Yes, you are right, the alpha channel would be the last one (RGBA).



 clbr:

Could you explain what you mean with  I don't think the alpha channel should just be the red channel from the same image?


In your previous example you are reusing the red channels as the alpha channel, or rather you were concatenating the red channel as the first channel and pushed the original RGB channels back so that you would have this layout [RRGB]=[RGBA]. The alpha channel is not a clone of the red channel, but contains own information.