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Comparing footage with difference mattes

It's a common question - how much quality do you lose when converting from Format X to Format Y or when you are forced to recompress footage? You can compare data rates and squint at it all day but that's not really telling you much.

Here's an easy way to tell exactly what has been lost.

Final Cut Pro
1. Take your Final Cut Pro sequence and export to the second format (the one that you plan to convert to). Or, if your footage didn't originate in FCP, import both versions (before and after) and place them into a new sequence.

2. Place the "before" version onto the bottom track and the "after" above it.



3. Ctrl-click on the "after" version and select Composite Mode > Difference.



Shake
1. Import the "before" and "after" versions into Shake using FileIn nodes.



2. Click on the Layer tab and click on MultiLayer.

3. Connect the noodles from the FileIns to the MultiLayer.



4. In the Parameters tab, change the composite mode of the second clip (the one listed at the top) to Difference.



Both applications will produce an image similar to the one below. This image tells you what has been lost from one version to the other. The clearer this image is, the more data has been lost. If you can clearly make out the edges of objects from the original image, this data loss is probably unacceptable. I personally would consider the image below unacceptable but how much data loss you can accept depends, of course, on the final destination of the clip. For example, web users will be much more forgiving than cinema-goers.