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The Phaser 3 format writes a single JSON file containing a textures array. Each entry in the array represents one atlas sheet and carries its own image, size, scale, and frames array. This matches the layout Phaser 3 expects from scene.load.multiatlas(). Single-sheet packs still produce a textures array with one entry, so you can always load with multiatlas regardless of whether multipack is on.

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Usage

fastpack pack sprites/ --data-format phaser3

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Output format

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Single sheet

Standard single-sheet output: 
{
  "textures": [
    {
      "image": "atlas.png",
      "format": "RGBA8888",
      "size": { "w": 512, "h": 256 },
      "scale": 1.0,
      "frames": [
        {
          "filename": "player/idle",
          "frame": { "x": 2, "y": 2, "w": 64, "h": 96 },
          "rotated": false,
          "trimmed": true,
          "spriteSourceSize": { "x": 4, "y": 0, "w": 64, "h": 96 },
          "sourceSize": { "w": 72, "h": 96 }
        }
      ]
    }
  ],
  "meta": {
    "app": "FastPack",
    "version": "3.0"
  }
}

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Multi-sheet (multipack)

Multi-sheet output writes two entries in textures but only one JSON file: 
{
  "textures": [
    {
      "image": "atlas.png",
      "format": "RGBA8888",
      "size": { "w": 4096, "h": 4096 },
      "scale": 1.0,
      "frames": [ "..." ]
    },
    {
      "image": "atlas1.png",
      "format": "RGBA8888",
      "size": { "w": 1024, "h": 512 },
      "scale": 1.0,
      "frames": [ "..." ]
    }
  ],
  "meta": { "app": "FastPack", "version": "3.0" }
}

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Loading in Phaser 3

function preload() {
  this.load.multiatlas('sprites', 'atlas.json', 'assets/');
}

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Schema reference

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Texture entry

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image
string
required
Texture filename (e.g., atlas.png, atlas1.png)
​
format
string
required
Pixel format (e.g., RGBA8888, RGB888)
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size
object
required
Atlas dimensions
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w
number
Width in pixels
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h
number
Height in pixels
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scale
number
required
Scale factor used during packing. Phaser uses this to apply sub-pixel corrections when mixing @1x and @2x atlases.
​
frames
array
required
Array of frame objects for this texture sheet

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Frame object

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filename
string
required
Sprite ID (source path without extension)
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frame
object
required
Rectangle in the atlas texture (x, y, w, h in pixels)
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rotated
boolean
required
true when the sprite was rotated 90° clockwise
​
trimmed
boolean
required
true when transparent borders were stripped
​
spriteSourceSize
object
required
Frame’s position and size relative to the original source image
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sourceSize
object
required
Full original image dimensions before trimming

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Meta object

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meta
object
required
Atlas metadata
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app
string
Always "FastPack"
​
version
string
Format version ("3.0" for Phaser 3)

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Multipack support

When multipack is active, the Phaser 3 exporter combines all sheets for a scale variant into a single JSON file. Secondary sheets do not get their own data files; only one .json is written per variant.
Each textures entry has its own scale field that reflects the ScaleVariant.scale used during packing. This allows Phaser to correctly handle mixed-resolution atlases.

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TexturePacker compatibility

TexturePacker also produces this format when targeting Phaser 3. The meta.app field reads "FastPack" instead of "TexturePacker"; Phaser ignores the app field. Frame fields use camelCase (spriteSourceSize, sourceSize) to match Phaser’s parser.

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