ffdshow is an open source codec that supports all major formats like DivX, Xvid, H.264, AVC, MPEG1, MPEG2 and many others. It also has filters for post-processing and enhancing the video quality.
Subtitles can be displayed as well, including those embedded in containers like MP4 or MKV. You can use ffdshow as an audio decoder too, but AC3Filter is way better in handling AC3, DTS or SPDIF output.
I have included a registry file that contains the settings I use to set ffdshow as the default decoder for all the supported files. It will display external or embedded subtitles and add post-processing to the video.
I use ffdshow, AC3Filter and DivX Tech Preview to play everything I want. You can use Haali Media Splitter instead of DivX Tech Preview to support thumbnails of MKV. For video playback, Media Player Classic is my favorite.
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Registry Settings