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Unmapped Pixels and Blue vs Alpha
In the example above, some pixels in the output image do not correspond to pixels in the original undistorted image—there is nowhere to pull those pixels from. There are several possible values to place into such pixels.
- White, no alpha (R=G=B=1, no A) This is a good choice, because no valid pixel can be white—the blue channel is zero for all valid pixels. The image is smaller, since no alpha channel must be stored. The alpha channel can be recreated for compositing, if needed, as 1-b.
- Black, with alpha. (R=G=B=A=0) Alpha is 1.0 for all used pixels. Alpha is available for compositing, but the image is larger due to the alpha. Software must pay attention to alpha, since black pixels could be valid or invalid depending on alpha. This option is pre-multiplied alpha: PMA.
- White, with alpha. (R=G=B=1, A=0) Combination of the above, where used and unused pixels are easy to distinguish. Requires alpha storage. These images are non-pre-multiplied alpha (nonPMA) , so compositing packages may need to know that.
- Unclipped. OpenEXR only! Any of the above, but with no clipping of out- of-range values. See below.
SynthEyes has preferences for each allowable image-map format (DPX, EXR, PNG, SGI, TIF) to configure which of the options above will be used for that specific image format.
The unclipped option is available only for OpenEXR images; it allows the red and green channels to go under zero or above one. (Blue is always used as a out-of- range marker.) With this option, compositing software that has pixels available in overscan/margin areas can bring those pixels into the final result.
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