Combining Mattes

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Intro

  • Matte's or alpha channels are combined in various different ways
  • The bit depth of the matte's will create slightly different results in the graduations of grey.
  • The viewer LUT that the matte is observed through will change it's appearance. The matte needs to be viewed in the same colour space that it is being used in to visually assess the result of the combination. (usually linear)
  • The colour space that the matte's are combined in changes the result greatly. i.e. an add in Log will create a different result than an add in linear. (linear is usually used)
  • Values beyond zero & one in a matte can occur in some operations when combining mattes in float bit depth. It is often clamped or a non float bit depth is used to negate this.

Using screen(or over) rather than max

screen or using an over to combine mattes combines the mid-tone grey values of the matte preferably
Using MaxUsing Screen
Image Image

Methods

Image Image
A Plate
B Plate
Image Image Image
Merge Over(similar to Screen)
Merge Under B
Merge Under A
Image
Add
Image Image
Subtract A-B
Subtract B-A
Image Image
Inverse Subtract A-B
Inverse Subtract B-A
Image Image
Max
Min
Image Image Image
Multiply
Divide A/B
Divide B/A

Notes

  • ""a more accurate approach to combining two separate alpha channels is to use MatteControl's Merge Over rather than Max, as this increases the opacity when two semi-transparent images are overlaid, as happens in real life"" . by Daniel Koch (2005) Pigsfly forum(external link) (cache)
  • Whats so good about the 'merge over' or 'merge under' matte operator is that it keeps the midrange grays areas the same and makes dark areas change - great for combing any matte with soft areas i.e. keys etc.. It seems that you get the same/similar result using a merge/over operator where the RGB is the same as the alpha or a screen opp.

Links

External Links


Created by ome. Last Modification: Monday 12 of January, 2009 07:10:17 GMT by ome.