A hit can make the view jolt without moving the player or the world. Keep the effect’s lifetime between updates, generate a small offset, and apply it only when converting world positions to the rendered view.
Include and initialise
Reserve public X/Y/R/A/O/T/S/D/N/M, private P/U/V and lines 3000–3250. There are no arrays. Combine routines.bas with a caller using build.py, keeping END, STOP or a jump before the helper. C64/BBC use GOSUB; Spectrum uses GO SUB. Call 3000 to clear public fields at a reset boundary.
R is remaining updates and A the retained amplitude. Supply trigger T, requested strength S, duration D and two caller-owned noise bytes N/M. All inputs and retained state are integral bytes. Strength is limited to zero through fifteen. Retained A must also be in that range unless a nonzero trigger is about to replace it.
Trigger once, then retain the effect
Call 3100 once per intended effect update. A nonzero T reloads A=S and R=D. With positive R, each noise value is reduced modulo 2*A+1 and shifted down by A. The helper returns signed X/Y offsets encoded in bytes, then decrements R. R=0 returns zero offsets. O=1 means valid input.
Reusable numbered routines
3000 LET X=0: LET Y=0: LET R=0: LET A=0: LET O=0: LET T=0: LET S=0: LET D=0: LET N=0: LET M=0: RETURN
3100 LET X=0: LET Y=0: LET O=0
3110 IF R<0 OR R>255 OR R<>INT (R) THEN RETURN
3120 IF A<0 OR A>255 OR A<>INT (A) THEN RETURN
3130 IF T<0 OR T>255 OR T<>INT (T) THEN RETURN
3140 IF S<0 OR S>15 OR S<>INT (S) THEN RETURN
3150 IF D<0 OR D>255 OR D<>INT (D) THEN RETURN
3160 IF N<0 OR N>255 OR N<>INT (N) THEN RETURN
3170 IF M<0 OR M>255 OR M<>INT (M) THEN RETURN
3180 IF T=0 AND A>15 THEN RETURN
3190 IF T<>0 THEN LET A=S: LET R=D
3200 LET O=1
3210 IF R=0 THEN RETURN
3220 LET P=2*A+1: LET U=N-P*INT (N/P)-A: LET V=M-P*INT (M/P)-A
3230 IF U<0 THEN LET U=U+256
3240 IF V<0 THEN LET V=V+256
3250 LET X=U: LET Y=V: LET R=R-1: RETURNDecode an output byte of 128 or more by subtracting 256 before adding it to a wider render coordinate. The maximum amplitude is fifteen, so an encoded 253 means minus three. Adding the encoded byte directly to an ordinary BASIC coordinate would move the camera the wrong way.
Invalid input clears offsets and O while preserving R/A and all caller inputs. A valid retrigger can replace an old amplitude byte of 255; without a trigger that retained amplitude is invalid. A trigger with duration zero deliberately cancels the effect. Strength zero produces zero offsets while its positive lifetime still counts down.
Follow the lifetime
The normal caller requests strength three for three updates. The custom caller requests strength five for four. Each triggers only on the first call and supplies different noise values on later calls. The saved second offset proves the effect survived beyond the trigger; a separate counter checks the intended number of active calls.
Example calls from the maintained caller
70 GOSUB 3000: LET AA=0: LET BB=0
80 LET S=3: LET D=3: LET NN=0
90 LET T=1: LET N=0: LET M=255
100 IF (T<>0 AND D>0) OR (T=0 AND R>0) THEN LET NN=NN+1
110 GOSUB 3100: GOSUB 2000
120 LET T=0: LET N=1: LET M=6
130 IF (T<>0 AND D>0) OR (T=0 AND R>0) THEN LET NN=NN+1
140 GOSUB 3100: GOSUB 2000
150 IF X=254 THEN LET AA=1
160 LET T=0: LET N=2: LET M=5
170 IF (T<>0 AND D>0) OR (T=0 AND R>0) THEN LET NN=NN+1
180 GOSUB 3100: GOSUB 2000
190 LET T=0: LET N=3: LET M=4
200 IF (T<>0 AND D>0) OR (T=0 AND R>0) THEN LET NN=NN+1
210 GOSUB 3100: GOSUB 2000
220 LET T=0: LET N=4: LET M=3
230 IF (T<>0 AND D>0) OR (T=0 AND R>0) THEN LET NN=NN+1
240 GOSUB 3100: GOSUB 2000
250 IF NN=3 THEN LET BB=1
260 PRINT "SHAK RESULTS";AA;BB
270 ?(MB+995)=AA: ?(MB+996)=BB: ?(MB+997)=GD: ?(MB+998)=WC-256*INT (WC/256): ?(MB+999)=INT (WC/256): ?(MB+1000)=239: STOPThe final active call can return R=0 after producing a nonzero offset. Consume that call’s X/Y; do not decide whether it was active solely from the decremented R. The caller counts active calls before the helper for precisely that reason. AA/BB/NN/JJ (Z on Spectrum), line 2000 and the mailbox are diagnostics.
Keep the offset out of authoritative world state
Add the decoded offset to a temporary render camera or screen coordinate, then discard it after drawing. Keep collision, movement and the stable camera origin separate. These native examples verify the logical offsets and lifetime; they do not implement hardware scrolling or demonstrate a rendered shaking scene.
The caller owns the clock and noise generator. Reducing a byte by a remainder does not guarantee an exactly uniform distribution for every span. Smoother motion can use a table or filtered noise; a decaying envelope can vary strength over time. Neither is secretly provided here. Repeated triggers restart this effect rather than accumulating impulses or choosing a stronger request.
Mistake and repair
The wrong caller clears R before every call. The first trigger still works, but later updates lose the effect. Both saved second-offset and lifetime checks fail. Retain R/A until expiry, an intentional reset or a new trigger. Repaired source and loaded program match normal exactly.
Cost and Maths for Games
Each valid active call validates bounded fields, computes two remainders and encodes two signed values. It allocates nothing and owns no timer, RNG or display register. More elaborate noise and decay need separate state and work; amplitude alone does not establish a visual-quality or frame-time budget.
Maths for Games connects modular arithmetic, signed representation and bounded displacement. The Craft connects an effect’s strength, duration and rendering ownership to readable feedback.
Global names and line numbers
BASIC numeric names, arrays and numbered lines are global. To combine helpers, rename public and private fields and renumber the helper together with every GO SUB/GO TO reference. Keep each state block under one owner. The maintained build.py rejects duplicate line numbers; it does not automatically namespace or renumber routines.
Machine and measured size
This helper uses scalar numeric fields and allocates no arrays. Its diagnostic uses MODE 7 and HIMEM=&6000 to separate program storage, mailbox and screen. The helper itself changes neither mode nor HIMEM.
The numbered helper occupies 526 tokenised program bytes on this interpreter, including line headers. This excludes the caller, test tables, numeric scalar/array storage, interpreter stacks and ROM. Tokenised size is not machine-code size or a hardware-cycle count. No real-time or whole-game performance budget is inferred from the diagnostic. The caller’s AA/BB/NN, JJ and RAM mailbox are observation machinery, not required game state.
Execution evidence
Working, custom, deliberately wrong and exact repaired callers executed through BBC Micro’s stock BASIC interpreter under Emu198x 0.44.2. Build198x 0.2.9 tokenises C64 PRG and Spectrum TAP programs. C64 enters stock RUN, Spectrum uses ROM tape loading, and BBC enters every numbered source line through MOS/BASIC. Actual loaded program bytes, complete state packets, positive completion/count/guards and visible result pixels are checked. After the positive completion marker, the observer advances two complete display frames before capturing, so a partially scanned final character cannot masquerade as a finished picture. Firmware is not distributed.
An independent mathematical/order model checks the trace and rejects altered source, configuration, loaded program, state, completion and image evidence. Host exhaustive checks complement the native boundary/sequential examples; those host ranges are not claimed as natively exhaustive. The supplied diagnostic callers deliberately end with STOP; the interpreter report is expected. The wrong caller prints 0,0 and the normal/custom/repaired callers print 1,1.
Public CI checks retained evidence; it does not execute restricted firmware. Refresh with python3 scripts/verify-basic-contacts.py camera-shake bbc-micro --tools /path/to/extracted/releases and inspect with python3 scripts/check-basic-contacts.py camera-shake.
Explore further
Compare the assembly technique. The BASIC game-flow routine makes session boundaries explicit. Complete source and native evidence.