[gst-devel] Need help using gst-launch to display a PNG image, Part 3+

[Tim-Philipp Müller]

On Thu, 2010-08-12 at 15:56 -0700, Bill West wrote:

> The command I'm using is:
> gst-launch-0.10 filesrc location=overlay.png ! pngdec !
> ffmpegcolorspace ! videoscale ! imagefreeze ! videomixer name=mix !
> autovideosink  videotestsrc ! video/x-raw-yuv, width=320, height=240 !
> mix.
> 

> Looking at the documentation, the only possible solution I could find
> is an element called "alphacolor", but nowhere placed in the pipeline
> does it work, so I'm stumped again.

Try the 'alpha' element.

Something like this works for me:

$ gst-launch-0.10 filesrc location=foo.png ! pngdec ! alpha alpha=0.5 !
videoscale ! imagefreeze ! videomixer name=mix ! ffmpegcolorspace !
autovideosink    videotestsrc ! alpha alpha=0.5 ! mix. 

or if the png already has an alpha channel:

gst-launch-0.10 filesrc location=alpha.png ! pngdec ! videoscale !
imagefreeze ! videomixer name=mix ! ffmpegcolorspace ! autovideosink
videotestsrc ! alpha alpha=0.5 ! video/x-raw-rgb,bpp=32,depth=32 ! mix. 


> 
> 2.  we have ALSA audio, and I see there's an element called
> "alsasink", though I couldn't find much documentation on it.  Is there
> a way with alsasink to specify the output channel?  (our card has 8,
> so 4 stereo pairs).

No. You can specify an output device though using the "device" property.
And you can probably do some .asoundrc magic to route stuff to the right
outputs (ie. expose the 4 stereo pairs as 4 stereo devices or 8 mono
devices or whatever).

> 
> 3.  Is there a way to have the other PNG images fade in and fade out
> of the window to/from black?

Yes, but for this it would be best to write some code (python script or
C or whatever).

The "alpha" property of the alpha element is controllable (see
GstController interface), and videomixer's pads also have properties
like "xpos", "ypos", "zorder" and "alpha", which are also controllable
and can be changed at runtime.


> 4.  Is there a way to loop between two particular frames on a video?
>
> 5.  Is videobox the best way to position my video behind the overlay
> PNG?  I got it to work kind of, but there were artifacts, like
> everything to the left and above the video is black.

videobox? No, just use videomixer and set xpos/ypos/zorder/alpha via the
pad properties.

> 
> 6.  I thought someone must have written a translator that takes a
> gst-launch command line and converts it to the equivalent C code, but
> couldn't find one.  Did I just not look in the right place?

There isn't one, and it's probably not a good idea either (RIP Glade
code generator).

You can use gst_parse_launch() from C code though (give elements a name
with name=foo and then use gst_bin_get_by_name() on the pipeline to
retrieve the elements by name to manipulate them).

Cheers
 -Tim