Filter Collision Pairs by Layer on Amiga

A projectile can cross its owner without hitting them, while still hitting an opponent in the same patch of screen.

Use it in a project

GROUP_A/B are eight-bit membership sets; MASK_A/B say which groups each object accepts. Both directions must accept the pair. Return PAIR_KIND=0 when either rejects it, 1 for an accepted sensor pair, or 2 when both accepted objects have nonzero SOLID flags. Preserve all inputs. This classifies policy; a separate spatial test must establish actual contact.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: PAIR_KIND, GROUP_A, MASK_A, GROUP_B, MASK_B, SOLID_A, SOLID_B. They occupy 7 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call collision_layer_filter_init once at the intended reset boundary. It clears inputs as well as outputs and workspace. Set your starting state and configuration afterwards, then call collision_layer_filter_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.

Intersect each object’s membership with the other’s acceptance mask. Requiring both intersections avoids one-sided friendly-fire rules. Solidity is considered only after acceptance.

Reusable initialisation and service

; SPDX-License-Identifier: MIT
; Filter Collision Pairs by Layer. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
collision_layer_filter_init:
    move.b #0,d0
    move.b d0,PAIR_KIND
    move.b d0,GROUP_A
    move.b d0,MASK_A
    move.b d0,GROUP_B
    move.b d0,MASK_B
    move.b d0,SOLID_A
    move.b d0,SOLID_B
    move.b d0,T0
    rts
collision_layer_filter_step:
; PAIR_KIND = 0
    move.b #0,d0
    move.b d0,PAIR_KIND
; T0 = GROUP_A & MASK_B
    move.b GROUP_A,d0
    move.b MASK_B,d1
    and.b d1,d0
    move.b d0,T0
; if T0 == 0:
    move.b T0,d0
    cmp.b #0,d0
    beq collision_layer_filter_L1
    jmp collision_layer_filter_L2
collision_layer_filter_L1:
; return
    rts
    jmp collision_layer_filter_L3
collision_layer_filter_L2:
collision_layer_filter_L3:
; T0 = GROUP_B & MASK_A
    move.b GROUP_B,d0
    move.b MASK_A,d1
    and.b d1,d0
    move.b d0,T0
; if T0 == 0:
    move.b T0,d0
    cmp.b #0,d0
    beq collision_layer_filter_L4
    jmp collision_layer_filter_L5
collision_layer_filter_L4:
; return
    rts
    jmp collision_layer_filter_L6
collision_layer_filter_L5:
collision_layer_filter_L6:
; PAIR_KIND = 1
    move.b #1,d0
    move.b d0,PAIR_KIND
; if SOLID_A != 0:
    move.b SOLID_A,d0
    cmp.b #0,d0
    bne collision_layer_filter_L7
    jmp collision_layer_filter_L8
collision_layer_filter_L7:
; if SOLID_B != 0:
    move.b SOLID_B,d0
    cmp.b #0,d0
    bne collision_layer_filter_L10
    jmp collision_layer_filter_L11
collision_layer_filter_L10:
; PAIR_KIND = 2
    move.b #2,d0
    move.b d0,PAIR_KIND
    jmp collision_layer_filter_L12
collision_layer_filter_L11:
collision_layer_filter_L12:
    jmp collision_layer_filter_L9
collision_layer_filter_L8:
collision_layer_filter_L9:
    rts
component_end:

Machine contract

D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four return-address stack bytes. The helper makes no nested calls and adds no other stack use. Its named scratch is private workspace, not a pointer, a ROM variable or shared zero-page space that the game may borrow concurrently. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. Give each block one mutation owner.

The A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.

Adapt the caller

demo.asm supplies the bindings and include order. exercise.inc first runs table cases, then a separate client retaining state between calls. A game can call the public step entry directly and remove the diagnostic packet and display. Do not copy the diagnostic’s hardware takeover into an existing game loop.

Build with make ASM198X=/path/to/asm198x and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.

Bindings and include order

    section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
PAIR_KIND: dc.b 0
GROUP_A: dc.b 0
MASK_A: dc.b 0
GROUP_B: dc.b 0
MASK_B: dc.b 0
SOLID_A: dc.b 0
SOLID_B: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XNIGHTONE000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

A client retaining state between calls

preview_rules:
    ifne CUSTOM
; GROUP_A = 1
    move.b #1,d0
    move.b d0,GROUP_A
; MASK_A = 2
    move.b #2,d0
    move.b d0,MASK_A
; GROUP_B = 2
    move.b #2,d0
    move.b d0,GROUP_B
; MASK_B = 1
    move.b #1,d0
    move.b d0,MASK_B
; SOLID_A = 1
    move.b #1,d0
    move.b d0,SOLID_A
; SOLID_B = 0
    move.b #0,d0
    move.b d0,SOLID_B
    ifne BROKEN
; MASK_A = 255
    move.b #255,d0
    move.b d0,MASK_A
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; GROUP_A = 1
    move.b #1,d0
    move.b d0,GROUP_A
; MASK_A = 2
    move.b #2,d0
    move.b d0,MASK_A
; GROUP_B = 4
    move.b #4,d0
    move.b d0,GROUP_B
; MASK_B = 1
    move.b #1,d0
    move.b d0,MASK_B
; SOLID_A = 1
    move.b #1,d0
    move.b d0,SOLID_A
; SOLID_B = 1
    move.b #1,d0
    move.b d0,SOLID_B
    ifne BROKEN
; MASK_A = 255
    move.b #255,d0
    move.b d0,MASK_A
    endif
    bsr checked_step
    bsr emit_state
    endif
    rts

Customise it

Make an accepted pair a sensor by clearing one solidity flag. The maintained custom caller changes its inputs or client sequence rather than hiding a different implementation behind the same label. Compare every state of the two clients, not only their final display.

Custom client selection

    section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
PAIR_KIND: dc.b 0
GROUP_A: dc.b 0
MASK_A: dc.b 0
GROUP_B: dc.b 0
MASK_B: dc.b 0
SOLID_A: dc.b 0
SOLID_B: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XNIGHTONE000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

The custom diagnostic row is 01 01 02 02 01 01 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller accepts every group for A. A pair excluded by MASK_A=2 becomes a solid candidate. Preserve the intended mask; overlapping objects do not automatically have permission to interact.

The BROKEN branch lives in the sequential client, not the reusable helper. wrong.asm selects it; repaired.asm restores the normal call sequence. The repaired executable matches the normal program (for Spectrum, compare loaded CODE rather than the tape filename).

Select the deliberately wrong client

    section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
PAIR_KIND: dc.b 0
GROUP_A: dc.b 0
MASK_A: dc.b 0
GROUP_B: dc.b 0
MASK_B: dc.b 0
SOLID_A: dc.b 0
SOLID_B: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XNIGHTONE000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

Cost and alternatives

The measured helper extent is 186 code bytes; public state is 7 bytes and private scratch is 1 bytes. There are no immutable helper tables. Caller data, the test cases, return-address stack, alignment and display buffers cost additional space. These are assembled sizes, not cycle measurements.

Two set intersections and bounded branches. Filter inexpensive policy before expensive shape tests; for a large object set, still use a broad phase to reduce candidate pairs.

A zero group or mask disables the relevant acceptance. This is neither collision detection nor damage response, and the eight group bits are a game-owned convention rather than hardware sprite-collision bits.

Execution evidence

Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. Each executes 64 boundary/varied/error table cases, dirty initialisation and the separate retained-state client. An independent mathematical/container model checks every public byte after every call. This is bounded example coverage, not exhaustive correctness for every possible state.

The records retain loaded immutable code, completion, register sentinels, full packets, source/tool/firmware hashes and captures. The public gate validates these retained observations and rejects altered evidence; it does not run restricted firmware. No firmware is distributed. The eight displayed bytes are the first eight public fields, zero-padded when shorter; the packet includes every field.

Where it fits

Rectangle overlap: Policy filtering and a geometric overlap test answer separate questions.

The Craft · Maths for Games · Source and native evidence.