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?
Read more … » [1]
Where I/Q lines come from
You may remember from my blog post about the Vectorscope [2] that the Vectorscope uses a color space different than RGB. In the image above it is YUV [3], in the image below it is YPbPr [4]. 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 [5]. 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).
[6]
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 [7].
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)