Vectorscope: What the I and the Q lines are good for

In the next kdenlive version (or in the current SVN version, if you dare compile it yourself :)) you will find a new option for the vectorscope: To draw I/Q lines. What are they good for?
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Where I/Q lines come from

You may remember from my blog post about the Vectorscope that the Vectorscope uses a color space different than RGB. In the image above it is YUV, in the image below it is YPbPr. They both share the property that the Y component represents Luma only (i.e. how bright a pixel is), and the other two components represent Chroma (colour) by expressing deviations from neutral color on the red-green and yellow-blue axis. (These are complementary colours each, so mixing them in equal parts results in neutral again – which is why they can be used for the deviation.)

YUV is the standard color space for analog PAL television. NTSC, the american analog TV standard, uses a color space I did not mention yet: YIQ. The special thing about this color space is that the I component was chosen such that skin tones (also known as flesh tones) lie on the I line (orange-blue), and it was given more than four times as much bandwidth as the Q component (which represents the green-purple line; The human eye is also less sensitive for changes on this line).

The Purpose of the I and the Q line

You might have guessed it already: The reason for displaying the Q and, especially, the I line is to help with skin tones. There is a rule of thumb in post production saying that all skin tones sould approximately lie on the I line. If it is not, you might want to color-correct your clip.

Why? If skin tones do not lie on the I line, they are likely to look unnatural. Our eye is trained on skin tones ;) End of the story.

Clip sources

Only one this time.

That's it! Thanks for reading.
Feel free to post your comments below.

Simon A. Eugster (Granjow)