Chapter 10 - The Overlay Plane
In this chapter, I'll start using the Overlay Plane I described in Chapter 8 to display the Score and Energy Status Bars.
I'm not aware of any 8-bit computer that had a graphics system with a hardware dual playfield
capability. The only one I am aware of is the Commodore Amiga which is
a 7.159 Mhz (NTSC) clock speed, 16 bit system with custom graphics
On the Amiga, you could create two (or more?) bit mapped displays
overlaying each other and both independently addressable. This was used
extensively in games, not only for overlaying Score and Game Status
information but also in parallax graphic effects.
8 bit systems such as the CoCo, such feats would have to be emulated in
software. This is what my graphics engine in GunStar does. It has some
CPU overhead to move large blocks of RAM around but the overall
operation is simple and the code size, using the 6309's TFM instruction
is quite small.
This dual playfield capability is magic-in-hardware on the Commodore Amiga but on the CoCo is only a software illusion.
But done right, to the untrained eye, it too looks like magic...
albeit, not as fast as the Amiga's hardware counterpart but pretty good
for an 8-bit system at a fraction of the clock speed.
GunStar, I have created the background plane which shows a window to a
slightly wider area. This is made to scroll down the screen by adding a
new line of graphics at the top of the page, to appear as if flying
over the background terrain.
As a simulated overlay plane, I
can place additional graphics in a fixed location that is unhindered by
the scrolling background behind. The diagram below hopefully clarifies
The image on the left shows the background plane with the ground scenery and the overlay plane with the Score and Energy Bars on a simulated separate plane. These planes are merged together to form the final composited screen as shown on the image at the right.
Plane and Overlay
Composited Plane to be displayed
don't need to concern myself with what is overwritten on the Background
Plane by the Overlay Plane. Nor do I need to concern myself with
erasing of the previous graphics on either plane, the graphics engine
does that. I just have to write the graphics data in whatever method I
choose as if it's a static display.
due to the bit mapping of 2 pixels per byte, if one of those pixels in
the byte I am writing on the Overlay Plane is actually black, it will
override the equivalent pixel on the Background Plane below unless I do
a bit-merge first. For speed, I can either ignore this black pixel
overwrite or design the graphics to fill a complete byte, avoiding the
Short but sweet
is the part of the chapter where I was to have a link to a video
demonstrating this overlay plane but things have been going so smoothly
and quickly that I have gone ahead and begun programming the actual
GunStar ship into the game. It is already working well but I will
postpone the video for the next chapter to include this. The next
chapter should be up in a week so it will only be a short wait to
actually see what I've been ranting about.