I posted this on ILP but thought some of you might find it helpful. I've learned so much, pretty much everything, from these boards that it makes me happy if this helps even just one person. Stephanie (stampasaurus) is just the coolest person ever - she drove out to my side of town, took all these photos, and gave me all the raw files to play with.
There were 25 of us with five under the age of one. I had 35 shots to work with, all taken with the camera on a tripod using the same settings, and some of the kids only looked at the camera just once. I swapped out 11 all together, including some bodies and just faces on others. This is my first time attempting head swapping and it turned out pretty well.
This is my starting picture. Be sure to make all of your global adjustments in RAW to all of your photos so they look the same. For this one, I just adjusted WB and exposure so that all 35 photos had the same values. Its not a bad photo but my goal was to get one final product with everyone looking at the camera with a decent look on their face.
I'll show you how I fixed the baby below.
This is the picture I want to copy from because sweet little chunka chunk is looking right at the camera. I realized that because her body was turned in the original, I'd have to do a full on body swap too. I had a hard time isolating baby so I swapped out mom's face as well because I liked this expression better. The one thing I had to be careful about was how close dad was in this photo compared to the original above.
Prior to starting I make sure both images are being seen at the same magnification. I think I was zoomed in at 50% for these. I went to the image I wanted to copy from and loosely selected mom and baby with the lasso tool and had it feathered at 5px. You can play with the feather value. It probably doesn't even really matter because you'll most likely end up brushing the edges away with a soft brush, but in case you don't, you have a nice feathered edge.
With my marching ants around mom and baby, I hit CTRL C to copy, go over to my main photo, hit CTRL V to paste. As long as the camera didn't move and both photos are at the same magnification, you shouldn't have to transform your new image. If it does need to be resized, use CTRL T to free transform to the size that you want and make sure you hold down the shift key while resizing to keep the same aspect ratio of the original.
Pasting this image creates a new layer. I made my other layers not visible in my layer palette below so you could just see this particular head swap. I highlight this new layer, keep the blending mode to normal, and reduce the opacity to 50%. Then I use my move tool to allign the new image over the old one. I try to allign the eyes but sometimes its better to allign other features. You can use the arrows to nudge for more precise movements.
This is what it looks like when they're alligned. You can see mom's face is alligned pretty well but I have to worry about masking out things like dad's second face and multiple hands.
With my layer still selected, I create a layer mask by clicking the little icon at the bottom of my layers palette that looks like a rectangle with a circle in it. I find it easier to bring my opacity back up to 100% to mask away the stuff I don't want. Use your brush tool with black to mask away any part of the new layer that you don't want to see and use the white color to mask in what you do want to see. Every image is different so the size and softness of the brush will vary. I circled some areas below where I had to decide which fit better - the original or the new layer.
As I was creating the images for this post I zoomed in to 200%. I'm glad I did because my original head swapping wasn't as nice as I thought it was. You can see on her arm where I didn't make it nice and clean.
I kept it zoomed in to 200% and used a small brush to clean it up. I gave her the skinnier version of the arm (not that she needed any help looking good). Then the skin didn't match up so well - there was an obvious vertical line between darker and lighter skin - so I used my soft brush at 20% opacity and just went over that line a couple times to blend it. This is what it looked like.
This is what my final image ended up looking like. You can see that I have a different layer with a mask for every person's face I fixed.
Head swapping is a lot easier than I thought it would be. I'm sure there are plenty of other methods out there that work but this is what I did and was very pleased with the result. If anyone has any corrections or pointers, please feel free to add.
I plan on printing this out as an 8x12 so I can look for other flaws before I commit to having 8 16x24 canvases printed.
Here's the final photo after I warmed it up, fixed the tilt, and slightly lightened it: