mirror of
https://frontier.innolan.net/github/AmigaExamples.git
synced 2026-09-12 21:52:00 +00:00
59 lines
1.8 KiB
Markdown
59 lines
1.8 KiB
Markdown
perform a masked blit
|
|
=====================
|
|
|
|
Extending [006.simple_blit](../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.
|
|
|
|
[From the hardware manual](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node011D.html):
|
|
> 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](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node011C.html):
|
|
|
|
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](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node001A.html) register setup.
|
|
|
|
[imagecon](../tools/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
|
|
----------
|
|
|
|
|
|
try it
|
|
------
|
|
* [Download disk image](bin/mask_blit.adf?raw=true)
|
|
* <a href="http://alpine9000.github.io/ScriptedAmigaEmulator/#amiga_examples/mask_blit.adf" target="_blank">Run in SAE</a>
|
|
|