Imagemagick flip channels and create an alpha channel (Unreal RMA to Unity RMA)

Question

So I have bunch of 3D models with RMAO maps to define the PBR channels. In unreal engine this is done so that I have the Roughness, metalness and ambient occlusion channels assigned as this in a mask texture:

• R = Ambient Occlusion
• G = Roughness
• B = Metalness

Now I want to transfer my assets to unity, but unity uses a different system and has the following setup:

• R = Metallic
• G = Ambient Occlusion
• B = empty
• A = Smoothness (Basically inverted roughness)

So basically this boils down to doing the following:

R <- B
G <- R
B <- pure black
A <- Inverted G

I've tried this command magick convert 'texture.png' -alpha opaque -separate -swap 0,2 -swap 1,2 -swap 3,2 -swap 2,2 -combine png32:texture_new.png but the result does not have a separate alpha channel, so the opacity is on the RGB channels.

So what I basically would want to do is to convert an RGB texture to RGBA texture, flip the channels around a bit and then invert the alpha channel. Is this possible with an imagemagick.

Answer 1

I don't use Unreal or Unity so I have no way of checking this but maybe it will help you see a way if it doesn't work.

I am laying it out so each line should produce one of your required output channels:

magick texture.png -write MPR:orig -channel B -separate \
    \( MPR:orig -channel R -separate \)                 \
    \( MPR:orig -channel R -separate -threshold 100% \) \
    \( MPR:orig -channel G -separate -negate \)         \
    -combine PNG32:result.png

Note by the way that magick convert ... is pretty much always wrong.

---

If it doesn't work, you can output each channel at each stage to see which is wrong like this:

magick texture.png -write MPR:orig -channel B -separate -write newR.png    \
    \( MPR:orig -channel R -separate                    -write newG.png \) \
    \( MPR:orig -channel R -separate -threshold 100%    -write newB.png \) \
    \( MPR:orig -channel G -separate -negate            -write newA.png \) \
    -combine PNG32:result.png