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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 will rely on these rules to determine how to behave with these images. However, it is possible to assign a different behavior:


  • File Alpha Premult: allows to override the rule adopted according to the file format.
  • Output Premult: defines the output mode of the reader.

Defining the background color for premultipled RGB channels

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



Two modifiers named "Premult" and "Unpremult" can be very useful to switch from one mode to the other, when you need to composite sources of different origin.