The DualPlayField Facility

DualPlayField is a hardware facility in the Amiga computer by which there is not one but two graphics areas, in which the front one is transparent in its 'background' colour (pen 0). This allows two independent graphics fields to be registered with each other, one over the other, as though one was a sheet of glass in front on the main 'background' picture on which someone has drawn something. Moreover, the two can be scrolled and moved independently of each other, allowing one to 'slide over' the other. This facility is built into the video display hardware, and requires no special software or programming.

Yet it matches several high level requirements, of which the last is perhaps the most important and most exciting.

View and control panel or cockpit, such as in a computer game of a flight simulator. This is probably how it has been mostly used.
Simple parallax motion, such as found in a number of games e.g. Worms. This is probably the second most common use for it, and makes use of the ability to move the two fields independently.
Picture being drawn or shown, with a grid or other framework behind it, such as showing gridlines in CAD or DTP. Also, today I saw something similar, with contour lines as background and sprinkled on top were a number of objects.
A simplified form of CAD layering. But, having only two layers
Picture being drawn or shown or manipulated, with a small collection of tools with which to manipulate it. This is the 'ToolGlass' (tm) or 'MagicLenses' (tm) idea that is gaining ground, yet which is proving a little difficult to implement on other platforms. DualPlayField was just made for this.
Information and its interpretation. This is perhaps the use with greatest potential. In many situations we have a visual field, a picture, say a photograph of Mars or a historical print, and we wish to highlight elements therein. That is, we wish to interpret the picture according to our own purposes, and imposing our own meaning. But, as many now argue, the interpretation should be kept separate from the thing being interpreted, owing to the dynamic and contingent nature of interpretations. Also because different people have or wish to make different interpretations. Therefore, the interpretation is fundamentally separate from the picture, and should not be merged with it or affect it in any way. Such interpretations include graphically delineating elements or areas of the picture, e.g. by drawing a rectangle or circle round them. Such graphics delineations should therefore be kept separate from the picture. However in most platforms, as they are drawn, they are drawn onto the picture, affecting it. DualPlayField avoids this, and keeps the graphical delineation (the interpretation) separate from the picture, right down to the hardware level. This is exciting: to find a hardware feature that closely matches an important notion of human interpretation. A crude example of it can be seen in the Annotator program.

Thus the DualPlayField facility of the Amiga is no mere technical curiosity, but a fundamentally important feature that matches well a significant number of application requirements.

It could do with updating, e.g. to more colours per field and also to allow more than two fields sometimes. But most of the above applications require exactly two fields.