How can i make an alpha channel available for channel mixer? I need it as output channel but there are only red, green and blue available. I need it for specular maps, where i have to set e.g Red: 60% Green: 20% Blue: 80% and Alpha 50%.
The Channel Mixer is for mixing color channels. It is not programmed to work with Alpha Channels (saved selections). See this help file:
I don't know if this step from Help is what you need or not:
But that is Affinity not Photoshop
Here is the screenshot for Photoshop. Dave answered as I was walking from my iPad to my computer.
Can you provide a sample image of the RGBA file before and after mixing so that the forum can see what is possible in Photoshop compared to Affinity?
Traditionally channel operations (CHOPS) were performed with Apply Image or Calculations commnds (Image menu). Channel mixer is designed to produce 100% while your values total 210%
As menioned by Jane-e and davescm, Photoshop works differently than Affinity.
Thinking laterally, you need a four channel mixer and the only colour mode that offers this is CMYK...
So, presuming that your channel order is R = C, G = M, B = Y & Alpha 1 = K.
* You can use Image > Mode > Multichannel (don't convert directly to CMYK)
* Which returns Cyan, Magenta, Yellow, Alpha 1
* Finally use Image > Mode > CMYK (don't use convert to profile)
You should have the same exact RGBA values in CMYK channels, which are available to the Channel Mixer. From there, who knows? You still have to deal with the mixed 100% issue and additive vs. subtractive colour models etc.
To return from CMYK, reversse the process where C=R, M=G, Y=B, K=Alpha 1
CMYK > Multichannel > RGB(A)