Ah, OK, I think I might know why you’re seeing what you’re seeing here, thanks (it might be a known bug that will likely be fixed in the next version)! Is the RGB data getting set to black only for those pixels that have an alpha value that is exactly 0? (The RGB data should be preserved for pixels that have any nonzero alpha value, even a value of 1/255.)
If so, here’s what’s going on: Because of some of the nice mathematical properties it has for blending colors more accurately (which was a major focus for this version), the plugin tries to internally store colors using premultiplied alpha as often as it can - i.e. it stores an unpremultiplied (r, g, b, a)
color in the form (r*a, g*a, b*a, a)
. (Many of the operators in the plugin, such as the RGBA scale/bias sliders, still work on unpremultiplied alpha colors, so the plugin converts between unpremultiplied and premultiplied colors often). Given a premultiplied color (r*a, g*a, b*a, a)
, so long as a isn’t equal to 0 (e.g. if a is 1/255), you can get the unpremultiplied color back by dividing by a. But when a is equal to 0, that information gets lost - (r, g, b, 0)
premultiplies to (0, 0, 0, 0)
. One of the solutions for this is to avoid multiplying by a when a is equal to 0 - i.e. having (r, g, b, 0)
premultiply to (r, g, b, 0)
instead of (0, 0, 0, 0)
, but this hasn’t been implemented yet in the current version. (I believe Photoshop stores colors using unpremultiplied alpha, so the RGB information doesn’t go away with a transparent layer mask or alpha channel, but this can also lead to some artifacts when blending transparent colors.)
The “Premultiplied Alpha Blending” setting only applies to export, instead of import, and affects how higher mips are generated so that colors are blended correctly when the alpha channel represents transparency (which isn’t the case in this case). To see what it does, you can take a texture with varying colors and transparency, slide the Mip slider to Mip 1 or higher, zoom in on a transparent edge, and toggle the setting on or off. John McDonald talks a bit about this here: Alpha Blending: To Pre or Not To Pre | NVIDIA Developer