The branch, master has been updated
via 0226b6fb2c0855eca417feb0a4e3bdf1f1193332 (commit)
from 8426622bb9584ec4fc32215f006a91e7ddc480cc (commit)
- Log -----------------------------------------------------------------
commit 0226b6fb2c0855eca417feb0a4e3bdf1f1193332
Author: Manuel Lauss <manuel.la...@gmail.com>
AuthorDate: Thu Aug 14 00:21:09 2025 +0200
Commit: Manuel Lauss <manuel.la...@gmail.com>
CommitDate: Mon Aug 18 09:11:09 2025 +0000
avcodec/sanm: bl16: fix artifacts in larger videos
The DOS/Windows decoder precomputes a table of linear offsets of
all motion vectors given the current image width.
For larger widths (>=762), the pairs starting at indices 1 and 254
of motion_vectors[] will overflow the int16_t, changing the sign.
Playing back the 800x600 "jonesopn_8.snm" video of "Indiana Jones and
the Infernal Machine" reveals a lot of artifacts and a lot of
"Ignoring invalid motion vector (149, -41)->(136, 0), block size = 8"
messages, hinting at the wrong direction of the motion vectors.
Fix this by doing the calculation that the DOS/Windows players do,
let the value overflow and reextract the "new" mvec x/y components.
diff --git a/libavcodec/sanm.c b/libavcodec/sanm.c
index 9e99aa9dd9..e4308af647 100644
--- a/libavcodec/sanm.c
+++ b/libavcodec/sanm.c
@@ -2074,6 +2074,20 @@ static int codec2subblock(SANMVideoContext *ctx, int cx,
int cy, int blk_size)
mx = motion_vectors[opcode][0];
my = motion_vectors[opcode][1];
+ /* The original implementation of this codec precomputes a table
+ * of int16_t all motion vectors for given image width.
+ * For larger widths, starting with 762 pixels, the calculation of
+ * mv table indices 1+ and 255- overflow the int16_t, inverting the
+ * sign of the offset. This is actively exploited in e.g. the
+ * "jonesopn_8.snm" video of "Indiana Jones and the Infernal Machine".
+ * Therefore let the overflow happen and extract x/y components from
+ * the new value.
+ */
+ if (ctx->width > 761) {
+ index = (int16_t)(my * ctx->width + mx);
+ mx = index % ctx->width;
+ my = index / ctx->width;
+ }
if (good_mvec(ctx, cx, cy, mx, my, blk_size)) {
copy_block(ctx->frm0 + cx + ctx->pitch * cy,
ctx->frm2 + cx + mx + ctx->pitch * (cy + my),
-----------------------------------------------------------------------
Summary of changes:
libavcodec/sanm.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
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