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

[FFmpeg-devel] [PATCH 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 | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavcodec/texturedsp.c b/libavcodec/texturedsp.c
index 5fb79937da..1b8237e9f7 100644
--- a/libavcodec/texturedsp.c
+++ b/libavcodec/texturedsp.c
@@ -175,9 +175,9 @@ 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;
+    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);
 }
 
 /**
-- 
2.40.1

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

[Connor Worley]

[-- Attachment #1: Type: text/plain, Size: 332 bytes --]

Attached is a comparison of the input and decoding with DXT4 and DXT5.

DXT5 produces dark outlines that are not present in the input.

Also, I have a prototype "Normal Quality, With Alpha" encoder that produces
nasty results in Resolume unless alpha is premultiplied. All of this points
to DXV misnomering DXT5.

-- 
Connor Worley

[-- Attachment #2: DXT4_5_comparison.png --]
[-- Type: image/png, Size: 602683 bytes --]

[-- Attachment #3: Type: text/plain, Size: 251 bytes --]

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