[gst-devel] Gstreamer, libv4l and YUV420

[Erik Andrén]

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Hi,

I'm developing a driver for stv06xx based webcams (Quickcam Web,
Dexxa Webcam, Quickcam Express etc.)

Some of these webcams contain a vv6410 sensor producing a raw bayer
image with a 356x292 resolution.

As the camera only supports bayer natively the libv4l [1] library is
used to convert the format to rgb or yuv.

RGB works fine but I'm having trouble to get yuv conversion working
together with gstreamer. This is due to that the U V plane widths
are rounded up by 8 in gstv4l2src.c

libv4l delivers 356*292+(356*292/2) = 155928 byte frames but
gstreamer expects 356+292+(360*292/2) = 156512 byte frames and thus
bails out.

If I've understood things correctly, gstreamer expects us to pad
each line with two bytes, but the v4l2 standard [2] says that the
driver may choose to ignore padding and this is as of current not
honoured by gstreamer.

I've hacked gstv4l2src.c to only round up by 4 but I still get an
corrupt image. My take is that the rounding by 8 is presumed
elsewhere in the gstreamer stack. Other yuv420 consuming
applications (skype for instance) work correctly and doesn't expect
this alignment.

My questions are as following:
1. Why is the rounding by 8 taking place? Is this defined somewhere
in yuv420 specification?
2. How do we resolve this issue? AFAICS the gstreamer stack is
currently violating the specs as it expects the padding. Would this
be easy to fix?

Best regards,
Erik Andrén

[1] http://hansdegoede.livejournal.com/3636.html
[2]
http://www.linuxtv.org/downloads/video4linux/API/V4L2_API/spec/c2030.htm#V4L2-PIX-FORMAT
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