Alpha Unblend is an image stacking operation that does the reverse of alpha blending – it takes two or more opaque images and uses them to deduce an image with an alpha channel such that it can be blended with solid backgrounds to yield the originals, or blended with a new background to effectively “transplant” the object of interest from one background to another. This technique can be very useful for creating sprites for use in other images, especially where transparency is tricky, such as objects with transparent or translucent parts (for example, a ball-point pen made of clear plastic or a glass bottle).
As an example, consider the two images below. They depict the same object with different backgrounds; white and black. The third image is the result of the Alpha Unblend operation on the two images.
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As you can see, the engine has been able to isolate the transparency information from the two images, resulting in a sprite that can be merged with any other background.
Here’s a real-life example to show you a practical use for this feature. The CD-R disk in the first two images was photographed using two backgrounds; a black table-top and a white paper placed against that table-top. The third image was generated by running the first two through this engine; this was then merged with the image of a mat to produce the fourth image – note the preservation of transparency information and the seamless merging. The last image was made by merging it with a photo of the planet Jupiter.
The “how” bit
You need at least 2 same-size images of the same scene to run this engine. The images need to be aligned; this can be done by either making sure the camera remains perfectly still or by using the Align Images for Stacking feature. The images need to be of the same object against different backgrounds. The backgrounds are evaluated in pairs and the members of each pair need to be as different as possible; it is therefore recommended that you alternate between black and white backgrounds (e.g. black, white, black, white, black, white, ...) when setting up the images. Neighboring images that are relatively similar will be disregarded by the engine.
This is the GUI for the Alpha Unblend feature:
The engine requires a reference point to use as the image background; this can be set by right-clicking on the preview. The area that is indicated as the background will be fully transparent in the resulting image if the unblend operation is successful. The sliders for the cut-off points determine the portion of the calculated alpha range to use; the default is to use the entire range 0 - 255.
For the best results when stacking images, keep the camera as steady as possible.
