[FFmpeg-user] alphamerge filter use problem
Tim Willison
timbot at oddlystudios.com
Fri Apr 12 14:43:25 CEST 2013
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.
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
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