[FFmpeg-devel] [PATCH] Fix chroma positioning for 4:2:0 pixel format

Maksym Veremeyenko verem at m1stereo.tv
Mon Feb 6 17:14:14 EET 2017


Hi,

Attached patch fixes chroma positioning during scaling interlaced 4:2:0.

On a first iteration default context value been overwritten by new value 
and not been update on next iterations for fields. This mean that 
vertical chroma position remain 128 for field#0 and field#1 instead of 
*64* and *192*.

Attached patch use local variable for storing this intermediate value of 
chroma vertical position not modifying default context value.

-- 
Maksym Veremeyenko

-------------- next part --------------
From 912ecf538b6b2f7a8df4afdfed2d34052162335c Mon Sep 17 00:00:00 2001
From: Maksym Veremeyenko <verem at m1.tv>
Date: Mon, 6 Feb 2017 17:03:17 +0200
Subject: [PATCH] Fix chroma positioning for 4:2:0 pixel format

---
 libavfilter/vf_scale.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
index 22bee96..a7dfd3d 100644
--- a/libavfilter/vf_scale.c
+++ b/libavfilter/vf_scale.c
@@ -374,6 +374,7 @@ static int config_props(AVFilterLink *outlink)
         int i;
 
         for (i = 0; i < 3; i++) {
+            int in_v_chr_pos = scale->in_v_chr_pos, out_v_chr_pos = scale->out_v_chr_pos;
             struct SwsContext **s = swscs[i];
             *s = sws_alloc_context();
             if (!*s)
@@ -406,17 +407,17 @@ static int config_props(AVFilterLink *outlink)
              * MPEG-2 chroma positions are used by convention
              * XXX: support other 4:2:0 pixel formats */
             if (inlink0->format == AV_PIX_FMT_YUV420P && scale->in_v_chr_pos == -513) {
-                scale->in_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
+                in_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
             }
 
             if (outlink->format == AV_PIX_FMT_YUV420P && scale->out_v_chr_pos == -513) {
-                scale->out_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
+                out_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
             }
 
             av_opt_set_int(*s, "src_h_chr_pos", scale->in_h_chr_pos, 0);
-            av_opt_set_int(*s, "src_v_chr_pos", scale->in_v_chr_pos, 0);
+            av_opt_set_int(*s, "src_v_chr_pos", in_v_chr_pos, 0);
             av_opt_set_int(*s, "dst_h_chr_pos", scale->out_h_chr_pos, 0);
-            av_opt_set_int(*s, "dst_v_chr_pos", scale->out_v_chr_pos, 0);
+            av_opt_set_int(*s, "dst_v_chr_pos", out_v_chr_pos, 0);
 
             if ((ret = sws_init_context(*s, NULL, NULL)) < 0)
                 return ret;
-- 
2.9.3



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