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The JSON Array format writes atlas metadata in the same structure as JSON Hash, but frames is a JSON array rather than an object. Each entry carries a filename field holding the sprite ID instead of using the sprite ID as an object key. Use this format when your runtime iterates frames in order, or when a flat array is more convenient than a keyed object.

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Usage

fastpack pack sprites/ --data-format json-array

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Output format

Output for a two-sprite atlas: 
{
  "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 }
    },
    {
      "filename": "ui/button",
      "frame": { "x": 68, "y": 2, "w": 48, "h": 32 },
      "rotated": false,
      "trimmed": false,
      "spriteSourceSize": { "x": 0, "y": 0, "w": 48, "h": 32 },
      "sourceSize": { "w": 48, "h": 32 }
    }
  ],
  "meta": {
    "app": "FastPack",
    "version": "1.0",
    "image": "atlas.png",
    "format": "RGBA8888",
    "size": { "w": 256, "h": 128 },
    "scale": "1"
  }
}

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With alias detection

When alias detection is on, duplicate sprites carry an aliasOf field pointing at the original sprite’s ID. The frame rect is still valid — it points to the same atlas region. 
{
  "filename": "enemies/goblin",
  "frame": { "x": 2, "y": 2, "w": 32, "h": 32 },
  "rotated": false,
  "trimmed": false,
  "spriteSourceSize": { "x": 0, "y": 0, "w": 32, "h": 32 },
  "sourceSize": { "w": 32, "h": 32 },
  "aliasOf": "enemies/orc"
}

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Schema reference

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Frame object

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filename
string
required
Sprite ID (source path relative to sprite root, without extension)
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frame
object
required
Rectangle in the atlas texture
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x
number
X position in pixels
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y
number
Y position in pixels
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w
number
Width in pixels
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h
number
Height in pixels
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rotated
boolean
required
true when the sprite was rotated 90° clockwise to fit
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trimmed
boolean
required
true when the sprite was packed with trim mode enabled
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spriteSourceSize
object
required
Frame’s position and size relative to the original source image
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x
number
X offset from original image
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y
number
Y offset from original image
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w
number
Width in pixels
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h
number
Height in pixels
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sourceSize
object
required
Full original image dimensions before any trimming
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w
number
Original width in pixels
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h
number
Original height in pixels
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aliasOf
string
Points to the original sprite’s ID for pixel-identical duplicates (when alias detection is enabled)

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Meta object

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meta
object
required
Atlas metadata
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app
string
Always "FastPack"
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version
string
Format version ("1.0")
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image
string
Texture filename
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format
string
Pixel format (e.g., "RGBA8888")
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size
object
Atlas dimensions
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scale
string
Scale factor used during packing

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TexturePacker compatibility

Matches TexturePacker’s JSON Array output. Engines using JSON Hash object-key lookup will not load this format — pick the right variant for your runtime loader.
Array order matches atlas packing order (largest-area-first). The order is stable across runs on the same input set.

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Technical notes

All field semantics — frame, rotated, trimmed, spriteSourceSize, sourceSize — match the JSON Hash format. See the JSON Hash documentation for detailed field-level descriptions.

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