Replace Sky Help

[ilya]

Can anyone give me some suggestions on how to quickly replace a sky; the operative word is 'quickly'.

I magic-tooled selected the sky I want replaced, and made it into a new layer. I then copied the sky that I want as another layer into the file. So now I have 3 layers - the background, the old sky, and the new sky.

Here's the main question - working with the old sky and the new sky layers - is there a way to overlay the new sky onto the old sky layer, but have it be take on the outline of the old sky exactly? In otherwords, I don't want to have to outline and exclude by hand the various objects protruding into the new sky, I need PS to do this automatically if it can.

Sorry if its confusing, but if anyone understands what I mean, please help.

TIA
Ilya

[john_houghton]

To replace the sky in an image you want to end up with the image on the top layer with the sky area transparent, and the substitute sky underneath on the background layer. In Photoshop, you should use a layer mask on the top image to achieve this. Regions of the mask that are black render the top image transparent. Regions that are white leave the image visble. Regions that are grey give partial transparency. This is how to do it, starting with your original image file opened:

1. Create a duplicate background layer (Layer->Duplicate Layer)
2. Make the top layer the active one.
3. Click the channels tab on the layers palette.
4. Clck and drag the blue channel down to the 3rd
icon from the left at the bottom of the palette (the square with the corner folded over). This creates an alpha channel "Blue copy".
6. Now use levels and paintbrush, selection tools, paintcan and eraser etc to make the sky area in the mask pure white and the rest solid black. The blue channel is a very good starting point for this since the sky is blue and comes out white-ish already, with leaves on trees nicely picked out in black. (Possibly finish with Image->Adjust->Threshold to ensure true blacks and whites but choose the threshold value with care while zoomed in on leaves in any trees on the skyline).
7. Click and drag the alpha channel down to the leftmost icon at the bottom of the layers palette (the dotted circle)
8. This will create a selection which can be applied to the rgb image. You might want to feather the edges very slightly (Select->Feather)
9. Click on the rgb channel. (The selection now behaves like any other selection you might make, e.g. with the marquee tool.)
10. Click on the Layers tab to display the layers palette.
11. With the top layer active, click on Layer->Add Layer Mask->Hide Selection. If you now switch off the background layer (by clicking on its eye) you
should verify that the sky is now transparent. Put your new sky in the bottom layer and there you are. (You should align the horizons in the two images to make the cloud perspectives look right).

You can readily make minor adjustments to the mask to tidy it up at the joins. Just click on the mask thumbnail in the layer on the layers palette (the right thumbnail of the pair). In the second column, the paintbrush
symbol will change to a circle). Now you can paint with a black or white brush onto the image to alter its transparency. (If you make a mistake, just switch the colour of the brush and paint over). Its just like using
the eraser except that you can bring back the erased bits at will. Pressing backslash will toggle the display of a red overlay to show the position of the mask. You can switch back to normal image editing mode by clicking on the image thumbnail to the left of the mask thumbnail.

John

[ssim]

I haven't jumped through quite so many hoops and I probably should but I'm still learning Photoshop.

I open both photos, the one I want the sky replaced in and the one that I am going to use the source sky from.

Select the area that you want to replace using the magic wand. Open the source file and open your layers tab. Drag the background onto your target photo and then click on vector mask at the bottom of your layers drop down.

Following is an example that took me less than five minutes to do. It could be cleaned up a little but generally is very easy to do.

Original photo where the sky is to be replaced.
http://www.pbase.com/image/23103582.jpg

The photo that I am using the sky out of:
http://www.pbase.com/image/23103656.jpg

The end result:
http://www.pbase.com/image/23103581.jpg

[john_houghton]

ssim wrote: Drag the background onto your target photo and then click on vector mask at the bottom of your layers drop down.

A vector mask will not always be the best way of doing this job because it gives a hard edge to the join. This can lead to an unfortunate cardboard cutout effect when the original skyline is not sharply delineated as in your example.

John

[ilya]

THANK you both for the advice. Gives me the quick and the robust ways to get there. I actually got to an even quicker and dirtier method -
- Select the entire "replacement" sky and copy into clipboard
- Magic-Wand the old sky
- Use the Paste-Into command to paste the new sky into the original. Use Opacity to tweak.
I learned also that as John says that anything more elaborate gives a cutout effect. For relatively simple lines, this works great though.
Thanks again!!
Ilya