Vectorscope I and Q Lines

The Vectorscope has an option to draw I/Q lines. So you may be wondering what are they good for?


I and Q lines in the Vectorscope

Where I/Q lines Come From

You may remember from the Vectorscope chapter 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).


Vectorscope showing skin tones along the I line

Purpose of the I and the Q line

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 should approximately lie on the I line. If it is not, you might want to color-correct your clip.

The simple reason for this is that our eyes are trained on skin tones, and if skin tones in your video do not lie in the I line they are very likely to look unnatural. There are very good picture examples in the Save our Skins article mentioned above.



skin1.avi (720p, 5.1 MB)

The original text was submitted by Simon A. Eugster (Granjow) on Fri, 11/26/2010 - 18:05 to the now defunct blog. For this documentation it has been lifted from, updated and adapted to match the overall style.