sprite-animation

name	sprite-animation
description	Use this skill when developing, validating, slicing, or managing 2D sprite sheets, animation sequences, frame durations, tag loops, and Aseprite files (.ase/.aseprite) for Ebitengine games. Serves as the specialized Animator Agent role for game development teams.

2D Sprite Animation & Aseprite Integration Guide (Animator Role)

This skill equips AI agents acting in the Animator Agent Role with the tools, specifications, and Go controllers required to design, validate, slice, and manage 2D character sprite sheets, animation frame sequences, tag loops, and Aseprite files (.ase / .aseprite) for Ebitengine v2 games.

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1. Animator Agent Role & Core Responsibilities

In a game development team, the Animator Agent owns all 2D sprite animation pipelines:

1. Asset Validation & Format Verification: Inspects sprite sheet images (from nano-banana or artist files) using file identification tools (file, mimetype, http.DetectContentType) to verify binary integrity and grid dimensions before slicing.
2. Animation Tag & State Specification: Defines animation tags (idle, walk, run, attack, hurt, death, cast), frame duration timings ($50\text{ms} - 200\text{ms}$), and loop directions (LoopForward, LoopReverse, LoopPingPong, LoopOnce).
3. Aseprite Integration & Slicing: Loads Aseprite files (.ase / .aseprite / .json) using recommended Go libraries (SolarLune/goaseprite) or pure-code Ebitengine SubImage grid slicing.
4. Animation Controller Code Generation: Authors clean, GC-friendly Ebitengine animation controllers that manage frame timers, state switches, directional flips, and completion callbacks (OnComplete).

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2. Technical Reference Standards & Specifications

For complete binary format specifications, GIMP RGBA palette specifications, and production Go code implementations, consult:

Module	Reference File	Key Topics Covered
Aseprite Binary Format & GPL	references/aseprite_format.md	Header, frame headers, cel chunks (0x2005), tag chunks (0x2018), 9-patch slices (0x2022), and GIMP .gpl RGBA palette format extension.
Go Animation Controller	references/animation_controller.go	Complete, production-grade Ebitengine AnimationController and GridSpriteSheet implementation with delta time ($dt$) updating and horizontal flipping.

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3. Asset Validation & Grid Slicing Workflow

Never assume a generated image is a valid PNG or has correct dimensions based solely on filename extension:

# 1. Validate actual file type using file identification tools
file assets/sprites/player_sheet.png
# Expected output: PNG image data, 256 x 128, 8-bit/color RGBA

# 2. Verify grid dimensions (e.g. 256x128 image with 32x32 frames = 8 columns x 4 rows = 32 frames)

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4. Animation State Machine Benchmark Table

When authoring animation sequences for characters and entities, enforce standard frame count and duration benchmarks:

Animation Tag	Frame Range / Count	Frame Duration	Loop Mode	Gameplay Trigger / Transition
idle	$4 - 8\text{ frames}$	$120\text{ms} - 180\text{ms}$	LoopForward	Default state when velocity is zero ($VX=0, VY=0$).
walk / run	$8 - 12\text{ frames}$	$60\text{ms} - 100\text{ms}$	LoopForward	Active when moving horizontally ($VX \neq 0$).
jump / fall	$2 - 4\text{ frames}$	$100\text{ms}$	LoopOnce / Hold	Triggered on jump start; holds final frame during airborne fall.
attack	$6 - 10\text{ frames}$	$40\text{ms} - 80\text{ms}$	LoopOnce	Triggered on attack keypress. Invokes OnComplete callback back to idle.
hurt	$3 - 5\text{ frames}$	$50\text{ms}$	LoopOnce	Triggered on damage hit. Flash red/white overlay.
death	$6 - 10\text{ frames}$	$100\text{ms}$	LoopOnce	Triggered on zero health. Holds final collapse frame without looping.

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5. Integration Patterns in Ebitengine

5.1 Using SolarLune's goaseprite Library

import "github.com/SolarLune/goaseprite"

type Player struct {
	Anim *goaseprite.File
	X, Y float32
}

func NewPlayer() *Player {
	return &Player{
		Anim: goaseprite.New("assets/player.json"),
	}
}

func (p *Player) Update(dt float32) {
	p.Anim.Update(dt)
}

func (p *Player) Draw(screen *ebiten.Image) {
	op := &ebiten.DrawImageOptions{}
	op.GeoM.Translate(float64(p.X), float64(p.Y))
	
	// Draw current frame sub-image from Aseprite atlas
	sub := p.Anim.Image.SubImage(p.Anim.CurrentFrameBounds()).(*ebiten.Image)
	screen.DrawImage(sub, op)
}

5.2 Using Pure-Code Grid Slicing Controller

sheet := &GridSpriteSheet{
	Image:       embeddedSpriteImage,
	FrameWidth:  32,
	FrameHeight: 32,
	Columns:     8,
	TotalFrames: 32,
}

controller := NewAnimationController(sheet)
controller.AddTag(AnimationTag{Name: "idle", StartFrame: 0, EndFrame: 5, FrameDuration: 150 * time.Millisecond, Loop: LoopForward})
controller.AddTag(AnimationTag{Name: "run", StartFrame: 8, EndFrame: 15, FrameDuration: 80 * time.Millisecond, Loop: LoopForward})
controller.Play("run")

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6. Animator Agent Pre-Flight Checklist

Before confirming animation code or sprite assets:

• Validated File Format: Verified image using file CLI tool (ensuring RGBA format and no corrupted/misnamed extensions).
• Grid Math Verified: Image width and height are exact integer multiples of frame width and height.
• All Animation Tags Defined: Included idle, run/walk, attack, and death states.
• No Allocation in Draw(): SubImage bounds calculations use pre-computed rectangles or persistent DrawImageOptions.
• Horizontal Flipping Handled: Configured negative matrix scale ($Scale(-1, 1)$) for left-facing direction without duplicating sprite assets.