Pixelation when creating mp4 for Vimeo

We’ve been using Fotomagico 4 for a while now to create slideshows and share them as mp4 video files through Vimeo. We notice that sometimes during playback of the video files, that some of the images briefly appear pixelated before sharpening. This pixelation does not appear in Fotomagico and always appears on the same images during playback so I have to assume it’s a problem that occurs in the rendering process, but re-rendering the video file doesn’t help. It’s by no means all the images (the vast majority are perfect) and it doesn’t ruin the video - it’s just kind of annoying. Anyone else had this problem?

Those artifacts are coming from the video compressor: When exporting a MP4 video you setup a certain maximum bit rate for streaming in order to fit into a certain internet bandwidth. Once the movie has big changes (like a cut from one image to another) then the video codec can’t transmit the full quality within this maximum bit rate and therefor only put a blurry image into the video stream. The next couple of frames are nearly the same as the blurry one and so the MP4 stream now contains data to improve the image quality until it is back to normal. There is some information about it here https://en.wikipedia.org/wiki/Compression_artifact .

The only way to avoid those artifacts is to increase the maximum bit rate in the exporter.