working with referenced images & masks

[Rogurt]

Hi all. I did some search but found no answer on this. Can I make a pixel layer to mask another pixel layer? Important: all layers need to be placed/linked files so they update automatically.

background info: I refer to 3D rendering where I produce one (or more) RGB images (beauty) and several b/w images as masks (passes). The passes would be used to a) define the compositions opacity b) select and manipulate areas of the RGB.

Thanks for your help
Rembert

[Rogurt]

addendum: What I want to achieve is much like compositing in nuke/fusion/flame and the like. You provide a new image in the mask "input" (or layer in AF) and instantly get updated results...

The thing is that I have tons and tons of iterations on a rendering (with at least half a dozen alphas/passes each time). I do not want to do the whole buildup process for my AP Composing each and every time. Just overwrite the linked images and the comp is ready to export...

But maybe I will end up misusing fusion for editing still images :-[

[RichardMH]

Not clear what you want to do. Can you post an example? Are you in Photo?

But if you right click on a layer you can rasterise to mask that might solve part of your problem.

[Rogurt]

Please keep in mind: I do not (!) want to lose the link to the original files. It´s the whole point to make this masking procedural. The moment I recalculate the images from my 3D program the AF comp should update accordingly...

So yes - with "raster as mask" I could make a b/w image become the mask of another image. But the whole "procedural" thing is gone

Just think "like in After Effects" -> everything stays linked the whole time.

[Rogurt]

Is it possible to do channel calculations? Like"take the luminance of layer x and add the values to the alpha of layer y"? Maybe if this then could be a live filter it would serve my needs...

[RichardMH]

Apply Image in the filters menu might help.

[Rogurt]

Unfortunately, the moment I drag a layer onto "Apply image" as a source it will be rasterized (= not linked anymore). So this aint the solution.

[Rogurt]

So here comes the ultimate solution: James Ritson (Product Expert Affinity Photo) developed a set of VERY useful macros that deal with problems CGI guys might face when comping their renderings in AP. Among various other tools one macro takes a layer ( b/w linked image) and preps it in a way that it can be dragged onto another layer as mask. EVERYTHING STAYES LINKED!! Horray!

Just head over here and look for JR Matting & Keying Macros v2. And make sure to show some love and leave a tip on gumroad. 
https://jamesritson.co.uk/resources.html

@James Ritson thanks again!