[FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[Connor Worley]

This bug causes the DXT5 decoder to produce incorrect block texture data.
After the fix, textures are visually correct and match data decoded by
Resolume Alley (extracted with Nvida Nsight for comparison). Current FATE DXT5
samples did not cover this case.

Signed-off-by: Connor Worley <connorbworley@gmail.com>
---
 libavcodec/dxv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/dxv.c b/libavcodec/dxv.c
index b29adf8ad9..be1216da86 100644
--- a/libavcodec/dxv.c
+++ b/libavcodec/dxv.c
@@ -755,7 +755,7 @@ static int dxv_decompress_dxt5(AVCodecContext *avctx)
                 break;
             case 2:
                 /* Copy two dwords from a previous index */
-                idx = 8 + bytestream2_get_le16(gbc);
+                idx = 8 + 4 * bytestream2_get_le16(gbc);
                 if (idx > pos || (unsigned int)(pos - idx) + 2 > ctx->tex_size / 4)
                     return AVERROR_INVALIDDATA;
                 prev = AV_RL32(ctx->tex_data + 4 * (pos - idx));
-- 
2.40.1

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Re: [FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[Connor Worley]

[-- Attachment #1: Type: text/plain, Size: 1368 bytes --]

Attached are the input file used for comparison with Alley, and
before-and-after images generated with
`ffmpeg -i dice_dxt5.mov -f image2 out.png`

On Mon, Jan 29, 2024 at 10:26 PM Connor Worley <connorbworley@gmail.com>
wrote:

> This bug causes the DXT5 decoder to produce incorrect block texture data.
> After the fix, textures are visually correct and match data decoded by
> Resolume Alley (extracted with Nvida Nsight for comparison). Current FATE
> DXT5
> samples did not cover this case.
>
> Signed-off-by: Connor Worley <connorbworley@gmail.com>
> ---
>  libavcodec/dxv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libavcodec/dxv.c b/libavcodec/dxv.c
> index b29adf8ad9..be1216da86 100644
> --- a/libavcodec/dxv.c
> +++ b/libavcodec/dxv.c
> @@ -755,7 +755,7 @@ static int dxv_decompress_dxt5(AVCodecContext *avctx)
>                  break;
>              case 2:
>                  /* Copy two dwords from a previous index */
> -                idx = 8 + bytestream2_get_le16(gbc);
> +                idx = 8 + 4 * bytestream2_get_le16(gbc);
>                  if (idx > pos || (unsigned int)(pos - idx) + 2 >
> ctx->tex_size / 4)
>                      return AVERROR_INVALIDDATA;
>                  prev = AV_RL32(ctx->tex_data + 4 * (pos - idx));
> --
> 2.40.1
>
>

-- 
Connor Worley

[-- Attachment #2: dice_dxt5.mov --]
[-- Type: video/quicktime, Size: 132318 bytes --]

[-- Attachment #3: bug.png --]
[-- Type: image/png, Size: 258957 bytes --]

[-- Attachment #4: fix.png --]
[-- Type: image/png, Size: 232816 bytes --]

[-- Attachment #5: Type: text/plain, Size: 251 bytes --]

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Re: [FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[Connor Worley]

Got some confirmation that this patch fixes
https://trac.ffmpeg.org/ticket/10264

-- 
Connor Worley
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Re: [FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[Paul B Mahol]

On Thu, Feb 1, 2024 at 9:41 PM Connor Worley <connorbworley@gmail.com>
wrote:

> Got some confirmation that this patch fixes
> https://trac.ffmpeg.org/ticket/10264
>
>
Thanks for the fix. I had encountered similar artifacts but failed to fix
it, and then removed files long ago, but i got them from the web so will
recheck if this indeed fixes it too.


> --
> Connor Worley
> _______________________________________________
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> ffmpeg-devel@ffmpeg.org
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>
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> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>
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Re: [FFmpeg-devel] [PATCH] lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

[Connor Worley]

Given that Paul is no longer a maintainer, can someone else please take a
look?

-- 
Connor Worley
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