On Sat, 26 Mar 2016 16:56:55 -0600 Nic Wolfe <n...@wolfeden.ca> wrote:
> The Matroska spec defines PixelCropTop, PixelCropBottom, PixelCropLeft, > and PixelCropRight elements: > https://www.matroska.org/technical/specs/index.html > > This commit adds support for demuxing these values so that > applications using libav* > are able to use them when playing the stream. They're added to the AVStream's > metadata if they are set to something non-zero. > > > My official patch is base64 encoded and attached but I will also > include the diff below for (hopefully) convenience. > To elaborate on why this change is bad (in its current state): - It's not clearly defined what the pixelcrop fields mean. Do they operate before or after aspect rasto is applied? Do they affect aspect ratio calculation? What if aspect ratio or video size change later? Does it get applied after h264 bitstream cropping, does it override it? AFAIK these issues were also discussed on the cellar mailing list, but I didn't follow it. - There should generally be a concept at least in libavformat's API how to handle cropping. For example, it could be some sort of well-defined AVStream side data. (Personally I'd be a fan of adding it to AVFrame too. There's no way around if it should work for hw decoding.) - Worst of all: it's exported as generic metadata. This means that: - API users could start interpreting the same metadata fields for other formats than Matroska too - Transcoders like ffmpeg CLI will copy the crop metadata to other containers (as normal metadata). - Non-Matroska files might be created that contain the "made up" libavformat Matroska demuxer metadata, and the creator of the file expects that programs respect it. (Something like this almost happened with the "old" libavformat rotation metadata, which is also exported as normal metadata.) While just adding a "hack" to export metadata for essentially 1 API user might be acceptable if adding "proper" API is too hard for now, at least the last point needs to be fixed. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
