Different algorithm for masks+stacks?

Recommended Posts

I recently tried stacking some photos of the same subject with different lighting (to eliminate glare in post) and ran into an annoying issue: stacks don't play well with masks. If you attempt to mask out an item inside a stack using a soft brush, you get an extremely sharp boundary instead of the gradual transition you want. I can see why it was implemented this way, but I would like to suggest a different algorithm that's slower but at least useful (and should get the result people want and probably intuitively expect).

Let's say you have 5 layers in the stack. You set the stack mode to "median". However, in one area of the image, layer 3 is causing an object to look too dark. You'd like to mask out layer 3 in that area, and that area should look like layer 3 isn't being included (and gradually transition as you'd expect using a soft brush on your mask layer). You'd like to mask out layer 4 elsewhere because it's making something look too light.

To get the intuitive/desired behavior, you would calculate the stack 4 times (2^<number of layers with masks>):

A: Stack color channels of layers 1,2,3,4,5

B: Stack color channels of layers 1,2,4,5

C: Stack color channels of layers 1,2,3,5

D: Stack color channels of layers 1,2,5

Let a_i be the alpha channel of image <i>

a_A = a_3*a_4

a_B = (1 - a_3) * a_4

a_C = a_3 * (1 - a_4)

a_D = (1 - a_3) * (1 - a_4)

To get the final color output of the stack, you calculate A*a_A + B*a_B + C*a_C + D*a_D

Or something along those lines.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×

×

• Affinity

×
• Create New...