Yes signal processing is not necessarily image processing but the most common uses on here are likely to be audio and image/video.
The computer vision group doesn't seem to have got much traction - that doesn't necessarily mean there is no demand for it, it may be that people simply haven't noticed or are just asking questions on SO.
Since the SO format is designed to handle large numbers of relatively obscure questions, with tagging etc, is it necessary to have such narrowly defined SE sites?
