[FFmpeg-user] alphamerge filter use problem

Thu Apr 11 17:36:38 CEST 2013

Hi Stefano!

Thanks very much. Your comment was actually very helpful.
I took a second look at how I was viewing the output of my alphamerge and
sure enough - the alpha information was correct. I am now tackling my last
problem.

My video output from the alphamerge is excellent, but consistently shows a
value of 16 in the alpha channel in areas that should be completely
transparent. It has the effect of putting a very slight gray layer on
everything.
I am betting that this is some compression on my output (which is rgba, png
codec into a mov).

You mentioned using rawvideo. I haven't tried that before, but it sounds
like what I might need.

Thanks again for your help.

-tim.

On Wed, Apr 10, 2013 at 11:51 AM, Stefano Sabatini <stefasab at gmail.com>wrote:

> On date Sunday 2013-04-07 15:44:07 -0400, Tim Willison wrote:
> > Hi, this is my first question ever to a mailing list (or anywhere!).
> > I have been using ffmpeg for about a year and a half and have figured out
> > many things on my own but I'm stumped regarding the alphaextract and
> > alphamerge filters.
> > I am able to get the proper result every time with images, but have yet
> to
> > succeed with videos. I have tried many formats and searched extensively
> but
> > have not found a single documented case of someone successfully using
> this
>
> > filter combination and sharing an example. The documentation on the
> > ffmpeg-filters page is very sparse, and does not give any details of what
> > formats/codecs are supported.
>
> Yes that's because ffmpeg-filters is supposed to only document filters.
>
> >
> > I think this filter would be very useful if I can figure it out.
> >
> > Step 1 - alphaextract
> > This seems to always work, and produces a grayscale video as expected. In
> > my tests I have used rgba video, as well as quicktime with animation
> codec
> > (bgra).
> >
> > Step 2 - applying the extracted alpha
> > This works every time for images. However while the encode completes with
> > video it never provides the masking expected by mapping the extracted
> alpha
> > information to the alpha channel of the target video. In my tests with
> > still images, I found that the output format must be a format that
> supports
> > an alpha channel, and I have tried several with video (which I determined
> > by looking at the source code of the filter).
>
> The question is, are you sure your output codec supports alpha (most
> don't, indeed I couldn't even tell which ones support alpha). You can
> try with rawvideo and make sure it is not a codec bug.
>
> > My command (taken from the docs) looks like this:
> >
> > ffmpeg -i timbotundelay.mp4 -vf "movie=timbotoverlayextracted.mov
> [alpha];
> > [in][alpha] alphamerge [out]" -an -y -pix_fmt rgba -vcodec png
> composite.mov
> > ffmpeg version git-2013-04-06-1177416 Copyright (c) 2000-2013 the FFmpeg
> > developers
> >   built on Apr  6 2013 17:31:47 with gcc 4.6.1 (Ubuntu/Linaro
> > 4.6.1-9ubuntu3)
> >   configuration: --enable-gpl --enable-libass --enable-libfaac
> > --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb
> > --enable-libopencore-amrwb --enable-libspeex --enable-librtmp
> > --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264
> > --enable-nonfree --enable-version3 --enable-libfreetype
> > --enable-filter=drawtext
> >   libavutil      52. 25.100 / 52. 25.100
> >   libavcodec     55.  2.100 / 55.  2.100
> >   libavformat    55.  1.100 / 55.  1.100
> >   libavdevice    55.  0.100 / 55.  0.100
> >   libavfilter     3. 49.100 /  3. 49.100
> >   libswscale      2.  2.100 /  2.  2.100
> >   libswresample   0. 17.102 /  0. 17.102
> >   libpostproc    52.  2.100 / 52.  2.100
> > Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'timbotundelay.mp4':
> >   Metadata:
> >     major_brand     : mp42
> >     minor_version   : 0
> >     compatible_brands: mp42mp41
> >     creation_time   : 2013-04-07 18:35:55
> >   Duration: 00:00:04.97, start: 0.000000, bitrate: 241 kb/s
> >     Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p,
> > 1280x720 [SAR 1:1 DAR 16:9], 35 kb/s, 30 fps, 30 tbr, 30k tbn, 60 tbc
> >     Metadata:
> >       creation_time   : 2013-04-07 18:35:55
> >       handler_name    : ?Mainconcept Video Media Handler
> >     Stream #0:1(eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo,
> > fltp, 189 kb/s
> >     Metadata:
> >       creation_time   : 2013-04-07 18:35:55
> >       handler_name    : #Mainconcept MP4 Sound Media Handler
> > Output #0, mov, to 'composite.mov':
> >   Metadata:
> >     major_brand     : mp42
> >     minor_version   : 0
> >     compatible_brands: mp42mp41
> >     encoder         : Lavf55.1.100
> >     Stream #0:0(eng): Video: png (png  / 0x20676E70), rgba, 1280x720 [SAR
> > 1:1 DAR 16:9], q=2-31, 200 kb/s, 15360 tbn, 30 tbc
> >     Metadata:
> >       creation_time   : 2013-04-07 18:35:55
> >       handler_name    : ?Mainconcept Video Media Handler
> > Stream mapping:
> >   Stream #0:0 -> #0:0 (h264 -> png)
> > Press [q] to stop, [?] for help
> > frame=  151 fps= 64 q=0.0 Lsize=    1952kB time=00:00:05.03
> > bitrate=3177.5kbits/s dup=2 drop=0
> > video:1951kB audio:0kB subtitle:0 global headers:0kB muxing overhead
> > 0.067323%
> >
> > After this, I see only my red background (timbotundelay.mp4) and no alpha
> > applied.
>
> I tried a variant of your command and seems to work fine here. Also note
> that you need a background to show an image with overlay. Something
> like:
> ffplay -f lavfi color -vf "movie=composite.mov, [in] overlay"
>
> should do.
>
> > Thanks so much to anyone who can help me out here. I'm running out of
> > things to try. I'd particularly like to see how people are using this
> > filter successfully.
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>

-- 
Tim Willison
Lead Product Designer
www.oddlystudios.com
495B Queen St. West
Toronto, ON.
647 345 6275