How can I fix the edges of foreground objects pasted onto a new background?
How can I cut out a foreground object, e.g. a person, from one picture and superimpose it onto a new background without making the edges look bad?
The problem I've come across is that if I have for example a white background and a dark blue object, there are some pixels at the edge of the object that are various lighter blue shades. If it were a black background instead, those pixels should be darker shades of blue instead of lighter; how can I change them appropriately when I want to replace the background? It's not too hard to fix this when I just have two colours, but what about when the imaged is varied, like with a photo of a person outdoors? I've tried blurring the edges of the object into the background, which is not awful but the edges don't look crisp enough.
I use Photoshop CS5 if there's a specific PS way of doing this.
This is called defringing, and Photoshop has a Defringe tool right under Layer > Matting.
However, you might not even need that tool in most cases if you're using the Quick Select with Refine Edge in CS5, or the Extract filter in earlier versions.
Its important to select the area you wish to cut/cutout very accurately. No automated tools will clean cut for you without having a good selection to start with.
that said this is my method:
I will regularly zoom in few hundred percent and using magnetic lasso to trace around the object or person. with add-to or subtract-from selection you can get it pretty accurate. Use quick mask and a brush to fine tune details.
Once you have a good selection Photoshop has a huge number of tools for improving it even more; personally i really like the improved refine edge tool with 'decontaminate colours'. Its especially great for fiddly selections around thinks like hair or fabric. Its also worth contracting your selection by 1 or 2 pixels, and feather the edge a very little bit.