[libbluray-devel] Clearing and wiping of YUV overlay

[Richard Hulme]

Hello,

How exactly should the BD_OVERLAY_CLEAR and BD_OVERLAY_WIPE commands be 
implemented?

The contents of the overlay are 8-bit indexes to a palette.  Each pixel 
cannot do anything other than reference an entry in the palette but if 
the contents of the palette can be set to anything, how can you ensure 
that a pixel has been cleared?

The palette could be scanned to find a transparent entry, but is there a 
guarantee that at least one entry will be transparent?  I could imagine 
it's likely but not necessarily guaranteed.

Even if a transparent entry is found, it may not remain transparent if 
the palette is updated.

Another way would be to maintain an additional 1-bit mask but that seems 
rather inefficient.

As far as I can tell from the code, these two commands are not directly 
linked to the structure of a Bluray disc, but have been created as part 
of the libbluray implementation.

I'd be very grateful for an explanation of the design considerations 
behind these commands (or a pointer to any documentation).

Thanks in advance!

Richard.