Alpa channel available for channel mixer?

New Here ,
Mar 13, 2021 Mar 13, 2021

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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%.  


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Adobe Community Professional ,
Mar 13, 2021 Mar 13, 2021

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Hi @Kinesshee 

The Channel Mixer is for mixing color channels. It is not programmed to work with Alpha Channels (saved selections). See this help file:

https://helpx.adobe.com/photoshop/using/color-monochrome-adjustments-using-channels.html

 

I don't know if this step from Help is what you need or not:

 
"(Optional) Drag the slider or enter a value for the Constant option.
"This option adjusts the grayscale value of the output channel. Negative values add more black, and positive values add more white. A ‑200% value makes the output channel black; a +200% value makes the output channel white."

 

~ Jane

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New Here ,
Mar 13, 2021 Mar 13, 2021

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Its possible to make alpha channel available as output for channel mixer.

https://forum.affinity.serif.com/index.php?/topic/112842-edit-rgb-channels-seperatly/&do=findComment... 

854875027_ScreenShot2020-04-12at11_13_24.png.afd2962eb14c4c97ead5e9d00d515782

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Adobe Community Professional ,
Mar 13, 2021 Mar 13, 2021

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But that is Affinity not Photoshop

Dave

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Adobe Community Professional ,
Mar 13, 2021 Mar 13, 2021

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Here is the screenshot for Photoshop. Dave answered as I was walking from my iPad to my computer.

 

jane-e_0-1615661376524.png

~ Jane

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Adobe Community Professional ,
Mar 13, 2021 Mar 13, 2021

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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)

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