Use the mouse to drag and explore the moiré interference effect

Initially there are two images overlaid, a circular and a square one. All the images are moveable, simply click and drag using the mouse. Try all the combinations you can, including on this text. For best effects, move the images very slowly when they are superimposed. For a closer look, enlarge the images using the button below.
What is going on here? The illusion is based on moiré fringing. The backgrounds are regularly spaced square dot patterns. When the patterns are superimposed the dots interact and new patterns are formed. The images are simple GIF files with white background set to transparent to allow the interaction to take place. The images were set using a screen display setting of 600x800. There is also another effect intruding - the screen display adds lines into the image display. If you are unlucky, then your monitor will exhibit Moiré effects without prompting!