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name: unity-2d-core description: | Unity 2D fundamentals — Sprite Renderer, Sprite Atlas v2 (master/variants), 9-slice rendering, Sorting Layers vs Order in Layer vs Sorting Group, sprite import settings, Pixels Per Unit (PPU), Pixel Perfect Camera, point filtering.

USE WHEN: importing sprites, packing atlases, controlling draw order in 2D, setting up pixel-art rendering, configuring 9-slice UI/decorations.

DO NOT USE FOR: Tilemap (use unity-2d-tilemap); 2D physics (use unity-2d-physics); animation (use unity-2d-animation); 2D Lights (use unity-2d-lighting). allowed-tools: Read, Grep, Glob, Write, Edit

Unity 2D Core

Sprite import settings (pixel art)

Texture Type        Sprite (2D and UI)
Sprite Mode         Single (or Multiple for sprite sheets)
Pixels Per Unit     32 (or 16 / 64 — pick ONE project-wide)
Mesh Type           Tight (saves overdraw on alpha sprites)
Pivot               Bottom for floor-anchored entities; Center for projectiles
Filter Mode         Point (no filter) for crisp pixel art; Bilinear for smooth art
Compression         None (pixel art) — Compression artifacts wreck pixels
Generate Mip Maps   Off for 2D

For HD vector-style art: keep PPU consistent; use Bilinear filter; allow ASTC/Crunch compression (mobile size).

Sprite Atlas v2

Atlas = combine many sprites into one texture so they share a draw call.

Window > 2D > Sprite Atlas
- Type: Master (default) or Variant (low-res derivation)
- Include in Build: ON
- Allow Rotation / Tight Packing: tune for empty space vs runtime cost
- Object for Packing: drag folders/sprites in

Variants: a 0.5 scale Variant of a Master atlas → mobile gets the smaller textures, desktop the master. Toggle by build target.

SpriteAtlasManager.atlasRequested for late-bound atlases (Addressables-friendly).

Sorting

Three orthogonal mechanisms, applied in this order:

1. Sorting Layer — broad strokes (Background, World, Characters, FX, UI).
2. Order in Layer — fine integer within a layer.
3. Sorting Group — wraps a parent + children so they sort as one unit (a knight + their weapon + their VFX).

Sorting Group is the key tool for multi-part 2D characters — without it, Z-fighting between sprite parts ruins the look.

// Optional dynamic order based on Y (top-down RPGs)
void LateUpdate() => _renderer.sortingOrder = -(int)(transform.position.y * 100);

9-slice

For UI panels, banners, expandable decorations:

Sprite Editor → Border (left/right/top/bottom in pixels)
Sprite Renderer → Draw Mode = Sliced (or Tiled)

Drag the Sprite Renderer scale → corners stay rigid, edges stretch / tile.

PPU consistency

Pick one PPU for all sprites in a level. Mixing 16 and 32 sprites in the same scene creates inconsistent pixel sizes and sub-pixel sampling artifacts.

For mixed art assets that genuinely have to coexist: use Sprite > Edit Sprite to set custom physics shapes and pivots, but keep PPU equal.

Pixel Perfect Camera

Add PixelPerfectCamera component to your Camera. Settings:

Reference Resolution    320 × 180  (or 480 × 270, 640 × 360 — base canvas)
Assets PPU              same as your sprites
Crop Frame              Letterbox (preserves aspect, adds black bars)
                        or Stretch Fill (stretches; only for tiny variations)
Pixel Snapping          ON  (snaps every renderer to integer pixels)
Upscale Render Texture  ON for clean integer scaling

Without Pixel Perfect Camera you get sub-pixel jitter as the camera moves.

AI-assisted sprite pipeline (Aseprite-native tools)

For static sprites (props, items, environments, tiles) AI generators are productive. No AI tool today produces ship-ready output zero-touch — every sprite needs an Aseprite cleanup pass before importing into Unity.

Tools that install as Aseprite extensions

Tool	Cost	Model	Output license	Offline
Retro Diffusion Full (astropulse.itch.io/retrodiffusion)	$65 one-time	Local pixel-art SD (Nvidia GTX 1050 Ti+ or Mac M1+)	Yours, commercial OK	Yes
PixelLab (pixellab.ai)	$10–50/mo	Cloud (zero hardware)	Yours, commercial OK (no model-training on outputs)	No
PixelAI (red335.itch.io/pixelai-local-ai-directly-in-aseprite)	Free / pay-what-you-want	Local SD (auto-download, Windows + Nvidia recommended)	Yours, commercial OK	Yes
Retro Diffusion Lite	$20+ one-time	Local pixel-art SD	Yours, commercial OK	Yes

All four install via Edit > Preferences > Extensions > Add Extension and expose AI panels inside the Aseprite UI.

License pitfalls (avoid for shipping)

• Leonardo.ai free tier — Leonardo retains rights to your outputs. Paid only for full ownership.
• Recraft free tier — outputs owned by Recraft, made public in gallery.
• Scenario.com free tier — "personal/evaluation only", do not ship.
• Midjourney / DALL-E / Recraft — produce anti-aliased "pixel-styled" output, not crisp pixels. Need 8× nearest-neighbour downscale + palette quantise to be usable as sprites.

Mandatory cleanup before Unity import

1. Spritefusion Pixel Snapper (spritefusion.com/pixel-snapper) — free Rust CLI that snaps off-grid pixels back onto integer coordinates and quantises the palette. Run on every AI-generated sprite before opening in Aseprite.
2. Aseprite touch-up — palette unification, edge cleanup, transparency. Even Retro Diffusion / PixelLab outputs benefit from a quick pass.
3. Unity import settings (set on the asset, not in code):

   Texture Type        Sprite (2D and UI)
   Pixels Per Unit     match project-wide PPU (see "PPU consistency" section)
   Filter Mode         Point  (no filter)        — anti-aliasing ruins pixel art
   Compression         None                       — JPEG-style compression destroys palette
   Generate Mip Maps   OFF                        — 2D doesn't need mips
   Wrap Mode           Clamp (or Repeat for tileable backgrounds)
   

Decision tree (what to use when)

Need	Pick
One-off props / items / icons / environments	Retro Diffusion Full ($65 once) — best ROI, offline, IP-clean
No budget / Windows + Nvidia	PixelAI (free)
Tilesets (47-tile autotile, rule-tile-ready)	PixelLab tileset gen OR Retro Diffusion + Aseprite cleanup
Hi-res concept / mood board (then trace by hand)	Midjourney / Flux / DALL-E (NOT for direct sprite output)
Full-control free + self-hosted (no SaaS)	ComfyUI + Pixel Art XL LoRA + 8× nearest-neighbour downscale + Pixel Snapper
Studio-grade "same character, every variant" consistency	Scenario.com Pro custom-trained character model ($30/mo)

For animation-specific AI tools (walk cycles, 4/8-directional sprites), see the unity-2d-animation skill — it's a different problem.

Anti-patterns

Anti-pattern	Fix
Mixing PPU values across sprites	Pick one PPU project-wide
Bilinear filter on pixel art	Set Filter Mode = Point
Compression on pixel art textures	None (or Crunch only after careful comparison)
Sprite atlas not included in build	Tick Include in Build
Sorting Layers not defined → all default	Define Background / World / Characters / FX / UI upfront
Z-fighting in multi-part character	Add Sorting Group on parent
Generating mipmaps on 2D	Disable mips

Production checklist

• Single project-wide PPU
• All gameplay sprites in a Sprite Atlas v2
• Sorting Layers + Sorting Group strategy documented
• Pixel Perfect Camera configured (if pixel art)
• Atlas Variants set up for mobile (if shipping mobile)
• Filter / Compression appropriate to art style
• No sub-pixel jitter when camera follows player