[theora-dev] pre 1.0 API issues.

[Romain Beauxis]

	Hi !

Le jeudi 19 novembre 2009 13:26:26, Timothy B. Terriberry a écrit :
> Romain Beauxis wrote:
> > Since the transcoding example in the binding works well, I am suspecting
> > some  sort of issue in the encoding parameters...
> 
> Well, one thing I notice is that create_encoder initializes an encoder
> of size video_x × video_y (rounded up to a multiple of 16, as required),
> but RGB.create_yuv is passed width and height (not rounded up to a
> multiple of 16), which is also what theora_yuv is initialized with. The
> old pre-1.0 API required a complete buffer, padded out to a multiple of
> 16 by the caller. The 1.0.2 encoder validated the luma plane's size, and
> 1.1.1 validates all of the planes' sizes, so I don't think this is the
> problem (assuming the input video in your example really was 320×240),
> but it is a problem.

Ok.

> As an aside, the new API guarantees it will never read outside of the
> picture region, so you don't really need to pad the buffer, but you do
> need to play pointer games as if you had a padded buffer (unfortunate,
> but it made it easier for existing code to be ported to the new API).
> See examples/encoder_example.c in the libtheora source for details.

Ok.

> I should also point out that the method used to end the stream in
> ocaml_theora_encode_eos() actually inserts an extra frame into the
> stream (which gets rendered as a duplicate of the previous frame). In
> addition the granule position assigned to it will be wrong if you
> happened to hit the maximum keyframe interval, producing an invalid
> stream (it may play straight through, but will certainly have its length
> mis-reported by anything that tries to calculate it, and seeking near
> the end of the stream may behave oddly). The correct way end a stream
> after you've already encoded all of the packets is to add a page with no
> packets, but with the e_o_s bit set on the page. It should use the same
> granule position as the last packet in the stream. This uses a few more
> bytes than just simply setting the e_o_s bit on the last packet itself,
> when it is generated.

Ok, thanks for this, it is really usefull.

However, I do not know how to forge a page with no packet, the ogg_page 
structure needs a header and a a body, so I would need to be able to forge 
them.. Do you know how to do so ?

> Now, I don't know ocaml, but the one line that does strike me as very
> suspicious is:
> http://savonet.rastageeks.org/browser/trunk/liquidsoap/src/ogg_formats/theo
> ra_encoder.ml?rev=7022#L48 quality = quality
> 
> The transcoder example has, instead
> http://savonet.rastageeks.org/browser/trunk/ocaml-theora/examples/thtransco
> de.ml#L114 quality = !quality
> 
> which I assume means it takes the second quality from an outer scope. If
> the former is really assigning the variable to itself, then it will be
> the default (zero), and if you're not specifying a rate, you could very
> well get the behavior you described from the 1.1.1 encoder. I don't
> recall what the 1.0.2 encoder would do if you set both quality and
> bitrate to zero, but I suspect that it would have continued to produce a
> recognizable picture (albeit of fairly poor quality). The new encoder,
> on the other hand, will drop frames and generally produce something
> horrible, in an attempt to give you what you asked for.

Ok, thanks. I don't think these parameters where wrong. However, I have 
sucessfully updated our binding to the new API which solved these issues, so 
for me at least this is ok now...



Romain