I have some glare effects on my image that looks fine on some places but ugly on others, in fact I don't even want it to appear on certain parts of the image. Is there a way to just mask out this effect using a black-white texture map for example, so that the image's lower parts get glare while the upper parts don't?
Ok, this should do the trick. Set the glare node's Mix value to 1. This will cause it to only output the glare, not the mixed image. Then, simply use an Add node to mix the glare onto the original image, using your black and white Mask imagein the Add node's Fac to Mask out where you want no glare. In the example below, black would be no glare and white would be full glare.