Layer masks in GIMP

[dweeble]

Shaming confession but I have a total and utter blind spot on these......trying to use one to replace background...the curves and invert bit to get a pure white on black is easy but I simply CANNOT get my head round which layer to select, paste, merge, alpha-ise or whatever.

Have watched endless tutorials and it simply doesn't sink in. Asked a couple of guys who offer paid training and they both said oh well it might takes HOURS whereas I hoped they would say they had taught bigger idiots inside two hours.

It can't be that hard, it's just a matter of steps....

Is there anyone out there who fancies seeing if they can drum the basic process into my thick skull?

 

[J Veitch]

Have you had a read through the gimp tutorial on it?

GIMP - Layer Masks

Intention¶ Layer masks are a fundamental tool in image manipulations. They allow you to selectively modify the opacity (transparency) of the layer they belong to. This differs from the use of the layer Opacity slider as a mask has the ability to selectively modify the opacity of different areas...

www.gimp.org

Short version - a mask is applied to the layer you want to make (parts of) transparent. So you will have another layer below that will now show through the "black" parts of the mask. In your example the "new" background would be at the bottom, the "old" image at the top with a layer mask set to black on the parts you want replace

 

[Tom Pinchenzo]

Not an answer to your questions but I found GIMP really difficult to get my head around. I bit the bullet and got Affinity Photo (like £30 or something) and never looked back. Much more intuitive but more importantly, loads of really good tutorials online.

 

[Stegosaurus]

I have no intention of trying to conduct a tutorial on a forum.. life is too short :-D

But..... perhaps you aren't seeing the wood for the trees. The logic is as follows:
1. Find a way to select the background in your image you want to eliminate
2. delete background to leave a hole
3. Put new background layer underneath the image so it shows through.
4. Tidy up.


The first part of the process is usually the most difficult bit and there are many ways of going about it, including painstakingly deleting manually. You say you know how to create a high contrast copy composed of pure black and pure white in a second layer (and I am assuming that you know that this will only work if your background is much lighter or darker than the foreground) . This is so that you can create a selection of either all the black or all the white pixels, which is basically making a really complicated cookie cutter. You then use that cookie cutter on your original image and use it to cut out the same areas (just select the image layer instead of the high contrast layer and press delete). BUT before you do that, you have to give your image layer an alpha channel, or the cut out area will not be transparent, it will be opaque. When you've done all this, you can delete the high contrast layer.

Having said that, I usually find that this technique is usually far too crude to give great results. Even if you feather the selection, you will invariably need to spend time with an eraser softening the edges up to make it look natural. So much so, that often you may as well just delete the unwanted bits manually in the first place.

 

[dweeble]

Thanks guys, the whole concept of masks or cookie cutters is simple...I just cannot work out the steps and what achieves what. I know I'm being thick but it's maddening. Have actually written down the steps from a Youtube vid and following them seems to get me where I need to be - but I'm parroting rather than actually understanding, which is sub optimal. Perhaps the light will dawn one day.....