v4l2src: Maintain downstream caps order

The g_list_insert_sorted() will behave like prepend when the compare
function returns 0. In our case, we want to maintain the order hence
append. This fixes this issue and improve the sorting algorithm to make
a 10x10 prefered over 10x200 with a preference of 10x8 (and similar
cases which was badly handled). This fixes generally fixes issue were a
sub-optimal format / size is picked.

https://bugzilla.gnome.org/show_bug.cgi?id=792435

1 files changed, 42 insertions, 10 deletions

diff --git a/sys/v4l2/gstv4l2src.c b/sys/v4l2/gstv4l2src.c
index 93c90536b..552e0d646 100644
--- a/sys/v4l2/gstv4l2src.c
+++ b/sys/v4l2/gstv4l2src.c
@@ -315,28 +315,58 @@ gst_v4l2src_fixed_caps_compare (GstStructure * a, GstStructure * b,
 {
   gint aw = G_MAXINT, ah = G_MAXINT, ad = G_MAXINT;
   gint bw = G_MAXINT, bh = G_MAXINT, bd = G_MAXINT;
+  gint ret;
 
   gst_v4l2_src_parse_fixed_struct (a, &aw, &ah, NULL, NULL);
   gst_v4l2_src_parse_fixed_struct (b, &bw, &bh, NULL, NULL);
 
   /* When both are smaller then pref, just append to the end */
   if ((bw < pref->width || bh < pref->height)
-      && (aw < pref->width || ah < pref->height))
-    return 1;
+      && (aw < pref->width || ah < pref->height)) {
+    ret = 1;
+    goto done;
+  }
 
   /* If a is smaller then pref and not b, then a goes after b */
-  if (aw < pref->width || ah < pref->height)
-    return 1;
+  if (aw < pref->width || ah < pref->height) {
+    ret = 1;
+    goto done;
+  }
 
   /* If b is smaller then pref and not a, then a goes before b */
-  if (bw < pref->width || bh < pref->height)
-    return -1;
+  if (bw < pref->width || bh < pref->height) {
+    ret = -1;
+    goto done;
+  }
+
+  /* Both are larger or equal to the preference, prefer the smallest */
+  ad = MAX (1, aw - pref->width) * MAX (1, ah - pref->height);
+  bd = MAX (1, bw - pref->width) * MAX (1, bh - pref->height);
+
+  /* Adjust slightly in case width/height matched the preference */
+  if (aw == pref->width)
+    ad -= 1;
+
+  if (ah == pref->height)
+    ad -= 1;
 
-  /* Both are strictly larger then the preference, prefer the smallest */
-  ad = (aw - pref->width) * (ah - pref->height);
-  bd = (bw - pref->width) * (bh - pref->height);
+  if (bw == pref->width)
+    bd -= 1;
 
-  return ad - bd;
+  if (bh == pref->height)
+    bd -= 1;
+
+  /* If the choices are equivalent, maintain the order */
+  if (ad == bd)
+    ret = 1;
+  else
+    ret = ad - bd;
+
+done:
+  GST_TRACE ("Placing %ix%i (%s) %s %ix%i (%s)", aw, ah,
+      gst_structure_get_string (a, "format"), ret > 0 ? "after" : "before", bw,
+      bh, gst_structure_get_string (b, "format"));
+  return ret;
 }
 
 static gboolean
@@ -387,6 +417,8 @@ gst_v4l2src_fixate (GstBaseSrc * basesrc, GstCaps * caps, GstStructure * pref_s)
     gst_structure_free (pref_s);
   }
 
+  GST_DEBUG_OBJECT (basesrc, "Prefered size %ix%i", pref.width, pref.height);
+
   /* Sort the structures to get the caps that is nearest to our preferences,
    * first */
   while ((s = gst_caps_steal_structure (caps, 0))) {