Need help finding a way to replicate this in Affinity Photo

[Frozen Death Knight]

There is this technique I want to replicate in Affinity Photo that can be done in Photoshop, but I can't get it to work in AP (see video link below). What I basically want to do is select all the pixels of an image from all the visible layers by using the Channels window, create a new layer, and then finally fill the entire selection with a single colour that has translated the shades of the selection into transparent pixels (i.e. pure white is 100% transparency, black is 0% transparency, and grey is everything in-between those extremes).

 

Video link in question: https://youtu.be/WZOJqbSBPxI?t=7m1s

[lepr]

.

[el.gato]

copy the main layer, invert and select that layer.

go to channel tab, right click select create spare channel.

right click the spare channel and select load to pixel selection.

create new layer, select that layer and fill w/ your color of choice w/ flood fill tool.

deselect, hide the main layer & delete invert layer.

 

[Alfred]

1 hour ago, owenr said:

You could use the Channels panel, but here's a method that doesn't require it:

1. group all the objects
2. shift-cmd-click the Group's thumbnail to get a luminosity selection
3. hide the group
4. press shift-cmd-i to invert the selection
5. do Layer > New Fill Layer and set the Fill object's colour
6. press cmd-d to deselect

 

From earlier threads it would seem that the OP is on Windows, where Ctrl is used instead of Cmd as a modifier for keyboard shortcuts.

 

[Frozen Death Knight]

Thank you both so much for the help! I ended up using mainly ownr's method, but I used the 1st paragraph of el.gato's as well. The issue I had with following the 1st method 100% was that it created a solid background (same colour as the one you used when filling the selection) around the character on the same layer, so if I needed to remove it I would have had to manually erase it with either a mask or with the Eraser Tool. Here's an image showing what I mean.
 

 

This became the final method I used in this specific order of events:

1. Duplicate the grouped layers of the character and rasterize them into a single image layer.
2. Use ctrl+I to turn all the dark areas of the rasterized image into light ones (this removed the background).
3. Use ctrl+shift+R click when pressing on the rasterized image, creating the desired selection with the correct opacity.
4. Created a new layer by either using New Fill Layer or simply adding a new one, then I used the Fill Tool to create the image with the added transparency I wanted.
5. Deselect everything.

Here is the final result.
 

[Frozen Death Knight]

On 5/6/2018 at 11:59 PM, Alfred said:

 

From earlier threads it would seem that the OP is on Windows, where Ctrl is used instead of Cmd as a modifier for keyboard shortcuts.

 

Yep, I am. I do know Mac commands as well since I used them while studying at university. I also own a MacBook Air, but I don't use it that much when I work, since my go to workstation is my custom made desktop PC.