Test Two Small Opaque Pixel Masks on NES | Pattern Library

Two sprite boxes can overlap while their visible shapes still miss each other.

NES shows the working diagnostic row 00 FF 01 00 80 00 00 00. — A completed native caller displays 00 FF 01 00 80 00 00 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

Use it in a project

P0–P7 and Q0–Q7 are eight MSB-left mask rows. Place Q SHIFT_X pixels right and SHIFT_Y pixels down relative to P. Return HIT=1 and the first overlapping P row when any opaque bits coincide; otherwise HIT=0 and ROW=255. A shift of eight or more on either axis is a valid empty overlap. Inputs are preserved.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: HIT, ROW, SHIFT_X, SHIFT_Y, P0, P1, P2, P3, P4, P5, P6, P7, Q0, Q1, Q2, Q3, Q4, Q5, Q6, Q7. They occupy 20 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5 (6 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call pixel_mask_collision_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 pixel_mask_collision_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.

Ownership	Fields
Retain between calls	none
Supply before service	SHIFT_X, SHIFT_Y and both mask arrays
Read after service	HIT and ROW

Shift each Q row into P’s horizontal alignment, then AND corresponding rows. Any surviving bit is a shared opaque pixel. Transparent corners of overlapping boxes no longer count as contact.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Test Two Small Opaque Pixel Masks. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
pixel_mask_collision_init:
    lda #0
    sta HIT
    sta ROW
    sta SHIFT_X
    sta SHIFT_Y
    sta P0
    sta P1
    sta P2
    sta P3
    sta P4
    sta P5
    sta P6
    sta P7
    sta Q0
    sta Q1
    sta Q2
    sta Q3
    sta Q4
    sta Q5
    sta Q6
    sta Q7
    sta T0
    sta T1
    sta T2
    sta T3
    sta T4
    sta T5
    rts
pixel_mask_collision_step:
; HIT = 0
    lda #0
    sta HIT
; ROW = 255
    lda #255
    sta ROW
; if SHIFT_X >= 8:
    lda SHIFT_X
    cmp #8
    bcs pixel_mask_collision_L4
pixel_mask_collision_L5:
    jmp pixel_mask_collision_L2
pixel_mask_collision_L4:
    jmp pixel_mask_collision_L1
pixel_mask_collision_L1:
; return
    rts
    jmp pixel_mask_collision_L3
pixel_mask_collision_L2:
pixel_mask_collision_L3:
; if SHIFT_Y >= 8:
    lda SHIFT_Y
    cmp #8
    bcs pixel_mask_collision_L9
pixel_mask_collision_L10:
    jmp pixel_mask_collision_L7
pixel_mask_collision_L9:
    jmp pixel_mask_collision_L6
pixel_mask_collision_L6:
; return
    rts
    jmp pixel_mask_collision_L8
pixel_mask_collision_L7:
pixel_mask_collision_L8:
; T0 = 0
    lda #0
    sta T0
; T5 = 8 - SHIFT_Y
    lda #8
    sec
    sbc SHIFT_Y
    sta T5
; while T0 < T5:
pixel_mask_collision_L11:
    lda T0
    cmp T5
    bcc pixel_mask_collision_L14
pixel_mask_collision_L15:
    jmp pixel_mask_collision_L13
pixel_mask_collision_L14:
    jmp pixel_mask_collision_L12
pixel_mask_collision_L12:
; T1 = Q[T0]
    ldx T0
    lda Q0,x
    sta T1
; T2 = SHIFT_X
    lda SHIFT_X
    sta T2
; while T2 != 0:
pixel_mask_collision_L16:
    lda T2
    cmp #0
    bne pixel_mask_collision_L19
pixel_mask_collision_L20:
    jmp pixel_mask_collision_L18
pixel_mask_collision_L19:
    jmp pixel_mask_collision_L17
pixel_mask_collision_L17:
; T1 = T1 >> 1
    lda T1
    lsr
    sta T1
; T2 = T2 - 1
    lda T2
    sec
    sbc #1
    sta T2
    jmp pixel_mask_collision_L16
pixel_mask_collision_L18:
; T3 = T0 + SHIFT_Y
    lda T0
    clc
    adc SHIFT_Y
    sta T3
; T4 = P[T3]
    ldx T3
    lda P0,x
    sta T4
; T4 = T4 & T1
    lda T4
    and T1
    sta T4
; if T4 != 0:
    lda T4
    cmp #0
    bne pixel_mask_collision_L24
pixel_mask_collision_L25:
    jmp pixel_mask_collision_L22
pixel_mask_collision_L24:
    jmp pixel_mask_collision_L21
pixel_mask_collision_L21:
; HIT = 1
    lda #1
    sta HIT
; ROW = T3
    lda T3
    sta ROW
; return
    rts
    jmp pixel_mask_collision_L23
pixel_mask_collision_L22:
pixel_mask_collision_L23:
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp pixel_mask_collision_L11
pixel_mask_collision_L13:
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. A direct JSR needs two 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.

Bindings and include order

demo.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HIT=$80
ROW=$81
SHIFT_X=$82
SHIFT_Y=$83
P0=$84
P1=$85
P2=$86
P3=$87
P4=$88
P5=$89
P6=$8a
P7=$8b
Q0=$8c
Q1=$8d
Q2=$8e
Q3=$8f
Q4=$90
Q5=$91
Q6=$92
Q7=$93
T0=$94
T1=$95
T2=$96
T3=$97
T4=$98
T5=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
.if CUSTOM
; SHIFT_X = 0
    lda #0
    sta SHIFT_X
; P0 = 128
    lda #128
    sta P0
; Q0 = 128
    lda #128
    sta Q0
.if BROKEN
; SHIFT_X = 0
    lda #0
    sta SHIFT_X
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; SHIFT_X = 1
    lda #1
    sta SHIFT_X
; P0 = 128
    lda #128
    sta P0
; Q0 = 128
    lda #128
    sta Q0
.if BROKEN
; SHIFT_X = 0
    lda #0
    sta SHIFT_X
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Align the two opaque pixels to produce an actual hit. 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

custom.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HIT=$80
ROW=$81
SHIFT_X=$82
SHIFT_Y=$83
P0=$84
P1=$85
P2=$86
P3=$87
P4=$88
P5=$89
P6=$8a
P7=$8b
Q0=$8c
Q1=$8d
Q2=$8e
Q3=$8f
Q4=$90
Q5=$91
Q6=$92
Q7=$93
T0=$94
T1=$95
T2=$96
T3=$97
T4=$98
T5=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

The custom diagnostic row is 01 00 00 00 80 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller ignores horizontal displacement and aligns the masks. It reports contact between opaque bits that are actually apart. Convert relative coordinates before the narrow-phase mask query.

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

wrong.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
HIT=$80
ROW=$81
SHIFT_X=$82
SHIFT_Y=$83
P0=$84
P1=$85
P2=$86
P3=$87
P4=$88
P5=$89
P6=$8a
P7=$8b
Q0=$8c
Q1=$8d
Q2=$8e
Q3=$8f
Q4=$90
Q5=$91
Q6=$92
Q7=$93
T0=$94
T1=$95
T2=$96
T3=$97
T4=$98
T5=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

NES shows the deliberate error row 01 00 00 00 80 00 00 00. — The wrong client completes with 01 00 00 00 80 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 209 code bytes; public state is 20 bytes and private scratch is 6 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.

At most eight row visits and seven shifts per row. Run an AABB broad phase first. Pre-shifted masks use more memory to reduce shifts; a box or circle may already provide the intended game feel.

This bounded helper covers Q right/down of P. Negative relative placement needs a deliberate swapped/normalised query, including conversion of the returned row to the desired space. It tests collision masks, not display colours or hardware collision latches.

Execution evidence

Four variants run on NTSC NROM, background only, DMC/NMI disabled, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 50 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: Use the cheap box test to avoid mask work for separated objects, then pay for precision only where it changes the game.

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