swscale: add two spatially stable dithering methods

Both of these dithering methods are from http://pippin.gimp.org/a_dither/ for
GIF they can be considered better than bayer (provides more gray-levels), and
spatial stability - often more than twice as good compression and less visual
flicker than error diffusion methods (the methods also avoids error-shadow
artifacts of diffusion dithers).

These methods are similar to blue/green noise type dither masks; but are
simple enough to generate their mask on the fly. They are still research work
in progress; though more expensive to generate masks (which can be used in a
LUT) like 'void and cluster' and similar methods will yield superior results

1 files changed, 2 insertions, 0 deletions

diff --git a/libswscale/swscale_internal.h b/libswscale/swscale_internal.h
index 5ce54d0dc3..8c2dc7998d 100644
--- a/libswscale/swscale_internal.h
+++ b/libswscale/swscale_internal.h
@@ -66,6 +66,8 @@ typedef enum SwsDither {
     SWS_DITHER_AUTO,
     SWS_DITHER_BAYER,
     SWS_DITHER_ED,
+    SWS_DITHER_A_DITHER,
+    SWS_DITHER_X_DITHER,
     NB_SWS_DITHER,
 } SwsDither;