I recently added a pattern to GStreamer’s videotestsrc that can be used to check YCbCr to RGB conversion is being done correctly as part of video output. It is the result of a clever hack — some YCbCr values, when converted to RGB, are out of range, so as part of the conversion process, they are clamped to the nearest RGB value. The pattern generator creates a checkerboard pattern of a color (say, red) and a YCbCr value that upon correct conversion will result in the same color. Thus the pattern should be invisible. Usefully, these out-of-gamut YCbCr values are preserved by video codecs, so I can present to you a Theora video demonstrating this:
Firefox does the conversion correctly, so it’s unlikely you’ll see the pattern. However, some video display drivers still get this wrong, so you might see the pattern when playing the video in a standalone program that uses XV. For those of you with working kit, I created a demonstration video that simulates a bad conversion:
Sometimes it’s possible to see the pattern very faintly due to rounding in even a correct conversion. This is unavoidable because the RGB->YCbCr->RGB round trip is lossy.