[Dave]

The following is an attempt to explain how to use Alpha channels for creating masks (I only sussed this afternoon, so please correct me if wrong). The example below is to show how this can be achieved & not a final image (which would require more work). This method is most suitable when you have a high contrast between the areas you want to mask.

In this example a photo I took of the house with a dull sky. This provides a good contrast between sky & house / trees, that I will use to add a more pleasing sky as follows



STEP 1 - Find greatest contrast & increase further
Via the "channels" tab select the colour which creates greatest contrast. In this case "blue" was best. Right click on "blue channel" & select "duplicate channel".
Select "Image" -> "Apply Image" with setting of "multiply" as follows.




This is to further increase contrast as follows:-



STEP 2 - Create Mask
The aim here is to create a simple black & white template. Using the "brush" in "overlay" mode first bleach out white area outside mask (eg. sky). Note you choose "white" as foreground colour & as it is in overlay mode, only the light pixels will be whitened. Now select "black" as foreground & blacken out template required (eg. House & trees)



continue to post 2 (due to limit on images in 1 post)

[Dave]

STEP 3 - Apply new Backround
On the layers tab duplicate the "background image" (right click as you did with channels above). With the sky & clouds image open on screen (tile images for ease) & already set to correct size, use the move tool to drag sky image onto house image, automtically creating a new layer



Right click on the new "sky" layer & select "blending options" . Set "blending mode" to "multiply" as follows :-



STEP 4 - Apply Mask

We now need to select the mask from the channels tab. Holding down "Ctrl" & "Alt" click on the "blue copy" channel. You will see the outline of the mask appear as follows :-



To apply the mask, select the "layers tab" & then click on the "apply mask" at the bottom as shown here :-



You now have your new background. Some final touches are whilst the mask icon is still active in the layer screen, apply Gausian Blur of 2 pixels for the final touch. Also play with levels to get the colours nearer to the original shot.

Trust me, after trying above a couple of times I managed to achieve same with 5 other shots of houses at about 90 secs a shot

There are lots of other touches to disguise the merging of 2 photo's, but I've yet to teach myself those yet

[nicklovick]

This method workes well providing you have hard edges to work with. I've tryed it with photos with soft edges and I always end up with 'halos' round these areas.

When this happens I have found that by using the burn tool you can remove these edges.