[gst-devel] plan to write a bayer filter

[Jan Schmidt]

On Mon, 2005-01-10 at 15:15 +0100, Benjamin Otte wrote:
> General comment: We need a good intro on what link and getcaps functions
> do, since a lot of people have a hard time understanding what this part of
> a plugin does.

http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/chapter-building-pads.html
and 
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-pads-getcapsfn.html
have something like this. Perhaps it could be expanded, or we could make
a plugin-writers FAQ somewhere nearby or in the PWG?

*snip*

> It might be useful to dump some output to a file (via 4vlsrc ! filesink or
> whatever) and use that. This is easy if Bayer frames are fixed size.
> Just run the pipeline like this:
> gst-launch gnomevfssrc location=/path/to/frames bytesperread=$framesize !
> bayerdec ! ...
> (You need to use gnomevfssrc here because filesrc's blocksizes need to be
> multiples of 2 iirc.)
> 
> Off Topic: This trick works nicely for raw video dumps, too. Just run
> something like
> gst-launch gnomevfssrc location=/path/to/frames bytesperread=153600 !
> video/x-raw-yuv,width=320,height=240,framerate=25.0,format=\(fourcc\)I420
> ! xvimagesink
> and you can view it.

*snip*

> Maybe have a look at a simple video decoder (if we have those - do we have
> simple ones, I can only come up with mpeg2dec and ffmpeg and they're both
> not simple) or colorspace transformation plugins (I believe there's an
> alpha plugin somewhere that does I420=>AYUV or so?)
> Anyway, some tips I find useful when doing caps stuff:
> 
> 1) the simplest solution to get a plugin working first and not need
> to care about link and getcaps is to only decode a fixed format. Elements
> that only decode a fixed format don't need link or getcaps functions, they
> specify the supported format in their fixed pad templates. Fixed caps can
> easily be found: They don't contain ranges ("[x, y]") or lists ("{x, y,
> ...}").
> 
> 2) when implementing caps stuff, particularly caps negotiation, the
> GST_PADS debugging category is important. So running your app with
> --gst-debug=GST_PADS:5 might clue you in on quite some stuff.
> It might also be useful for general understanding to just watch what
> happens when you run simple GStreamer pipelines with that debugging
> output, like gst-launch videotestsrc ! xvimagesink --gst-debug=GST_PADS:5
> or gst-launch v4lsrc ! xvimagesink --gst-debug=GST_PADS:5
> 
> 3) Get to know gst-launch filtercaps syntax. It's very helpful if you can
> just force a particular caps.
> 

This is good stuff. I think it should go in the PWG somewhere too.

J.
-- 
Jan Schmidt <thaytan at noraisin.net>