Bug 22370

Created attachment 26959 [details]
sample code to see the phenomenon

Description:
------------

Using the XV texture adapter in YV12 format.

I fill a rectangle with many 1 pixel-high horizontal lines. Odd lines are black and even lines are white.  I send this rectangle using XVShmPutImage to a window with the same size so that no scaling occurs.

I would expect to see the black/white horizontal lines pattern. But I get a uniformly gray rectangle! Same phenomenon appears on either VGA, DVI or CVBS-TV monitors: it doesn't seem to be monitor/video encoder issue-related.


System environment:
------------------
-- chipset: 945G
-- system architecture: i686
-- xf86-video-intel/xserver/mesa/libdrm version: 2.7.0/1.6.0/7.4/2.4.7
-- kernel version: 2.6.29.2
-- Linux distribution: homemade
-- Machine or mobo model: 945G 
-- Display connector: VGA (but does not seem to matter)


Repro steps:

Compile the attached xv_tester.c file with the command
gcc xv_tester.c -lXv -lX11 -lXext -o xv_tester

Run xv_tester on a XV texture adapter port

(use xv_tester -h to see how to change the output window size or specify the Xv port to use)

The pattern that was used to create the YV12 image is the attached bitmap.


Additional Info:
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1) I also saw that on much older drivers. Maybe it has always been this way.

2) When the output window is one pixel higher(upscaling) or one pixel shorter(downscaling) than the source rectangle, I do see the expected pattern, but with some scaling distortion (as I expected). 

3) Opening a bitmap or .jpeg with the same pattern with an image viewer (GIMP), everything is fine. The phenomenon seems to be Xv-specific.

4) With the XV overlay adapter, if the pattern has the same size as the window, I do get all lines but with an unexpected scaling distortion.  If the pattern is one pixel higher than the window (downscaling case), I do get the same phenomenon as when the texture adapter is used with no scaling. There seems to be a skew of 1 pixel on the overlay adapter vs. the texture one.

5) [sroland@tungstengraphics.com]) wrote on the xorg mailing list:

"Not sure here, but as a wild guess (at least for textured xv) I would guess there's some half-pixel offset somewhere causing the bilinear texture filter to produce grey out of 2 black, 2 white pixels. Maybe either coords are set up slightly wrong, or chip somehow set up incorrectly (for instance d3d and ogl have a half-pixel offset difference in sampling positions - not sure though if that's actually changeable on this chip).
"