[FFmpeg-devel] [PATCH] avcodec/vc1: correct aspect ratio calculation

[Jerome Borsboom]

According to VC-1 spec:
* Display size defaults to max coded size when not explicitly set in
sequence header
* Aspect ratio in the sequence header refers to the Display size elements.
Therefore, the aspect ratio for the coded samples (SAR) needs to take into
account the scaling from coded size to display size, and the aspect ratio
of the display size elements.

Signed-off-by: Jerome Borsboom <jerome.borsboom at carpalis.nl>
---
VC-1 spec assumes that the output of the decoder, i.e. the pixel matrix with
dimensions coded_width x coded_height, is scaled to display size, i.e. a pixel
matrix of display_horiz_size x display_vert_size. This may introduce part of
the sample aspect ratio when the sizes of the two pixel matrices are not equal.
A further part of the sample aspect ratio is optionally specified in the sequence
header as the aspect ratio of the display size pixels. This patch takes both
aspect ratios into account and aims to be correct even when the coded image
includes overscan regions.

 libavcodec/vc1.c | 38 +++++++++++++++++++++-----------------
 libavcodec/vc1.h |  2 ++
 2 files changed, 23 insertions(+), 17 deletions(-)

diff --git a/libavcodec/vc1.c b/libavcodec/vc1.c
index 3581d87b57..efc6edc4b0 100644
--- a/libavcodec/vc1.c
+++ b/libavcodec/vc1.c
@@ -442,30 +442,24 @@ static int decode_sequence_header_adv(VC1Context *v, GetBitContext *gb)
     }
     v->s.max_b_frames = v->s.avctx->max_b_frames = 7;
     if (get_bits1(gb)) { //Display Info - decoding is not affected by it
-        int w, h, ar = 0;
+        int ar = 0;
         av_log(v->s.avctx, AV_LOG_DEBUG, "Display extended info:\n");
-        w = get_bits(gb, 14) + 1;
-        h = get_bits(gb, 14) + 1;
-        av_log(v->s.avctx, AV_LOG_DEBUG, "Display dimensions: %ix%i\n", w, h);
+        v->disp_horiz_size = get_bits(gb, 14) + 1;
+        v->disp_vert_size = get_bits(gb, 14) + 1;
+        av_log(v->s.avctx, AV_LOG_DEBUG, "Display dimensions: %ix%i\n",
+               v->disp_horiz_size, v->disp_vert_size);
         if (get_bits1(gb))
             ar = get_bits(gb, 4);
         if (ar && ar < 14) {
-            v->s.avctx->sample_aspect_ratio = ff_vc1_pixel_aspect[ar];
+            v->aspect_ratio = ff_vc1_pixel_aspect[ar];
         } else if (ar == 15) {
-            w = get_bits(gb, 8) + 1;
-            h = get_bits(gb, 8) + 1;
-            v->s.avctx->sample_aspect_ratio = (AVRational){w, h};
+            v->aspect_ratio = (AVRational){get_bits(gb, 8) + 1, get_bits(gb, 8) + 1};
         } else {
-            av_reduce(&v->s.avctx->sample_aspect_ratio.num,
-                      &v->s.avctx->sample_aspect_ratio.den,
-                      v->s.avctx->height * w,
-                      v->s.avctx->width * h,
-                      1 << 30);
+            v->aspect_ratio = (AVRational){1, 1};
         }
-        ff_set_sar(v->s.avctx, v->s.avctx->sample_aspect_ratio);
-        av_log(v->s.avctx, AV_LOG_DEBUG, "Aspect: %i:%i\n",
-               v->s.avctx->sample_aspect_ratio.num,
-               v->s.avctx->sample_aspect_ratio.den);
+        av_log(v->s.avctx, AV_LOG_DEBUG, "Aspect ratio: %i:%i\n",
+               v->aspect_ratio.num,
+               v->aspect_ratio.den);
 
         if (get_bits1(gb)) { //framerate stuff
             if (get_bits1(gb)) {
@@ -490,6 +484,10 @@ static int decode_sequence_header_adv(VC1Context *v, GetBitContext *gb)
             v->transfer_char = get_bits(gb, 8);
             v->matrix_coef   = get_bits(gb, 8);
         }
+    } else {
+        v->disp_horiz_size = v->max_coded_width;
+        v->disp_vert_size = v->max_coded_height;
+        v->aspect_ratio = (AVRational){1, 1};
     }
 
     v->hrd_param_flag = get_bits1(gb);
@@ -544,6 +542,12 @@ int ff_vc1_decode_entry_point(AVCodecContext *avctx, VC1Context *v, GetBitContex
         av_log(avctx, AV_LOG_ERROR, "Failed to set dimensions %d %d\n", w, h);
         return ret;
     }
+    av_reduce(&avctx->sample_aspect_ratio.num,
+              &avctx->sample_aspect_ratio.den,
+              v->disp_horiz_size * v->aspect_ratio.num * h,
+              v->disp_vert_size * v->aspect_ratio.den * w,
+              1 << 30);
+    ff_set_sar(avctx, avctx->sample_aspect_ratio);
 
     if (v->extended_mv)
         v->extended_dmv = get_bits1(gb);
diff --git a/libavcodec/vc1.h b/libavcodec/vc1.h
index 69f6ca9e4d..7674b0f9a1 100644
--- a/libavcodec/vc1.h
+++ b/libavcodec/vc1.h
@@ -209,6 +209,8 @@ typedef struct VC1Context{
     int hrd_param_flag;   ///< Presence of Hypothetical Reference
                           ///< Decoder parameters
     int psf;              ///< Progressive Segmented Frame
+    int disp_horiz_size, disp_vert_size;
+    AVRational aspect_ratio;
     //@}
 
     /** Sequence header data for all Profiles
-- 
2.13.6