I have a large series of aligned images of an object in motion (camera and background stationary) I want to overlay into a single image to show the object in each frame without any alpha blending.
I could do this in Photoshop by importing each image as a separate layer, and then unmasking the object in each layer, as noted in How do I combine multiple exposures for action shots? But with dozens of images per composite I'm wondering if there is a more efficient method to do this.
Answer
There is a simple method to automate the process using Photoshop. It could be recorded as an action or scripted.
Load all of the images into a stack and take the median of each pixel (there is a built in function to do this, might just be in Photoshop extended though). This should give you an image of just the background.
Load up the first image and paste the background image over the top as a new layer. Set the blending mode to difference. Copy the result, paste it to a new layer, desaturate (under the image menu) it then apply a threshold (image menu again) of about 5-10. This should give you a black and white mask of just the object in motion. Create a new layer mask and past the black and white image in. Delete the other layers. You now have a cut-out of the object.
Create a new document and paste in the background. Paste in the cut-out. Repeat the above steps to cut out each image and paste them in turn.
Flatten the result, and you have your montage. It would take some work but if the number of image was constant you could create a Photoshop action to do all this.
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