As often discussed, the Motorola 6847 VDG offers little in terms of assistance for game programmers or other software authors. While many contemporary video chips offered assistance in scrolling backgrounds or in moving objects in the foreground of a display, the 6847 hardware merely provides access to a static framebuffer. As we have seen prviously, anything beyond a still image requires work from the CPU. Fortunately for CoCo people, the Motorola 6809 CPU is reasonably capable, and when used properly it can provide at least some of these video effects.
Get Things Moving
Of course the only way to start is to draw something in the framebuffer memory by using the CPU's usual load and store instructions to copy data out of program storage and into the framebuffer. To create the illusion of motion this must be done rapidly and repeatedly. If care is not taken between draws to remove the previous image then "movement" will create a trail across the screen, corrupting the background image. Various methods exist for doing that cleanup (e.g. saving and restoring the background as part of the sprite draw operation), but this takes both program development time and execution time. The latter in particular is at a premium when so much data is already being moved for the background.
the 6847's RG3 (4 color, 2 bit per pixel) graphics mode, each byte of
buffer memory represents 4 pixels. Since memory writes happen one byte
at a time, any image that uses less than 4 pixels in each byte it
occupies will interfere with those extra pixels unless care is taken. This is typically observable as the movable object appearing a bit like a postage stamp laid on top of the display. Unless the object being displayed is rectangular with a pixel width that is a multiple of 4 pixels, then part of the draw operation must include masking. This involves reading the background, using a bitwise mask of the object to combine the object's pixels with the background pixels then writing the resulting data. This operation performs the trimming necessary to preserve the background. There is some execution time lost for the masking, and storing the mask data doubles the amount of memory necessary for storing the graphics objects. But there really is no way around this part.
No Background Check
With the scrolling background demonstration as a base, we can now show a foreground item on top. Since both the foreground item and the background image move in relation to each other, this is not a single draw (or mask and draw) operation. With a normal static framebuffer, displaying a moving foreground itme would require some action to restore any background bits eclipsed by the foreground when the foreground moves. This might require regenerating the background algorithmically or saving the background before drawing the foreground and restoring the background after the foreground is moved. If we draw the foreground item in our source or "render" buffer, then all of the above would be true. But by drawing the foreground item directly in the current "display" buffer, we know that the background will be copied from the render buffer to the next display buffer at the next buffer switch. Since the background is already being redrawn for every frame, we don't need to maintain the background at all, saving all that time that would have been used for maintaining the background image!
So while redrawing the entire background initially seems expensive, that operation turns out to be cheaper than tracking damage to the background image and redrawing the background over and over again. It took me a while to wrap my head around this feature, but hopefully the explanation above makes things clear for you? I hope so.
Hopefully I can keep finding time to turn this discussion into an actual CoCo game. If you want to see where this all goes, then be sure to stay tuned!