After Effects Pixilation

GPU accelerated effects can fail on some systems. Decoding MP4 original footage can also fail. If these problems always happen at the same place select the problem layer, press the 'u' key twice to reveal all modified properties, and start resetting properties one at a time or turning off effects to see what effect is causing the problem. Sometimes pre-rendering parts (a pre-comp) of complex composites can solve the problem. Sometimes turning off GPU acceleration or updating drivers can solve problems.

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If it is a decoding issue with your original footage then, as Andrew Yoole suggested. transcoding your footage to a frame-based (intraframe) compressed production format will almost always solve a problem from interframe compressed MP4 files. Interframe compression always takes longer to decode and it is much more likely to fail when used in a production pipeline.

If your source files look fine when played outside AE, it may be some kind of decoding issue.  My first suggestion is to transcode your MP4 source files to a high quality intermediate codec. High quality DNxHD, Quicktime Animation etc.  (On a Mac you'd use ProRes 422HQ or better).  Then play the files in a media player to see if they look okay.  If so, replace your MP4s in AE with the new files and see if it solves the problem.

If it doesn't fix the issue, you need to tell us a lot more about your project in terms of the effects you are using etc.

This is not pixelation, but compression artefacts. Check you source files, changes are high, this dirt is already in the footage.

*Martin

heavy* not hevy