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wimpysworld/nix-config/using-ebitengine
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version: "1.0.0"

Using Ebitengine

Ebitengine is a 2D game engine for Go. This skill covers sharp rendering, efficient drawing, Kage shaders, and headless runs.

Freshness. Upstream wrote this guidance against Ebitengine at commit 345100b0e4b75ed7e06d2a1cd84571814a5e6140.
exp/vmhost is experimental, and its API can change. If this guidance disagrees with the source, the source wins.
Use the godoc and source of the target Ebitengine version for API and platform behaviour.
Report drift to the user. Do not edit the installed skill.

Core facts

These facts apply to every Ebitengine task.

  • Render detail at the resolution where it is displayed. Implement LayoutF and return the outside dimensions times ebiten.Monitor().DeviceScaleFactor().

A low-resolution stage anywhere in the path loses detail that later scaling cannot recover.

  • The number of Go drawing calls is not the number of GPU draw commands. Ebitengine packs eligible images into automatic texture atlases and merges compatible successive draws.

Check the real commands with -tags=ebitenginedebug before you change drawing code for performance.

  • Kage coordinates are atlas-absolute, in pixels. Put //kage:unit pixels in every shader.

Subtract the image origin before you scale a coordinate, and add the origin back after.

  • Ebitengine uses premultiplied alpha everywhere, shaders included. A shader must return a premultiplied vec4.
  • Prefer a headless run for screenshots, golden-image tests, and input repros. An app built with -tags ebitenginevmguest runs as an exp/vmhost guest of a hidden host.

The host driver is scripts/headless-driver/main.go. A Guigui app is an ebiten.Game, so the same procedure applies to it.

  • Compile against the target Ebitengine version before you use a newer API such as FilterPixelated or imageSrcNAtFromSrc0Pos.

References

All reference paths in this skill are relative to the skill root. Read the reference for the task before you write code.

TaskReference
A new app, layout, text, UI, image scaling, final presentation, blurry output, pixel art, high-DPI displays, input coordinatesreferences/crisp-rendering.md
Drawing code, sprites, offscreen passes, batching, texture atlases, unmanaged images, rendering performancereferences/efficient-rendering.md
A headless run with no display or visible window: screenshots, golden images, input injection, multi-tick repros, audio checksreferences/headless-run.md and scripts/headless-driver/main.go
Writing, reviewing, or debugging a Kage shader: coordinate model, sampling, premultiplied alpha, language limits, a compile checkreferences/kage-shaders.md
Porting a shader from GLSL, Shadertoy, HLSL, Unity, or Godot to Kage, and the CPU-side draw callreferences/kage-porting.md
Kage snippets: normalisation, addressing modes, bilinear sampling, pixelation, blur, branchless selectors, debugging rampsreferences/kage-recipes.md

For a shader port, read references/kage-shaders.md first, then references/kage-porting.md.

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