An impact can shake the view without moving the player, the level or the collision geometry.
Use it in a project
TRIGGER reloads AMPLITUDE from STRENGTH 0–15 and REMAINING from DURATION. While a lifetime tick remains, map each caller-supplied noise byte into −AMPLITUDE..+AMPLITUDE, return two’s-complement OFFSET_X/Y, then decrement REMAINING. Expired shake returns zero offsets. Invalid STRENGTH clears only outputs/VALID and leaves retained lifetime/amplitude unchanged. Existing AMPLITUDE must also be 0–15 when not retriggering.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: OFFSET_X, OFFSET_Y, REMAINING, AMPLITUDE, VALID, TRIGGER, STRENGTH, DURATION, NOISE_X, NOISE_Y. They occupy 10 contiguous bytes. Also reserve the private workspace: T0, T1 (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call camera_shake_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 camera_shake_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 | REMAINING and AMPLITUDE |
| Supply before service | TRIGGER, STRENGTH, DURATION and NOISE_X/Y |
| Read after service | OFFSET_X/Y and VALID |
Separate the lifetime from the world camera. Reduce noise into a bounded span, centre it around zero and leave authoritative world state untouched.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Keep Camera Shake out of World State. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
camera_shake_init:
ld a,0
ld (OFFSET_X),a
ld (OFFSET_Y),a
ld (REMAINING),a
ld (AMPLITUDE),a
ld (VALID),a
ld (TRIGGER),a
ld (STRENGTH),a
ld (DURATION),a
ld (NOISE_X),a
ld (NOISE_Y),a
ld (T0),a
ld (T1),a
ret
camera_shake_step:
; OFFSET_X = 0
ld a,0
ld (OFFSET_X),a
; OFFSET_Y = 0
ld a,0
ld (OFFSET_Y),a
; VALID = 0
ld a,0
ld (VALID),a
; if STRENGTH > 15:
ld a,(STRENGTH)
cp 15
jp z,camera_shake_L2
jp nc,camera_shake_L1
jp camera_shake_L2
camera_shake_L1:
; return
ret
jp camera_shake_L3
camera_shake_L2:
camera_shake_L3:
; if TRIGGER == 0:
ld a,(TRIGGER)
cp 0
jp z,camera_shake_L4
jp camera_shake_L5
camera_shake_L4:
; if AMPLITUDE > 15:
ld a,(AMPLITUDE)
cp 15
jp z,camera_shake_L8
jp nc,camera_shake_L7
jp camera_shake_L8
camera_shake_L7:
; return
ret
jp camera_shake_L9
camera_shake_L8:
camera_shake_L9:
jp camera_shake_L6
camera_shake_L5:
camera_shake_L6:
; VALID = 1
ld a,1
ld (VALID),a
; if TRIGGER != 0:
ld a,(TRIGGER)
cp 0
jp nz,camera_shake_L10
jp camera_shake_L11
camera_shake_L10:
; AMPLITUDE = STRENGTH
ld a,(STRENGTH)
ld (AMPLITUDE),a
; REMAINING = DURATION
ld a,(DURATION)
ld (REMAINING),a
jp camera_shake_L12
camera_shake_L11:
camera_shake_L12:
; if REMAINING == 0:
ld a,(REMAINING)
cp 0
jp z,camera_shake_L13
jp camera_shake_L14
camera_shake_L13:
; return
ret
jp camera_shake_L15
camera_shake_L14:
camera_shake_L15:
; REMAINING = REMAINING - 1
ld a,(REMAINING)
sub 1
ld (REMAINING),a
; if AMPLITUDE == 0:
ld a,(AMPLITUDE)
cp 0
jp z,camera_shake_L16
jp camera_shake_L17
camera_shake_L16:
; return
ret
jp camera_shake_L18
camera_shake_L17:
camera_shake_L18:
; T0 = AMPLITUDE << 1
ld a,(AMPLITUDE)
sla a
ld (T0),a
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
; T1 = NOISE_X
ld a,(NOISE_X)
ld (T1),a
; while T1 >= T0:
camera_shake_L19:
ld a,(T1)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp nc,camera_shake_L20
jp camera_shake_L21
camera_shake_L20:
; T1 = T1 - T0
ld a,(T1)
ld b,a
ld a,(T0)
ld c,a
ld a,b
sub c
ld (T1),a
jp camera_shake_L19
camera_shake_L21:
; OFFSET_X = T1 - AMPLITUDE
ld a,(T1)
ld b,a
ld a,(AMPLITUDE)
ld c,a
ld a,b
sub c
ld (OFFSET_X),a
; T1 = NOISE_Y
ld a,(NOISE_Y)
ld (T1),a
; while T1 >= T0:
camera_shake_L22:
ld a,(T1)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp nc,camera_shake_L23
jp camera_shake_L24
camera_shake_L23:
; T1 = T1 - T0
ld a,(T1)
ld b,a
ld a,(T0)
ld c,a
ld a,b
sub c
ld (T1),a
jp camera_shake_L22
camera_shake_L24:
; OFFSET_Y = T1 - AMPLITUDE
ld a,(T1)
ld b,a
ld a,(AMPLITUDE)
ld c,a
ld a,b
sub c
ld (OFFSET_Y),a
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL 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 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
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
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
OFFSET_X: defb 0
OFFSET_Y: defb 0
REMAINING: defb 0
AMPLITUDE: defb 0
VALID: defb 0
TRIGGER: defb 0
STRENGTH: defb 0
DURATION: defb 0
NOISE_X: defb 0
NOISE_Y: defb 0
T0: defb 0
T1: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; TRIGGER = 1
ld a,1
ld (TRIGGER),a
; STRENGTH = 15
ld a,15
ld (STRENGTH),a
; DURATION = 1
ld a,1
ld (DURATION),a
; NOISE_X = 255
ld a,255
ld (NOISE_X),a
; NOISE_Y = 0
ld a,0
ld (NOISE_Y),a
if BROKEN
; REMAINING = 0
ld a,0
ld (REMAINING),a
endif
call checked_step
call emit_state
; TRIGGER = 0
ld a,0
ld (TRIGGER),a
if BROKEN
; REMAINING = 0
ld a,0
ld (REMAINING),a
endif
call checked_step
call emit_state
ret
else
; TRIGGER = 1
ld a,1
ld (TRIGGER),a
; STRENGTH = 3
ld a,3
ld (STRENGTH),a
; DURATION = 3
ld a,3
ld (DURATION),a
; NOISE_X = 0
ld a,0
ld (NOISE_X),a
; NOISE_Y = 6
ld a,6
ld (NOISE_Y),a
if BROKEN
; REMAINING = 0
ld a,0
ld (REMAINING),a
endif
call checked_step
call emit_state
; TRIGGER = 0
ld a,0
ld (TRIGGER),a
; NOISE_X = 6
ld a,6
ld (NOISE_X),a
; NOISE_Y = 0
ld a,0
ld (NOISE_Y),a
if BROKEN
; REMAINING = 0
ld a,0
ld (REMAINING),a
endif
call checked_step
call emit_state
; NOISE_X = 1
ld a,1
ld (NOISE_X),a
; NOISE_Y = 5
ld a,5
ld (NOISE_Y),a
if BROKEN
; REMAINING = 0
ld a,0
ld (REMAINING),a
endif
call checked_step
call emit_state
endif
retCustomise it
Use the largest supported amplitude for a single-tick impulse. 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
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
OFFSET_X: defb 0
OFFSET_Y: defb 0
REMAINING: defb 0
AMPLITUDE: defb 0
VALID: defb 0
TRIGGER: defb 0
STRENGTH: defb 0
DURATION: defb 0
NOISE_X: defb 0
NOISE_Y: defb 0
T0: defb 0
T1: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe custom diagnostic row is 00 00 00 0F 01 00 0F 01. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears the lifetime on every tick. The trigger produces one offset, then shake disappears even though its configured duration has not elapsed. Retain REMAINING and apply offsets only to rendering coordinates.
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
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
OFFSET_X: defb 0
OFFSET_Y: defb 0
REMAINING: defb 0
AMPLITUDE: defb 0
VALID: defb 0
TRIGGER: defb 0
STRENGTH: defb 0
DURATION: defb 0
NOISE_X: defb 0
NOISE_Y: defb 0
T0: defb 0
T1: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 292 code bytes; public state is 10 bytes and private scratch is 2 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.
Modulo by subtraction takes at most 85 iterations per nonzero-amplitude axis. A lookup table or masked power-of-two distribution is faster with different storage or visual distribution. No frame budget is claimed.
The helper does not generate random values, write a camera position or fade amplitude. Noise reduction is biased when the span does not divide 256; that is an explicit visual choice, not an unbiased random-selection routine. Supply a separate visual noise stream if gameplay randomness must remain unchanged.
Execution evidence
Four variants run on 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. 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
Seeded random values: Choose a separate stream or deterministic samples when visual effects must not perturb game rules.