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

Browse source 
'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.
This commit is contained in:
Lane Brooks 2011-01-11 10:34:33 -07:00 committed by Sebastian Dröge
parent e73f293ee5
commit e2150b11ff
1 changed files with 14 additions and 5 deletions
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19
ext/pango/gsttextoverlay.c
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@ -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;