If you export with RGBA for the sole purpose of avoiding to define a proper background color: yes, it would work. But better make sure it is really transparent and really not. Any value which is not really transparent will cause the whole image to be blend, which is noticable slower and may not produce the expected results (for a render).
Still, not only shadows (and lights) but also player colors are useful targets for blending.
Has anyone actually tried this in practice and tested the results?
I ask because I have done this for pedestrians. I initially attempted to make pedestrians the usual way for Pak128.Britain, by exporting from Blender with a black background in plain RGB, and then flood filling the special background colour with a tolerance of zero . However, because pedestrians are very small, the anti-aliased blending between their edges and the black background dominated them and they looked very dark.
I then attempted to use a transparent background in Blender and export them with RGBA. The results were much better: the pedestrians have much better edges and look much more natural. Performance does not seem noticeably degraded even with very large numbers of pedestrians, although I have not actually tested the performance.
Looking at the images in the GIMP, what I see is that there is a 1 pixel (approximately) wide halo around the pedestrians of blended pixels. Despite the sub-optimal blending algorithm in the game itself discussed by Dr. Supergood, this is not noticeable in practice, whereas the blending with the black background in the GIMP makes the edges of the images look noticeably wrong.
So far as pedestrians are concerned, therefore, it seems that, from an image quality perspective, using the alpha blending to export images improves the image quality over the conventional method rather than worsens it, and there is no noticeable impact upon performance (at least not with graphics multi-threaded).
One great advantage of this process is that it eliminates the need for all post-processing, which can greatly improve productivity. With the conventional method, I need to export from Blender, move the exported images to a post-processing directory, consolidate them all with ImageMasker, then open each consolidated image in The GIMP and flood fill the background. Sometimes, extra work is needed on an individiaul image, such as where there are pure black pixels representing, for example, windows on the border of the image itself, which need to be re-filled with black pixels manually; other times, there are islands of black where the image has transparent centres, as with semaphore signals or sailing ships. With the new method, all that is needed is to move the images from their output directory to the relevant pakset directory.
Such disadvantages as there are with this method may well end up being outweighed by advantages, therefore. Has anyone actually tested producing vehicles with this system to see whether it is workable? I am planning to rescale all of Pak128.Britain's road vehicles over Christmas, and it would be a great advantage if no post-processing were required afterwards. Smaller vehicles (such as the private cars) are likely to benefit particularly from the better looking borders.