[FFmpeg-soc] [PATCH] updated! vf_overlay alpha patch and watermarking using PNG with alpha

[Michael Niedermayer]

On Wed, May 12, 2010 at 09:00:46AM +0200, Vitor Sessak wrote:
> Baptiste Coudurier wrote:
>> On 12/06/2009 08:12 AM, Vitor Sessak wrote:
>>> Hi and sorry for the delay.
>>>
>>> Artur Bodera wrote:
>>>> On Tue, Dec 1, 2009 at 12:50 AM, Vitor Sessak <vitor1001 at gmail.com>
>>>> wrote:
>>>>
>>>>> While I normally oppose making non-committed code more complex, I think
>>>>> this feature is so often requested that it is worth the extra work in
>>>>> the
>>>>> future. Stefano, Michael, any strong opinion about this?
>>>>>
>>>>
>>>> I think the vf_overlay should be modified altogether. Although
>>>> mathematically alpha-blending is more expensive than opaque pixel
>>>> replacement, I think that it should be automatically decided by 
>>>> analyzing
>>>> the overlay format.
>>>>
>>>> So the alpha-blending should be a "built-in" functionality (not a
>>>> switchable
>>>> parameter) and should be implicitly functional with any overlay
>>>> stream/image
>>>> that has alpha channel (i.e. rgba). If there is no alpha channel, then
>>>> pixel
>>>> overriding would be used. This makes much more sense.
>>>
>>> I agree that this would be nice, but there is no way to make it work
>>> with the current format negotiation in libavfilter. For example, there
>>> is no way to have a filter that accepts either "input: rgb, output rgba"
>>> or "input: yuv, output: yuva", so I suggest you just do as your present
>>> patch for the time been.
>> How much harm does doing yuv -> yuva or rgb -> rgba in all cases ?
>> To my knowledge it would only be a matter of adding one component and 
>> memset it to 0xff.
>
> This wouldn't do much harm, but there is no way of asking for such kind of 
> inputs in lavfi (i.e., either "rgb,rgba" or "yuv,yuva"). And changing lavfi 
> to accept such constraints would probably make colospace negotiation a NP 
> problem.

finding a solution with a minimum of convertion could become NP, finding any
solution wont

heres a quick option (i do not know how good / bad this suggestion is,
if someone like loren could look at this before its implemented this
would probably be a good idea)

1. we extend query_format to query_format(int alternative) where alternative
selects which of several (small number normally and for most filters just 1)
alternative colorspace lists each link supports.
Euch such alternative is like the current case that is 2 links either must
have the same list of possible pixelformats/colorspaces or they are
independant.

2. we run the current avfiltergraph.c query_formats() over the graph skiping
all filters that have alternatives.

3. we go over all filters that have alternatives
   for each such filter we find the 2 best alterantives, best based on
   score= (A<<32) + B;
   A= the number of scale filters the specific alterantive would require
   B= the number of degrees of freedom lost (aka fewer colorspaces)
   we then calculate a score2 that is the difference of the scores of
   the best 2 alterantives.
   And last lock in filter alterantives iteratively based on best score2

The idea here is that we lock in the filter with the most clearly supperior
alterantive first. That is if we have a filter that has 2 alterantives and
neither on its own would require an additional convertion filter and another
filter that also has 2 alternatives and one of these would require 1
convertion and the other case would require 2 convertions then the 1
convertion alternative would be locked in and we would restart from the begin

This is not optimal, not even for a simple linear filter chain with 2 filters.
But it might be an acceptable heuristic for real filter graphs.
For a single linear chain its possible to find the optimal case with
viterbi

Another method would be to only use avfiltergraph.c query_formats() and
consider all the alternative values inputs to it and the number of convertions
and degrees of freedom left over the whole graph its score. And then use some
generic optimization technique that minimizes this score.

btw, off topic a little but simply favoring the first colorspace in each
list like its done in svn is not a good idea, it likely would make more
sense to favor a colorspace that preserves all information that is there
and is simple. that is if either upstream or downstream is greyscale we
shouldnt select yuv similar issues exist with 16/8 bit and rgb/yuv and alpha

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is not what we do, but why we do it that matters.
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