Date: Tue, 1 Mar 2016 10:08:50 +0000 From: dereksalt1159@gmail.com
To: gimp-user-list@gnome.org
Subject: [Gimp-user] Problems creating layer masks

I hope that you can help me. For a long time I have been trying to make layer masks of fine details, such as leafless trees full of twigs, using Gimp 2.8. I can make what looks like a perfect black and white image for the mask but when I apply it to the photo all the twigs come out with a white halo around them. I have tried selecting the image, growing, shrinking, and feathering it, the copying and pasting but nothing works. Working on solid areas such as a building gives no problems.

Any suggestions please.

Derek Salt _______________________________________________ gimp-user-list mailing list
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Not quite sure what you're trying to do based on this description but let's see ... is it safe to assume that the object you're trying to mask was originally shot against some kind of white backdrop? Then the white "halos" you see are pixels that are originally some mix (think alpha blending) of foreground and background, and GIMP has no way of automatically 'knowing' what the original, non-blended color of that pixel was supposed to be.

So you'll probably need to do something along the lines of a color-to-alpha transition. However, instead of going to the menus to look for this function, switch to your Paintbrush tool and select the "color erase" painting mode from the toolbox -- color erase is effectively a color-to-alpha function but as a painting mode. Very useful! Make sure you've added an alpha channel to the layer. Then eyedrop your background color (the one to be erased) and start using the brush to "paint" around the edges of your object. If you make any mistakes, you can undo this by setting the painting mode to "behind" (Behind and Color Erase are opposite modes) and painting back over the area.

-- Stratadrake strata_ranger@hotmail.com
--------------------
Numbers may not lie, but neither do they tell the whole truth.

dereksalt1159@gmail.com writes:
I hope that you can help me. For a long time I have been trying to make layer masks of fine details, such as leafless trees full of twigs, using Gimp 2.8. I can make what looks like a perfect black and white image for the mask but when I apply it to the photo all the twigs come out with a white halo around them. I have tried selecting the image, growing, shrinking, and feathering it, the copying and pasting but nothing works. Working on solid areas such as a building gives no problems.

Richard writes:

Not quite sure what you're trying to do based on this description but let's see ... is it safe to assume that the object you're trying to mask was originally shot against some kind of white backdrop? Then the white "halos" you see are pixels that are originally some mix (think alpha blending) of foreground and background, and GIMP has no way of automatically 'knowing' what the original, non-blended color of that pixel was supposed to be.

So you'll probably need to do something along the lines of a color-to-alpha transition.

[more suggestions snipped]

The problem may boil down to that phrase "a perfect black and white image." If you really mean black and white, not greyscale, then that's the problem. A layer mask for photographic detail almost always needs to fade from black to white to look reasonable, for the reason Richard gives: real-world photos don't have hard boundaries where one pixel is the leaf and the one next to it is the sky. There's always a bit of fade from one to the next.

So instead of using, for example, Threshold, try using Levels and using the middle slider to get something that looks almost like a black-and-white threshold, but includes a little bit of grey at the boundaries. If you're using something other than Threshold to get that black and white, adjust accordingly.

If you're already using greyscale and didn't really mean black and white, then ignore that advice: but in that case, it might help to post a sample image and mask, and details of how you're generating the mask.

Good luck!

...Akkana