[Xvid-devel] RGBA output images have their alpha values set to 0 (fully transparent)

[delthas]

Hey,

I noticed that when decoding a frame to the XVID_CSP_RGBA color space, 
the alpha values of all pixels are set to 0. In the RGBA color space an 
alpha value of 0 means fully transparent and 255 means fully opaque, so 
the decoder effectively returns an image that is fully transparent: the 
RGB values are correct but all the pixels are fully transparent.

Setting the alpha to 0 is done in src/image/colorspace.c, in the 
WRITE_RGB macro (at line 389 in the current revision, 2176).

The code is as follows:
=====
#define WRITE_RGB(SIZE,ROW,UV_ROW,C1,C2,C3,C4)	\
	rgb_y = RGB_Y_tab[ y_ptr[(ROW)*y_stride + 0] ];						\
	x_ptr[(ROW)*x_stride+(C3)] = MAX(0, MIN(255, (rgb_y + b_u##UV_ROW) >> 
SCALEBITS_OUT));	\
	x_ptr[(ROW)*x_stride+(C2)] = MAX(0, MIN(255, (rgb_y - g_uv##UV_ROW) >> 
SCALEBITS_OUT));	\
	x_ptr[(ROW)*x_stride+(C1)] = MAX(0, MIN(255, (rgb_y + r_v##UV_ROW) >> 
SCALEBITS_OUT));	\
	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(C4)] = 0;									\
	rgb_y = RGB_Y_tab[ y_ptr[(ROW)*y_stride + 1] ];									\
	x_ptr[(ROW)*x_stride+(SIZE)+(C3)] = MAX(0, MIN(255, (rgb_y + 
b_u##UV_ROW) >> SCALEBITS_OUT));	\
	x_ptr[(ROW)*x_stride+(SIZE)+(C2)] = MAX(0, MIN(255, (rgb_y - 
g_uv##UV_ROW) >> SCALEBITS_OUT));	\
	x_ptr[(ROW)*x_stride+(SIZE)+(C1)] = MAX(0, MIN(255, (rgb_y + 
r_v##UV_ROW) >> SCALEBITS_OUT));	\
	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(SIZE)+(C4)] = 0;
=====

The lines responsible for setting the value to 0 are:
if ((SIZE)>3) x_ptr[(ROW)*x_stride+(C4)] = 0;
and
if ((SIZE)>3) x_ptr[(ROW)*x_stride+(SIZE)+(C4)] = 0;

The values 0 could be replaced by 255 to output fully opaque data rather 
than fully transparent data.

It seems to me that outputting fully transparent data is a bug and that 
the alpha values should be set to 255 instead. Is it the case, or is 
this intended?

A simple diff is provided below in case this is actually a bug.

Thanks,
delthas

Index: xvidcore/src/image/colorspace.c
===================================================================
--- xvidcore/src/image/colorspace.c	(revision 2176)
+++ xvidcore/src/image/colorspace.c	(working copy)
@@ -391,12 +391,12 @@
  	x_ptr[(ROW)*x_stride+(C3)] = MAX(0, MIN(255, (rgb_y + b_u##UV_ROW) >> 
SCALEBITS_OUT));	\
  	x_ptr[(ROW)*x_stride+(C2)] = MAX(0, MIN(255, (rgb_y - g_uv##UV_ROW) >> 
SCALEBITS_OUT));	\
  	x_ptr[(ROW)*x_stride+(C1)] = MAX(0, MIN(255, (rgb_y + r_v##UV_ROW) >> 
SCALEBITS_OUT));	\
-	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(C4)] = 0;									\
+	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(C4)] = 255;									\
  	rgb_y = RGB_Y_tab[ y_ptr[(ROW)*y_stride + 1] ];									\
  	x_ptr[(ROW)*x_stride+(SIZE)+(C3)] = MAX(0, MIN(255, (rgb_y + 
b_u##UV_ROW) >> SCALEBITS_OUT));	\
  	x_ptr[(ROW)*x_stride+(SIZE)+(C2)] = MAX(0, MIN(255, (rgb_y - 
g_uv##UV_ROW) >> SCALEBITS_OUT));	\
  	x_ptr[(ROW)*x_stride+(SIZE)+(C1)] = MAX(0, MIN(255, (rgb_y + 
r_v##UV_ROW) >> SCALEBITS_OUT));	\
-	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(SIZE)+(C4)] = 0;
+	if ((SIZE)>3) x_ptr[(ROW)*x_stride+(SIZE)+(C4)] = 255;