499c7feed7839915da665c376cc1257cd82721ee8d454dc2f425c1bfc16490ae4607465b3b9545fc4741ce0a2f
build/test/example1-icon-png-to-dds/sample-icon-unvanquished-64x64.dds ce98b21c574b4883e4aff4a0b92fd0f533f22e00ab753686f353111fa7503287c2c4dba25f2c2670508630d79b59464578e094461bad81ad803e1d58ed5e69f8
build/test/example1-icon-png-to-crn/sample-icon-unvanquished-64x64.crn a3b526e95f44b6a4c919bc8eb79f4debf592f0bccc163c900cd1b6f9add0ce58c3db9fc8dac1828d350bbcd915823d7743a0c114a533fbc08c163fcb4846d2f5
build/test/example1-icon-crn-to-dds/sample-icon-unvanquished-64x64.dds 2aae5c5bd8c6a82d1cf3fc9d9da12fe3fc5be8651261ff1cb359e754308fd81e93bf304aeb55ada0bad50794d6e97d3e48084e5a4f48fa7caec4ed954152bae9
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Binary file added test/sample-grayscale-sampling1x1.jpg
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Binary file added test/sample-grayscale-sampling2x2.jpg
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2 changes: 2 additions & 0 deletions test/test.py
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Expand Up @@ -425,6 +425,8 @@ def start_from(items, value):
"format": "jpg",
"samples": [
"sample-black-64x64",
"sample-grayscale-sampling1x1",
"sample-grayscale-sampling2x2",
],
},
]
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