[FFmpeg-devel] [RFC][PATCH] Use correct chroma position in YUV420P interlaced conversions

[Kieran Kunhya]

I think this is right but would be useful to get it checked. Visually the output looks ok

---
 libavfilter/vf_scale.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
index 64b88c2..83a0666 100644
--- a/libavfilter/vf_scale.c
+++ b/libavfilter/vf_scale.c
@@ -373,6 +373,13 @@ static int config_props(AVFilterLink *outlink)
             av_opt_set_int(*s, "dst_format", outfmt, 0);
             av_opt_set_int(*s, "sws_flags", scale->flags, 0);
 
+            /* Override interlaced YUV420P settings to have the correct (MPEG-2) chroma positions
+             * MPEG-2 chroma positions are used by convention
+             * Set the context up for tff */
+            if (i && scale->interlaced && inlink->format == AV_PIX_FMT_YUV420P){
+                scale->in_v_chr_pos = (i == 1) ? 64 : -64;
+            }
+
             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, "dst_h_chr_pos", scale->out_h_chr_pos, 0);
@@ -418,7 +425,12 @@ static int scale_slice(AVFilterLink *link, AVFrame *out_buf, AVFrame *cur_pic, s
         int vsub= ((i+1)&2) ? scale->vsub : 0;
          in_stride[i] = cur_pic->linesize[i] * mul;
         out_stride[i] = out_buf->linesize[i] * mul;
-         in[i] = cur_pic->data[i] + ((y>>vsub)+field) * cur_pic->linesize[i];
+
+        /* Chroma planes are shared in interlaced 4:2:0 */
+        if (i && cur_pic->format == AV_PIX_FMT_YUV420P && scale->interlaced)
+            in[i] = cur_pic->data[i];
+        else
+            in[i] = cur_pic->data[i] + ((y>>vsub)+field) * cur_pic->linesize[i];
         out[i] = out_buf->data[i] +            field  * out_buf->linesize[i];
     }
     if(scale->input_is_pal)
@@ -520,8 +532,8 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
               INT_MAX);
 
     if(scale->interlaced>0 || (scale->interlaced<0 && in->interlaced_frame)){
-        scale_slice(link, out, in, scale->isws[0], 0, (link->h+1)/2, 2, 0);
-        scale_slice(link, out, in, scale->isws[1], 0,  link->h   /2, 2, 1);
+        scale_slice(link, out, in, scale->isws[!in->top_field_first], 0, (link->h+1)/2, 2, 0);
+        scale_slice(link, out, in, scale->isws[in->top_field_first],  0,  link->h   /2, 2, 1);
     }else{
         scale_slice(link, out, in, scale->sws, 0, link->h, 1, 0);
     }
-- 
1.9.1