# Amiga Agnus Blitter

The Amiga's graphics multitool. Lets you do:

* copies from up to three areas of Chip RAM with logical operations
* line drawing
* horizontal fills

## About Horizontal Fills

The fill operation has to be done as part of a copy. If you want to draw a polygon and then fill it, the order of operations is:

* Draw all the lines
* Blitter copy the area with all the lines to a scratch area, enabling fill
* Blitter copy the scratch area elsewhere

## Gotchas

* Before you do blitter stuff, swap out your own dummy copper list, otherwise an existing copper list can blow away what you're trying to do.
  * http://eab.abime.net/showthread.php?t=99892
  * Symptom: A large blitter copy works up to a certain scanline height, and trying to do any blitter stuff with a larger size fails.
* Wait for one blitter operation to finish before starting another. You have these options:
  * Use `graphics/WaitBlit`
  * Wait for bit 14 (`DMAB_BLTDONE`) to become 0. The Amiga Hardware Manual gives the first half of this code. Here's your own basic blitter wair.

```asm
_MyWaitBlit:
  LEA _custom,A0

  BTST.B #DMAB_BLTDONE-8,dmaconr(A0)
  BTST.B #DMAB_BLTDONE-8,dmaconr(A0)
  BEQ .done

.loop:
  BTST.B #DMAB_BLTDONE-8,dmaconr(A0)
  BNE .loop

.done:
  RTS
```

* Blit wait (cont'd)
  * Use Interrupts. I haven't done this yet.

## Links

* https://www.markwrobel.dk/post/amiga-machine-code-letter6-blitter2/
* https://www.markwrobel.dk/post/amiga-machine-code-letter6-blitter3/
* https://www.markwrobel.dk/post/amiga-machine-code-letter7/
* http://coppershade.org/articles/AMIGA/Agnus/Programming_the_Blitter/

## How-Tos

### Set up BLTSIZE programmatically

```asm
  MOVEQ #0,D0
  MOVEQ #0,D1
  MOVE.W #320,D0 ; width
  ASR.W #4,D0 ; in words
  MOVE.W #256,D1 ; height
  ASL.W #6,D1 ; shifted 6 bits
  ADD.W D1,D0
  MOVE.W D0,BLTSIZE ; start blit
```

### Draw a line across multiple bitplanes

This draws heavily from:

* https://www.markwrobel.dk/post/amiga-machine-code-letter12-linedraw2/
* http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0128.html

```c
struct BlitDrawLineSetup {
  int dx, dy;
  int sud, sul, aul, accumulator, sign;
  int bltapt;
  int bltamod;
  int bltbmod;
  unsigned char *target;
};

void BlitDrawLine(
  struct ScreenSetup *screenSetup,
  struct BlitDrawLineSetup *setup,
  uint16_t sx,
  uint16_t sy,
  uint16_t ex,
  uint16_t ey,
  uint8_t color
) {
  unsigned char *target = screenSetup->memoryStart + (((screenSetup->width * sy + sx) >> 3) & 0xfffe);
  int i;

  setup->dx = ex - sx;
  setup->dy = ey - sy;

  if (setup->dx == 0 && setup->dy == 0) return;

  setup->sud = abs(setup->dy) < abs(setup->dx);
  if (!setup->sud) { i = setup->dx; setup->dx = setup->dy; setup->dy = i; }

  setup->sul = setup->dy < 0;
  setup->dy = abs(setup->dy);

  setup->aul = setup->dx < 0;
  setup->dx = abs(setup->dx);

  setup->accumulator = 4 * setup->dy - 2 * setup->dx;
  setup->sign = setup->accumulator < 0;

  for (i = 0; i < screenSetup->bitplanes; ++i) {
    custom.bltadat = 0x8000;
    custom.bltbdat = color & 1 == 1 ? 0xffff : 0;

    custom.bltafwm = 0xffff;
    custom.bltalwm = 0xffff;

    custom.bltamod = 4 * (setup->dy - setup->dx);
    custom.bltbmod = 4 * setup->dy;
    custom.bltcmod = screenSetup->width / 8;
    custom.bltdmod = screenSetup->width / 8;

    custom.bltapt = setup->accumulator;
    custom.bltcpt = target + screenSetup->nextBitplaneAdvance * i;
    custom.bltdpt = target + screenSetup->nextBitplaneAdvance * i;

    custom.bltcon0 = ((sx % 16) << 12) + (11 << 8) + 0xca;
    custom.bltcon1 = 1 + (setup->aul << 2) + (setup->sul << 3) + (setup->sud << 4) + (setup->sign << 6);

    custom.bltsize = 0x02 + ((setup->dx + 1) << 6);
    WaitBlit();

    color >>= 1;
  }
}
```