Replacing a background in photoshop

[lostmind]

This may be an easy task for you photoshop experts and I'm really not anywhere close to that yet so maybe you can tell me the best way to proceed. I've got an image of a building that has numerous trees in the foreground going up into the sky. The artist wants me to add blue to the sky. Problems is, my CMYK build in the sky is 0%. I'd like to be able to make the 0% sky background transparent and drop in a blue sky from another image behind it. If it wasn't for the leaves and openings between the branches it might not be a problem..any suggestions? Thanks!

[cari]

just an idea:
Make background into a layer...

Creat a rough lasso around your sky areas
Use Select > Colour Range command to select sky within this selection

Then jump to quick mask mode and get rid of other bits and piece that you didn't want as part of your selection

Now Alt/Option click the Layer Mask icon in your layers palette

Then place your other image on layer below

Further editing can be done using the layermask, adding of adjustment layers that take masked area incosideration etc.

[T-Rex]

Just as cari says, it should be fairly easy, considering the sky is 0%. That would make it very easy to select from any other element in the picture, provided there are no other areas that are totally blown out to 0%.

And color range would be the best place to start, or go to Calculations and do a simple combination of two of the channels with the most contrast between the sky and everything else. This usually makes great masks to work from.

[Joe]

If you can select the 0% sky and mask it off you can create your own sky. Once you get the mask made set the forground color to something like 50% Cyan & 10% Magenta. Then set the background color to 10% Cyan and 3% Magenta. Select the "Filter" menu, "Render", "Clouds". There you have a blue sky with wispy white clouds. You can experiment with the colors to adjust the sky to your liking. Also try reversing the foreground & background colors and running the "Clouds" more than once. Blue sky on a budget...