Trapezoidal Window: Sometimes things are not as they appear.
Trapezoidal Window animation Small Trapezoidal Window QTVR (160 x 120 - 1.4Mb)

Large Trapezoidal Window QTVR (320 x 240 - 5.5Mb)

Small Trapezoidal Window MPEG (160 x 120 - 413kb)

To Do and Notice

Download one of the two QuickTime trapezoidal window movies above. Note that you will need a QuickTimeVR player or the QuickTime plugin to view it. You can download QuickTime here . If you are running a system that does not support QuickTime VR try the MPEG version. If you are bandwidth-challenged, just watch the animated GIF above.

If you click and drag near the center of the movie you can rotate the window to any position you like. Dragging horizontally rotates the window. Dragging the window vertically lets you see the window from 45 degrees up or from directly overhead. If you click and hold near the edge of the frame, the window will rotate continuously. Dragging outward will cause it to rotate faster.

- Watch the rotating window. Notice that the window seems to swing back and forth.

- Grab the window and tilt it down until you can see it from above. Now notice that the window is actually going around in circles.

- Notice that one vertical edge of the window is shorter than the other. Most windows are rectangles; this window is a trapezoid.


Rectangle and Trapezoid

When you look at a rectangular window that is angled away from you, the farther edge of the window appears smaller, and so the whole window appears to be trapezoidal.

From past experience, your brain assumes that all windows are rectangular and that the shorter edge is always the edge farthest away. But the rotating window in front of you really is a trapezoid. When the shorter edge of this trapezoidal window moves closer to you, your brain refuses to see it as being closer; it assumes that the window is not rotating in the opposite direction. The window therefore appears to swing back and forth.