ChromaKey Blues

edited September 2016 in Pro Support

HFP4 ChromaKey. I love the flexibility this key gives but I have always had a problem when keying out green screen where there are blues - especially light blues or turquoises in the  sections I want to keep. The colour is just drained out of the - sometimes turning it a sort of purple.

I assume this is something to do with there being either a lot of "G" in the colour I want to leave, or a lot of "B" in the sections I want to remove. However I do not get thi issue with the Colour Difference Key.

I'd put it down to my camera (although its a Canon XF100 - which shoots 4:2:2) and/or lighting, but I've since tried this with different kit and outdoor overcast lighting and have the same issue.

Any suggestions as to how to overcome this? (I've hitherto been working around it by copying a layer and rotoscoping over the blue bits - rather tedious)

Comments

  • Color Difference and Chroma Keying are entirely different methods of removing a color. In some cases one will work better than another, which is why it is useful to have multiple options available. 

    Is the key removing areas you want to keep, or is it just shifting the colors? Color shifts will be the result of the Spill Suppression tool, and you should adjust its settings to only remove the colors you want it to.

  • My go-to method is to jiggle the Clip Foreground and Clip Background values, plus a bit of Color Correction if necessary.  Ironically, I usually have the exact opposite issue you do; I get better keying with Chroma Key than I do with the Color Difference.

  • @Axel - Thanks. Spill suppression it is! I had erroneously thought that Spill Suppression worked only around the edges of the matte. What's more, because the default setting provide for excellent suppression, I've never had cause to play with it, not appreciating that it s set to maximum and over a very wide range.

    @Steve - Like you I do get a far better key with Chroma rather than Colour Difference. It's just the problems with the blues I have (had) with Chroma .

    I use the same method as you except I use the Gain contol to remove most of the background first. The reference manual states: 

    Gain is the base sensitivity of the chroma key. This is best used for the initial background removal. It should be adjusted until the subject is isolated, though you don't want to push the gain too far as you will start to lose edge detail.

    I've never really understood whether (or when) one should use Gain rather than Clip Background.

    Anyone?

     

  • My approach is to always use Gain first, before Clip Background. Get the key as clean as possible using Gain and Gamma before you clip anything. Especially if you have any fine edge detail (like hair), I save the clip sliders (background and foreground) as a last resort. Because fine edge details are likely to get lost once you start clipping.

  • From the title of this thread, I was expecting to see a music video. "I got those low down, good for nothin' ChromaKey Blues. Doo-wah"  ;^)

  • @StormyKnight giving me reason to miss the "Like" button... 

  • Axel having solved my immediate problem back in September I forgot to check back here. Thanks for the additional comments.

    Music video to follow ... or perhaps not (have you heard me sing!!!?)

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