Swap RGBA channels around in Gimp?

Question

I have a weird texture format of a game which stores the RGB color and alpha channel in an uncommon way like this:

• Real R maps to game's R
• Real G maps to game's R
• Real B maps to game's R
• Real A maps to game's G

Thus, it basically can only contain grayscale (R channel) and alpha information (in the B channel). The G and A channels can be safely ignored. Is it possible to remap the color channels and alpha channel in Gimp accordingly?

This is the input image:

Answer 1

In general, the Decompose and Compose filters may be able to do this.

Decomposes can split an image into grayscale images or layers for the R, G, B and A component.

Compose allows to use grayscale images or layers for the R, G, B and A component of a new image.

For the new G and A, you probably want to use either plain white or black.

You will have to clarify how you expect more than one component to be mapped to the new R, though.

Answer 2

The necessary tools are in the Colours > Components submenu:

• The "Channel Mixer" allows to assign linear combinations of all input channels to all output channels. This is the fastest way to swap e.g. red and blue:
  • Select red output channel, configure red to 0% and blue to 100%.
  • Select blue output channel, configure blue to 0% and red to 100%.
  • Apply with "OK".
• As Michael suggested, one may also decompose the channels and compose them again later, which may open additional possibilities (than just linear combinations).