[maemo-developers] Help with YV12->YUV420 converter

[From: Simon Pickering]

Hello everyone,

I've been hacking together a YV12 to YUV420 converter to run on the DSP. I
have running code, but there are some artefacts on the screen. I was hoping
that some of you with more experience of video stuff might spot an obvious
error.

If you want to run the code, you need to make some changes to your setup
(including a kernel patch). I'm happy to give instructions, but thought in
the first instance more eyes looking at the output and the code might turn
up some things I've missed.

I have two images showing the sorts of visual artefacts I'm getting [1,2],
as you can see they are different frames but very similar. The green regions
seem to stay in those general locations. Other than that, the image looks
quite reasonable. Any thoughts on what might cause this sort of localised
glitching?

The source video is here [3]. It was converted to a file called stream.yuv
by the following mplayer command: "mplayer -vo yuv4mpeg comet_jupiter_1.mpg"

I have checked the resultant YV12 file and there are no glitches in any of
the planes.

The DSP code is here [4] and ARM code is here [5]. The code is messy (read
development), and un-optimised (lots of divisions of unchanging variables
within the main loop for example). If anyone has any thoughts about
optimisation I would be interested to hear them, but the immediate problem
is getting it to output correctly.

The ARM code takes two input arguments, a filename and a letter which
indicates how to output the data. The case we're interested in is a letter
code of "y" indicating that we want to do YUV420 output. Note that there is
a typo in the ARM code [5] on line 198 (should read 45 uchars rather than
46), however this is luckily dealt with by the code which follows so
shouldn't cause a problem.

The general code flow is: 
ARM side construct a struct to pass to the DSP to tell it the input/output
formats and the size of the frame; 
ARM side read a frame's worth of data from file and place it in shared
memory; 
ARM side call ioctl to trigger DSP code; 
DSP side catch ioctl in yv12_convert_rcv_tctl(), call
yv12_convert_draw_buffer() which in turn calls convert_YV12_to_YUV420() in
this specific case. In this function the data are copied to the framebuffer
and converted en route. The framebuffer update is then called and it's
output to the screen.

Any thoughts, comments, etc. gladly accepted.

Thanks,


Simon

P.S. If there's a comment/piece of code you don't understand it's probably
because I've fiddled with the code so much and it got left behind. Please
ask :).

[1] http://people.bath.ac.uk/enpsgp/nokia770/dsp/PICT0050.JPG
[2] http://people.bath.ac.uk/enpsgp/nokia770/dsp/PICT0052.JPG
[3]
http://hubblesource.stsci.edu/sources/video/clips/details/images/comet_jupit
er_1.mpg
[4]
http://people.bath.ac.uk/enpsgp/nokia770/dsp/test-yv12-convert-and-output/N8
x0/yv12_convert.c
[5]
http://people.bath.ac.uk/enpsgp/nokia770/dsp/test-yv12-convert-and-output/ar
m/test_yv12_output.c