I am playing back a Theora video via xvimagesink with:
   gst-launch-1.0 playbin uri=file:///home/olpc/test.ogg

And it appears all-black on OLPC XO-1.5. This is a regression over gstreamer-0.10. Here's what happens:

The underlying X11 graphics driver offers the XV_BRIGHTNESS attribute with range 0 to 10000, default value 5000 (normal brightness). At the extremes, value 0 is totally black (dark), value 10000 is totally white (bright). This range and default value gets correctly communicated from the X server into the color balance channel created by xvimagesink.

When the video is about to be played, in gstplaysink.c gen_video_chain() gets called. This finds the colorbalance element of the xvimagesink and then calls update_colorbalance().

update_colorbalance goes through all the channels, including brightness, and sends values down to xvimagesink by calling gst_color_balance_set_value().

Which values does it use at this point? Well, values have not been set by any user, so we are sending down the ones set in gst_play_sink_init():

  channel->label = g_strdup ("BRIGHTNESS");
  channel->min_value = -1000;
  channel->max_value = 1000;
  playsink->colorbalance_channels =
      g_list_append (playsink->colorbalance_channels, channel);
  playsink->colorbalance_values[1] = 0;

and we send that value *without any normalization* to what the proxy might accept.

So xvimagesink receives a request to set brightness=0 (where the valid values are from 0(dark) to 10000(bright)) which means "all black" and then it goes ahead and plays back the video all black. :(