Nan,
You can overlay text on an image using the "Draw Text at Point.vi" which is pretty straight forward to use. However, if you want to write to an avi file, this is where things get a bit tricky without vision. The Vision VIs for writing AVIs require a different image type (the IMAQ image data type) in order to be able to do this, you would have to convert the data into that type, which is NOT straight forward in LabVIEW. If you really want to write to AVI files, I'd highly encourage that you use the Vision Development module, which not only makes it possible for you to create AVI files, but manipulate, analyze, and process many types of images as well. Feel free to give us a call if oyu have any questions!
Dan
National Instruments
