avcodec_find_best_pix_fmt_of_2: favor formats with fewer components if it does not incur a loss.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/imgconvert.c

@@ -551,6 +551,8 @@ enum AVPixelFormat avcodec_find_best_pix_fmt_of_2(enum AVPixelFormat dst_pix_fmt
 {
     enum AVPixelFormat dst_pix_fmt;
     int loss1, loss2, loss_order1, loss_order2, i, loss_mask;
+    const AVPixFmtDescriptor *desc1 = av_pix_fmt_desc_get(dst_pix_fmt1);
+    const AVPixFmtDescriptor *desc2 = av_pix_fmt_desc_get(dst_pix_fmt2);
     static const int loss_mask_order[] = {
         ~0, /* no loss first */
         ~FF_LOSS_ALPHA,
@@ -577,7 +579,11 @@ enum AVPixelFormat avcodec_find_best_pix_fmt_of_2(enum AVPixelFormat dst_pix_fmt
         loss_order2 = loss2 & loss_mask_order[i];
 
         if (loss_order1 == 0 && loss_order2 == 0 && dst_pix_fmt2 != AV_PIX_FMT_NONE && dst_pix_fmt1 != AV_PIX_FMT_NONE){ /* use format with smallest depth */
-            dst_pix_fmt = avg_bits_per_pixel(dst_pix_fmt2) < avg_bits_per_pixel(dst_pix_fmt1) ? dst_pix_fmt2 : dst_pix_fmt1;
+            if(avg_bits_per_pixel(dst_pix_fmt2) != avg_bits_per_pixel(dst_pix_fmt1)) {
+                dst_pix_fmt = avg_bits_per_pixel(dst_pix_fmt2) < avg_bits_per_pixel(dst_pix_fmt1) ? dst_pix_fmt2 : dst_pix_fmt1;
+            } else {
+                dst_pix_fmt = desc2->nb_components < desc1->nb_components ? dst_pix_fmt2 : dst_pix_fmt1;
+            }
         } else if (loss_order1 == 0 || loss_order2 == 0) { /* use format with no loss */
             dst_pix_fmt = loss_order2 ? dst_pix_fmt1 : dst_pix_fmt2;
         }