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Defining the rules regarding Alpha channel

Autograph has been designed to handle images containing an Alpha layer, whose RGB channels may have been recorded in Straight/Unpremultied or Premultiplied mode.


Some image formats have been designed to work only in Straight/Unpremultied mode like PNG for example, while others like EXR, specifically records its RGB channels Premultiplied by the Alpha.

In Auto mode, Autograph relies on these rules to determine how to behave with these images. However, it is possible to assign a different behavior:


  • File Alpha Premult: lets you override the rule adopted, according to the file format.
  • Output Premult: defines the Reader's output mode.

Defining the background color for premultipled RGB channels

For images with an Alpha whose RGB channels have been Premultiplied, it is possible to define the background color if it is not black.



Two modifiers called "Premult" and "Unpremult" can be very useful for switching from one mode to the other, when you need to composite sources of different origins.