lavf: add av_guess_frame_sample_aspect_ratio function

Guesses the sample aspect ratio of a frame, based on both the stream and the
frame aspect ratio.

Since the frame aspect ratio is set by the codec but the stream aspect ratio
is set by the demuxer, these two may not be equal. This function tries to
return the value that you should use if you would like to display the frame.

Basic logic is to use the stream aspect ratio if it is set to something sane
otherwise use the frame aspect ratio. This way a container setting, which is
usually easy to modify can override the coded value in the frames.

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

1 files changed, 22 insertions, 0 deletions

diff --git a/libavformat/utils.c b/libavformat/utils.c
index d19e4270bf..c0f6d45c2a 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -4414,3 +4414,25 @@ const struct AVCodecTag *avformat_get_riff_audio_tags(void)
 {
     return ff_codec_wav_tags;
 }
+
+AVRational av_guess_sample_aspect_ratio(AVFormatContext *format, AVStream *stream, AVFrame *frame)
+{
+    AVRational undef = {0, 1};
+    AVRational stream_sample_aspect_ratio = stream ? stream->sample_aspect_ratio : undef;
+    AVRational frame_sample_aspect_ratio  = frame  ? frame->sample_aspect_ratio  : undef;
+
+    av_reduce(&stream_sample_aspect_ratio.num, &stream_sample_aspect_ratio.den,
+               stream_sample_aspect_ratio.num,  stream_sample_aspect_ratio.den, INT_MAX);
+    if (stream_sample_aspect_ratio.num <= 0 || stream_sample_aspect_ratio.den <= 0)
+        stream_sample_aspect_ratio = undef;
+
+    av_reduce(&frame_sample_aspect_ratio.num, &frame_sample_aspect_ratio.den,
+               frame_sample_aspect_ratio.num,  frame_sample_aspect_ratio.den, INT_MAX);
+    if (frame_sample_aspect_ratio.num <= 0 || frame_sample_aspect_ratio.den <= 0)
+        frame_sample_aspect_ratio = undef;
+
+    if (stream_sample_aspect_ratio.num)
+        return stream_sample_aspect_ratio;
+    else
+        return frame_sample_aspect_ratio;
+}