Playfield and flow: vertical versus horizontal layouts, HUD placement, scrolling, waves, score and recovery.
Bullets and encounters: patterns, pools, collision, power-ups, bombs and boss phases.
SGDK rendering and budgets: verified APIs, shared sprite frames, scanlines, DMA, palettes and runtime validation.
Shooter Lab: a playable vertical example with a side HUD, two stages, weapon upgrades, shields, Twins, delayed bombs and patterned bosses.
Carpet of the Crescent: a playable horizontal example with three loadouts/stages, parallax, a gem-driven upgrade track, returning crescents, a sustained piercing beam and chaining magic.

The labs use ordinary C game logic and SGDK directly. SGDK provides rendering, input, sound and frame processing; wave scripting, bullet patterns, hitboxes, scoring and boss state are original game code. Use ewrogers/sgdk:latest, pull before building, and check unfamiliar APIs in the selected SDK. The reference was checked against SGDK 2.11.

Design for readable pressure

Keep player, enemy, projectile and pickup collision separate. A ship's damaging core can be smaller than its artwork. Make a focus/slow mode display that core if it helps the chosen game. Keep HUD pixels outside movement, spawn and culling bounds. Horizontal play is not just vertical coordinates swapped: reconsider reaction distance, terrain and enemy entrances.

Snapshot aimed shots when fired unless homing is explicitly intended. Build spreads, rings and spirals from bounded tables or verified SDK math. Pick a pool exhaustion policy, such as skipping a complete volley, and track refusals while tuning. Do not allocate per projectile or silently turn missing visuals into invisible damaging bullets.

Separate stage progress from actor lifetime. Use stable spawn order, bounded per-tick work and explicit rules when enemy slots are busy. Boss entry, damage windows, phase changes and defeat should be visible. Give recovery room after a player hit and clear only the objects the game's rules intend to clear.

Budget the busy moment

A bullet is not free because its art is small. Budget simulation, collision, RAM, sprite count, sprite pixels per scanline, VRAM and DMA separately. Culling and retirement are different operations. Reuse resident animation frames where appropriate and avoid needless uploads for repeated bullets.

Measure an optimized ROM with the busiest attack, player fire, effects and music together. Check focused and normal movement, pause, power changes and nominal PAL/NTSC timing. Input edges must not repeat on PAL's occasional second simulation step. A fixed clock does not recover real time lost to missed frames.

Test the rules, then play

Execute tests for pattern symmetry/direction, full pools, collision at maximum speed, one hit per projectile, invulnerability, score bounds, wave ordering, phase transitions, restart and pause. Compare broad-phase collision candidates against an independent reference before optimizing. Include long deterministic runs and target-code checks where host arithmetic cannot establish behavior.

Build tests establish API/resource compatibility; they do not prove VDP timing, art readability or sound quality. Use the lab checklist in an emulator and on hardware when available. Optional core skills such as sgdk-graphics, sgdk-input, sgdk-math, sgdk-audio and sgdk-performance can help when installed. This skill's resources work independently.

sgdk-shmup

SGDK Shmup

Choose the reference