Computer Graphics with Photoshop - Selections and Gray Pixels

Author

Selections and Gray Pixels

JSB

2003-11-19, 11:00 pm

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.

Warren Sarle

2003-11-19, 11:00 pm

"JSB" <jblye@nassau.cv.net> wrote in message
news:9felrvo9m70e0v8tvr0di9q6hpdetp38f1@4ax.com...
> 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).

A layer mask does not require any painting. Make your selection
as usual and use Layer>Add Layer Mask to convert the selection
to a layer mask. Use Filter>Other>Maximum, Filter>Other>Maximum,
and Filter>Blur>Gaussian Blur to fine-tune the mask.

Peter Wollenberg

2003-11-19, 11:00 pm

JSB <jblye@nassau.cv.net> 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.

If the trees are darker than the replacement sky, try and set the mode
of the top layer to darken. If not, try to improve your mask by using
either the green or the blue channel to create the selection.
HTH, Peter

JSB

2003-11-19, 11:01 pm

This is great. With a little modification, this technique does the
job. I seemed to get better results using the Minimum filter and was
able to really get it right when I applied a Fade Minimum right after
the filter was applied. Thank you for your time and help.

On Wed, 19 Nov 2003 06:35:57 GMT, "Warren Sarle" <saswss@unx.sas.com>
wrote:

>"JSB" <jblye@nassau.cv.net> wrote in message
>news:9felrvo9m70e0v8tvr0di9q6hpdetp38f1@4ax.com...
>
>A layer mask does not require any painting. Make your selection
>as usual and use Layer>Add Layer Mask to convert the selection
>to a layer mask. Use Filter>Other>Maximum, Filter>Other>Maximum,
>and Filter>Blur>Gaussian Blur to fine-tune the mask.
>