[FFmpeg-devel] [PATCH v3 1/2] lavc/texturedsp: fix premult2straight inversion

[FFmpeg-devel] [PATCH v3 1/2] lavc/texturedsp: fix premult2straight inversion

[Connor Worley]

This function should convert premultiplied alpha to straight, but does the opposite.

Signed-off-by: Connor Worley <connorbworley@gmail.com>
---
 libavcodec/texturedsp.c | 9 ++++++---
 tests/ref/fate/dds-dxt2 | 2 +-
 tests/ref/fate/dds-dxt4 | 2 +-
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/libavcodec/texturedsp.c b/libavcodec/texturedsp.c
index 5fb79937da..721bfc17f5 100644
--- a/libavcodec/texturedsp.c
+++ b/libavcodec/texturedsp.c
@@ -175,9 +175,12 @@ static av_always_inline void premult2straight(uint8_t *src)
     int b = src[2];
     int a = src[3]; /* unchanged */
 
-    src[0] = (uint8_t) r * a / 255;
-    src[1] = (uint8_t) g * a / 255;
-    src[2] = (uint8_t) b * a / 255;
+    if (a == 0)
+        return;
+
+    src[0] = (uint8_t) FFMIN(r * 255 / a, 255);
+    src[1] = (uint8_t) FFMIN(g * 255 / a, 255);
+    src[2] = (uint8_t) FFMIN(b * 255 / a, 255);
 }
 
 /**
diff --git a/tests/ref/fate/dds-dxt2 b/tests/ref/fate/dds-dxt2
index b5bdfbadc3..1744ef41f6 100644
--- a/tests/ref/fate/dds-dxt2
+++ b/tests/ref/fate/dds-dxt2
@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 64x64
 #sar 0: 0/1
-0,          0,          0,        1,    16384, 0x11cebeb0
+0,          0,          0,        1,    16384, 0xd7f7241b
diff --git a/tests/ref/fate/dds-dxt4 b/tests/ref/fate/dds-dxt4
index 136dfd8006..f22878da56 100644
--- a/tests/ref/fate/dds-dxt4
+++ b/tests/ref/fate/dds-dxt4
@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 64x64
 #sar 0: 0/1
-0,          0,          0,        1,    16384, 0x31aaacd6
+0,          0,          0,        1,    16384, 0xf18d4216
-- 
2.40.1

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[FFmpeg-devel] [PATCH v3 2/2] lavc/dxv: treat DXT5-tagged files as DXT4

[Connor Worley]

DXV files seem to misnomer DXT5 and really encode DXT4 with
premultiplied alpha. At least, this is what Resolume alley does.

To check, encode some input with alpha as "Normal Quality, With Alpha"
in Alley, then decode the output with this change -- results are true
to the original input compared to git-master.

Signed-off-by: Connor Worley <connorbworley@gmail.com>
---
 libavcodec/dxv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libavcodec/dxv.c b/libavcodec/dxv.c
index 5923811b29..1e6791e63f 100644
--- a/libavcodec/dxv.c
+++ b/libavcodec/dxv.c
@@ -1074,7 +1074,8 @@ static int dxv_decode(AVCodecContext *avctx, AVFrame *frame,
         break;
     case MKBETAG('D', 'X', 'T', '5'):
         decompress_tex = dxv_decompress_dxt5;
-        ctx->tex_funct = ctx->texdsp.dxt5_block;
+        /* DXV misnomers DXT5, alpha is premultiplied so use DXT4 instead */
+        ctx->tex_funct = ctx->texdsp.dxt4_block;
         ctx->tex_rat   = 4;
         ctx->tex_step  = 16;
         msgcomp = "DXTR5";
-- 
2.40.1

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Re: [FFmpeg-devel] [PATCH v3 2/2] lavc/dxv: treat DXT5-tagged files as DXT4

[Lynne]

Feb 8, 2024, 20:07 by connorbworley@gmail.com:

> DXV files seem to misnomer DXT5 and really encode DXT4 with
> premultiplied alpha. At least, this is what Resolume alley does.
>
> To check, encode some input with alpha as "Normal Quality, With Alpha"
> in Alley, then decode the output with this change -- results are true
> to the original input compared to git-master.
>
> Signed-off-by: Connor Worley <connorbworley@gmail.com>
> ---
>  libavcodec/dxv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/dxv.c b/libavcodec/dxv.c
> index 5923811b29..1e6791e63f 100644
> --- a/libavcodec/dxv.c
> +++ b/libavcodec/dxv.c
> @@ -1074,7 +1074,8 @@ static int dxv_decode(AVCodecContext *avctx, AVFrame *frame,
>  break;
>  case MKBETAG('D', 'X', 'T', '5'):
>  decompress_tex = dxv_decompress_dxt5;
> -        ctx->tex_funct = ctx->texdsp.dxt5_block;
> +        /* DXV misnomers DXT5, alpha is premultiplied so use DXT4 instead */
> +        ctx->tex_funct = ctx->texdsp.dxt4_block;
>  ctx->tex_rat   = 4;
>  ctx->tex_step  = 16;
>  msgcomp = "DXTR5";
>

Pushed along with the rest of the DXT patches.
Thanks.
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