perform a masked blit
Extending 006.simple_blit, we now add a mask to the blit.
Using the "cookie-cut" blitter logic funtion we set up the blitter to use all four DMA channels.
To draw the car, we might use the A DMA channel to fetch the car mask, the B DMA channel to fetch the actual car data, the C DMA channel to fetch the background, and the D DMA channel to write out the new image.
A = mask
B = blitter object (sprite)
C = background
D = destination (background)
Now we calculate the logic function minterm:
blitter logic function minterm truth table
We fill in the desired logic values in the D column. In this case D is set if:
- The mask bit is set and the bob bit is set
- The mask bit is not set and the background bit is set
| A(mask) | B(bob) | C(bg) | D(dest) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Then we read D column from bottom up to give us the logic function minterm:
11001010 = $ca
This is then ready to be used in the LF1-7 bits of the BLTCON0 register setup.
imagecon generates the interleaved mask bitplanes for the A channel.
We then blit the sprite to the background and notice it now has a shape!
screenshot
