In Composite, it is typically not desirable to work with images that have their alpha premultiplied with the colors. If you are working with such an image, then you should manually add an Unpremultiply node from the Image Import tab or create a post-import script that does.
The Unpremultiply button is off by default for most formats, unless there is information in the file that indicates the presence of an alpha channel that was premultiplied. OpenEXR files have their alpha channel premultiplied, so it is set to on by default. The following table summarizes the different cases.
| Format | Unpremultiply | Value |
|---|---|---|
| Format without alpha | Off | -- |
| Format with alpha, without premultiplied info | On | Off |
| Format with alpha, with premultiplied info | On | Depends on info |
| Format with alpha, conventional premultiplied | On | Depends on convention |
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