[gst-devel] Text rendering

Gergely Nagy gergely.nagy at neteyes.hu
Tue Feb 22 03:08:07 CET 2005

On Mon, 2005-02-21 at 15:34 +1100, Jan Schmidt wrote:
> On Sun, 2005-02-20 at 21:58 +0100, Maciej Katafiasz wrote:
> >Dnia 20-02-2005, nie o godzinie 18:51 +0100, Gergely Nagy napisał:
> >> Note, that the text renderer element would only create a buffer that
> >> is large enough to hold the text, not as large as the whole frame
> >> it will be rendered onto. This way the extra overhead coming from
> >> the fact that renderer and blender are separated is insignificant.
> >
> >Not really, you kinda need to render to full frame (at least logically
> >so), because subtitles are to be positioned precisely. If we were to
> >render only "necessary" parts, it will make correct positioning
> >impossible. As I refuse to pass positioning info via caps, it pretty
> >much reduces to either RLE or what Jan proposed -- equivalent of X
> >damage regions. I favor the latter, having RLE decoder inside imagemixer
> >feels wrong :)
> Actually, I think an RLE input format will be useful - in fact I'm
> planning to implement one for DVD subtitling, because RLE is the format
> the subtitles are given as on DVD. I'm planning to do this with a helper
> library in gst-libs to avoid duplicating the code everywhere, of course.

I'd think an RLE decoder would be more useful.. The only case where an
RLE input format might be better, is when you can memcpy a bunch of
pixels over the background. Which is the most common case for subtitles,
true, but in that case, blitting an uncompressed image over the picture
is pretty fast too (and has an added benefit of making it possible to do
some tricks, like transparency, anti-aliasing and stuff like that).

> Also, I don't think we need anything as complex as a full XDamage
> structure, we just need some values in the first bytes of the buffer
> that specify the x/y offset and have each buffer represent a single
> output rectangle to draw. To draw multiple regions in an output frame,
> the text renderers could supply a stream of buffers with 0 duration and
> identical timestamps. Only the last buffer of the set would have a
> non-zero duration.

I don't really like this... this places additional burden on the mixer,
which will have to care about buffer timestamps and durations. Couldn't
we have the text renderer output a container format that has a header
prepended to each region-buffer, and have a regionmixer that parses the
format, and calls into imagemixer appropriately? This way, the mixer
would not need extensive modifications to deal with 0 duration buffers.
It wouldn't even know about it, as the regionmixer takes care of that

I mean something like, text_renderer outputs this container format,
regionmixer parses it into individual buffers, then passes each one in
turn to gst_imagemixer_mix(). This sounds like pretty easy to do.

However, what would be best, is to push a whole buffer set over on a
pad. That way, one wouldn't have to do the splitting into individual
buffers part (far more resource friendly, and faster, since it means far
less buffer copying), nor would we need the 0-duration checking.

Gergely Nagy <gergely.nagy at neteyes.hu>
NetEyes Kft.

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