textoverlay: converted AYUV to use 'A OVER B' alpha compositing

'A OVER B' compositing is explained at
http://en.wikipedia.org/wiki/Alpha_compositing.

Previously, overlaying text on a transparent background image left the
text overlay also transparent. This pipeline shows such an example:

gst-launch videotestsrc pattern=white ! video/x-raw-yuv,format=\(fourcc\)AYUV ! alpha alpha=0.0 ! textoverlay text=Testing auto-resize=False font-desc=60px ! videomixer ! ffmpegcolorspace ! autovideosink

With this patch, text is composited "OVER" the background image and
thus is visible regardless of the alpha of the background image. The
overlay in the above pipeline works after applying this patch.

diff --git a/ext/pango/gsttextoverlay.c b/ext/pango/gsttextoverlay.c
index ca38dc1..6cd9db7 100644 (file)
--- a/ext/pango/gsttextoverlay.c
+++ b/ext/pango/gsttextoverlay.c
@@ -143,6 +143,12 @@ GST_DEBUG_CATEGORY (pango_debug);
        ret = (v0 * alpha + v1 * (255 - alpha)) / 255; \
 }
 
+#define OVER(ret, alphaA, Ca, alphaB, Cb, alphaNew)    \
+{ \
+    ret = (Ca * alphaA + Cb * alphaB * (255 - alphaA) / 255) / alphaNew; \
+    ret = CLAMP (ret, 0, 255); \
+}
+
 #if G_BYTE_ORDER == G_LITTLE_ENDIAN
 # define CAIRO_ARGB_A 3
 # define CAIRO_ARGB_R 2
@@ -1617,7 +1623,7 @@ static inline void
 gst_text_overlay_blit_AYUV (GstTextOverlay * overlay,
     guint8 * rgb_pixels, gint xpos, gint ypos)
 {
-  int a, r, g, b;
+  int a, r, g, b, a1;
   int y, u, v;
   int i, j;
   int h, w;
@@ -1649,14 +1655,17 @@ gst_text_overlay_blit_AYUV (GstTextOverlay * overlay,
 
       CAIRO_UNPREMULTIPLY (a, r, g, b);
 
+      // convert background to yuv
       COMP_Y (y, r, g, b);
       COMP_U (u, r, g, b);
       COMP_V (v, r, g, b);
 
-      a = (a * dest[0] + 128) >> 8;
-      BLEND (dest[1], a, y, dest[1]);
-      BLEND (dest[2], a, u, dest[2]);
-      BLEND (dest[3], a, v, dest[3]);
+      // preform text "OVER" background alpha compositing
+      a1 = a + (dest[0] * (255 - a)) / 255 + 1; // add 1 to prevent divide by 0
+      OVER (dest[1], a, y, dest[0], dest[1], a1);
+      OVER (dest[2], a, u, dest[0], dest[2], a1);
+      OVER (dest[3], a, v, dest[0], dest[3], a1);
+      dest[0] = a1 - 1;         // remove the temporary 1 we added
 
       pimage += 4;
       dest += 4;