I've recently been taking a lot videos of swimming cells, with the aim of calculating their velocity. In each video there is inevitably some background dirt or stuck cells, which can hinder the macro I use to automatically track cells. To deal with this problem I've written a very simple ImageJ macro (ImageJ is an excellent, free, open source image processing program) to remove static objects from an AVI or stack of images, which has proved surprisingly effective.
The macro works by creating a median image based on all the images in a stack and subtracting this from each image in the stack. The logic is that the background will be relatively constant in all the images in a stack, so the median intensity will be the background intensity. Moving objects will be darker or lighter than the general background, but since the median effectively ignores extreme values (in a way that the mean doesn't), then these objects don't show up in the average image.
Here is the macro which can be copied into a text file and put into your ImageJ macro folder. It takes an 8-bit grayscale AVI; if your AVI isn't 8-bit, you will probably want to remove the last line.
avi = getImageID; run("32-bit"); run("Z Project...", "projection=Median"); median = getImageID; imageCalculator("Subtract stack", avi, median); run("8-bit");
This is a short video of Chlamydomonas reinhardtii swimming.
Running the macro on this video creates this median image:
Subtracting this image from every frame of the original video, creates a new video, in which the cells can be seen more clearly (though there are a few artefacts in regions with slow-moving cells):