Selections and Gray Pixels

J
Posted By
JSB
Nov 19, 2003
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242
Replies
1
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Closed
I need some help with this Photoshop problem.

I’ve taken a number of digital photographs which show a forest of trees against a bald sky (no clouds – basically light blue to almost all white). I would like to use PS to replace the bald sky with another digital photograph of a deep blue sky containing an assortment of clouds. My preferred technique is to carefully make a selection of the bald sky, delete it and then place the new sky image as a layer underneath the original layer of the trees.

Here’s the problem: In any technique I’ve tried that uses a precise selection, I always seem to wind up with a small halo of gray pixels around the edges of the leaves and branches where the treetops meet the new sky. All my selections seem to be quite accurate yet the composite always has a small amount of gray pixels showing. I’ve tried the Matting>Defringe command but it doesn’t seem to help too much. (Just for the record, I have also used other techniques such as using a layer mask to bring in the new sky but I’ve found this to be too time consuming and the results are dependent on my ability to do a good job on "painting" away the bald sky).

I need something which gives me a result which looks natural and not obviously composited. Are there any tricks or alternate techniques which could produce the results I’m looking for? Any suggestions for helping me with this problem would be greatly appreciated. Thanks.

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H
Hecate
Nov 19, 2003
On Wed, 19 Nov 2003 00:31:20 GMT, JSB wrote:

I need some help with this Photoshop problem.

I’ve taken a number of digital photographs which show a forest of trees against a bald sky (no clouds – basically light blue to almost all white). I would like to use PS to replace the bald sky with another digital photograph of a deep blue sky containing an assortment of clouds. My preferred technique is to carefully make a selection of the bald sky, delete it and then place the new sky image as a layer underneath the original layer of the trees.

Here’s the problem: In any technique I’ve tried that uses a precise selection, I always seem to wind up with a small halo of gray pixels around the edges of the leaves and branches where the treetops meet the new sky. All my selections seem to be quite accurate yet the composite always has a small amount of gray pixels showing. I’ve tried the Matting>Defringe command but it doesn’t seem to help too much. (Just for the record, I have also used other techniques such as using a layer mask to bring in the new sky but I’ve found this to be too time consuming and the results are dependent on my ability to do a good job on "painting" away the bald sky).

I need something which gives me a result which looks natural and not obviously composited. Are there any tricks or alternate techniques which could produce the results I’m looking for? Any suggestions for helping me with this problem would be greatly appreciated. Thanks.

Well, I can help a bit if you don’t need the clouds 🙂

Normally sky is graduated from a dark blue at the top to a lighter blue at the horizon. You can approximate this using a gradient set to darken mode. The foreground and background are sampled from the image and darken mode works by checking the pixel densities in each colour channel replacing those lighter than the blend colour. (Which means the trees, being darker than the fill colours, are almost untouched.

So, what you do is this:

Load the blue channel as a selection. Click the fill layer button (bottom of the layers palette) and chose gradient, foreground to background. Change the fill layer blending mode to darken. Then play with the gradient fill dialog. Make the angle 90 degrees to give you a top to bottom fill. Use a linear gradient. That should give you the dark to light blue sky and shouldn’t give you any colour fringing on the trees.



Hecate

veni, vidi, relinqui

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