I had cut the background out of an image, with the person in the foreground, and wanted a background that mimicked the SLR view of a low f-stop, where the foreground is sharp but background is blurred.
To do this in Gimp is interesting. I found an image with a very sharp background.
Here are the steps:
- duplicate the image with a sharp background
- copy the image into the clipboard
- with copy 2, blur this image with a Gaussian setting of 5px, then turn this copy off. Put this layer behind the second copy.
- with the other copy, make a layer mask, switch to it and paste the clipboard (sharp background image) into the layer mask
- make the foreground colour white
- use the blend tool, set to foreground to transparent, draw from the most sharp area to the least sharp area
The mask layer will be sharp, but only from the foreground and will fade to transparent (unsharp). The second background image will be blurred to 5px. This will have the effect of a low f-stop depth of field, the foreground being sharp but blending to a blurry background.
I got some hints from this video, but he had the model in his photo already, so the technique is a bit different.