PR #21263 opened by Niklas Haas (haasn)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21263
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21263.patch
This function was assuming that the bits are MSB-aligned, but they are
LSB-aligned in both practice (and in the actual backend).
Also update the documentation of SwsPackOp to make this clearer.
Fixes an incorrect omission of a clamp after decoding e.g. rgb4, since
the max value range was incorrectly determined as 0 as a result of unpacking
the MSB bits instead of the LSB bits:
bgr4 -> gray:
[ u8 XXXX -> +XXX] SWS_OP_READ : 1 elem(s) packed >> 1
[ u8 .XXX -> +++X] SWS_OP_UNPACK : {1 2 1 0}
[ u8 ...X -> +++X] SWS_OP_SWIZZLE : 2103
[ u8 ...X -> +++X] SWS_OP_CONVERT : u8 -> f32
[f32 ...X -> .++X] SWS_OP_LINEAR : dot3 [...]
[f32 .XXX -> .++X] SWS_OP_DITHER : 16x16 matrix + {0 3 2 5}
+ [f32 .XXX -> .++X] SWS_OP_MIN : x <= {255 _ _ _}
[f32 .XXX -> +++X] SWS_OP_CONVERT : f32 -> u8
[ u8 .XXX -> +++X] SWS_OP_WRITE : 1 elem(s) planar >> 0
(X = unused, + = exact, 0 = zero)
>From d93ed0e7ada69c32541a0688d50527cb62a68637 Mon Sep 17 00:00:00 2001
From: Niklas Haas <[email protected]>
Date: Mon, 22 Dec 2025 15:31:01 +0100
Subject: [PATCH] swscale/ops_internal: fix ff_sws_pack_op_decode()
This function was assuming that the bits are MSB-aligned, but they are
LSB-aligned in both practice (and in the actual backend).
Also update the documentation of SwsPackOp to make this clearer.
Fixes an incorrect omission of a clamp after decoding e.g. rgb4, since
the max value range was incorrectly determined as 0 as a result of unpacking
the MSB bits instead of the LSB bits:
bgr4 -> gray:
[ u8 XXXX -> +XXX] SWS_OP_READ : 1 elem(s) packed >> 1
[ u8 .XXX -> +++X] SWS_OP_UNPACK : {1 2 1 0}
[ u8 ...X -> +++X] SWS_OP_SWIZZLE : 2103
[ u8 ...X -> +++X] SWS_OP_CONVERT : u8 -> f32
[f32 ...X -> .++X] SWS_OP_LINEAR : dot3 [...]
[f32 .XXX -> .++X] SWS_OP_DITHER : 16x16 matrix + {0 3 2 5}
+ [f32 .XXX -> .++X] SWS_OP_MIN : x <= {255 _ _ _}
[f32 .XXX -> +++X] SWS_OP_CONVERT : f32 -> u8
[ u8 .XXX -> +++X] SWS_OP_WRITE : 1 elem(s) planar >> 0
(X = unused, + = exact, 0 = zero)
---
libswscale/ops.h | 4 ++++
libswscale/ops_internal.h | 4 +++-
tests/ref/fate/sws-ops-list | 2 +-
3 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/libswscale/ops.h b/libswscale/ops.h
index 6392d0ffdf..6fc7e60a02 100644
--- a/libswscale/ops.h
+++ b/libswscale/ops.h
@@ -109,6 +109,10 @@ typedef struct SwsReadWriteOp {
} SwsReadWriteOp;
typedef struct SwsPackOp {
+ /**
+ * Packed bits are assumed to be LSB-aligned within the underlying
+ * integer type; i.e. (msb) 0 ... X Y Z W (lsb).
+ */
uint8_t pattern[4]; /* bit depth pattern, from MSB to LSB */
} SwsPackOp;
diff --git a/libswscale/ops_internal.h b/libswscale/ops_internal.h
index 0071f78558..ba4c9b39da 100644
--- a/libswscale/ops_internal.h
+++ b/libswscale/ops_internal.h
@@ -39,7 +39,9 @@ static inline AVRational ff_sws_pixel_expand(SwsPixelType
from, SwsPixelType to)
static inline void ff_sws_pack_op_decode(const SwsOp *op, uint64_t mask[4],
int shift[4])
{
- const int size = ff_sws_pixel_type_size(op->type) * 8;
+ int size = 0;
+ for (int i = 0; i < 4; i++)
+ size += op->pack.pattern[i];
for (int i = 0; i < 4; i++) {
const int bits = op->pack.pattern[i];
mask[i] = (UINT64_C(1) << bits) - 1;
diff --git a/tests/ref/fate/sws-ops-list b/tests/ref/fate/sws-ops-list
index b49f944794..a7d6149d8b 100644
--- a/tests/ref/fate/sws-ops-list
+++ b/tests/ref/fate/sws-ops-list
@@ -1 +1 @@
-e910ff7ceaeb64bfdbac3f652b67403f
+ef1dd10af970984495f6008e43d0fe1b
--
2.49.1
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