[FFmpeg-user] alphamerge filter use problem
Paul B Mahol
onemda at gmail.com
Fri Apr 12 16:38:48 CEST 2013
On 4/12/13, Tim Willison <timbot at oddlystudios.com> wrote:
> That's interesting Paul, you might be on to something there. I found it
> very coincidental that the alpha from pure black areas was always 16.
>
> I'm actually doing two passes, and using rgba for everything.
> On my first pass, I have a moving gradient which is rgb24, and I apply a
> mask to it (with alphamerge) that was created using the alphaextract filter
> on an rgba png image (png color-type 6).
>
> I am using the output of that (which is now a nice animated gradient in the
> shape of text) as the luma source and doing another alphamerge against my
> original text image. Now my nice moving gradient reveals the original
> image.
>
> The only place I can think that YUV might be introduced would be in the
> alphaextract filter. But even that I encode into rgba png before I use it.
> I wonder if there is a better format to use than rgba png encoded.
Does your rgb source have pure black 0 pixels?
(you can see it with histogram filter)
When converting it to gray8 with alpha extract, is it preserved?
0 values should remain 0.
>
> -t.
>
> On Fri, Apr 12, 2013 at 9:12 AM, Paul B Mahol <onemda at gmail.com> wrote:
>
>> On 4/12/13, Tim Willison <timbot at oddlystudios.com> wrote:
>> > Hello folks,
>> > I thought I could share a tip I've used in my experiments with the
>> > alphaextract and alphamerge filters.
>> >
>> > I have found that the result of the alphamerge filter often leaves
>> > areas
>> > with some non-zero alpha. Comparing the mask file, areas that are pure
>> > black are not always reproduced as pure alpha after the merge.
>>
>> Looks like you used YUV source for mask. And result is expected because
>> YUV (limited range, not YUVJ) values can not be 0-16 or 235-255.
>>
>> >
>> > However, I found a way to deal with this by using the lut filter. It
>> > was
>> as
>> > simple as adding this to my alphamerge filter chain:
>> >
>> > [text2][out]alphamerge,format=rgba,lutrgb=a=if(gte(val\,18)\,val)
>> >
>> > The lutrgb is using an expression that lets me add a threshold for the
>> > alpha channel. In this case, I'm saying that if the alpha drops below
>> > 18,
>> > it should likely be zero, otherwise keep it the same.
>> > This cleaned up any alpha artefacts very nicely.
>> >
>> > Hope that is helpful for somebody.
>> >
>> > -t.
>> >
>> >
>> > On Thu, Apr 11, 2013 at 11:36 AM, Tim Willison
>> > <timbot at oddlystudios.com>wrote:
>> >
>> >> 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.
>> >>> _______________________________________________
>> >>> ffmpeg-user mailing list
>> >>> ffmpeg-user at ffmpeg.org
>> >>> http://ffmpeg.org/mailman/listinfo/ffmpeg-user
>> >>>
>> >>
>> >>
>> >>
>> >> --
>> >> Tim Willison
>> >> Lead Product Designer
>> >> www.oddlystudios.com
>> >> 495B Queen St. West
>> >> Toronto, ON.
>> >> 647 345 6275
>> >>
>> >
>> >
>> >
>> > --
>> > Tim Willison
>> > Lead Product Designer
>> > www.oddlystudios.com
>> > 495B Queen St. West
>> > Toronto, ON.
>> > 647 345 6275
>> > _______________________________________________
>> > ffmpeg-user mailing list
>> > ffmpeg-user at ffmpeg.org
>> > http://ffmpeg.org/mailman/listinfo/ffmpeg-user
>> >
>> _______________________________________________
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>
>
>
> --
> Tim Willison
> Lead Product Designer
> www.oddlystudios.com
> 495B Queen St. West
> Toronto, ON.
> 647 345 6275
> _______________________________________________
> ffmpeg-user mailing list
> ffmpeg-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-user
>
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