… for Codecs & Media

Tip #1324: Test Compressed Image Quality – FAST!

Larry Jordan – LarryJordan.com

The Difference blend mode is a fast way to see how much your compression settings are damaging your images.

The most important concept you need to understand about video compression is that the process of compressing a file ALWAYS removes data during compression. Always. This means that the more you compress a file to reduce it’s file size, the more data is removed.

Once removed, you can’t put this data back. This is the reason you don’t want to re-compress an already compressed file. Another important note is that different movies, codecs and bit rates yield different results.

There’s a very fast way to compare the quality of a source file with the compressed image.

The process is simple: Using Photoshop, Final Cut Pro, or Premiere Pro, compare a frame from the source file with a frame from the compressed file using the Difference blend mode.

Perfectly matched frames are solid black. Frames with lots of differences – such as the screen shot – show lots of ghosting, especially around edges. This technique is a good way to test different compression technology and see which one works the best for your projects.

Here’s an article the explains this technique in detail and provides illustrations of the results from a variety of compression settings and software.