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FastPack opens as a native desktop window when launched with no subcommand. All packing settings, sprite management, and output configuration live in the GUI without needing a .fpsheet file.

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Launching the GUI

Open the GUI with any of these commands: 
fastpack
Opens the GUI with an empty project.

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Window layout

The window is divided into five panels:
Below the menu bar. Contains:
  • Export button
  • Add Sprites button
  • Preview Animation button (enabled when two or more sprites are selected)
  • Source count indicator
  • Light/Dark theme toggle
Shows source directories at the top and packed frame list below. Resizable by dragging its border.
All packing configuration options organized into collapsible sections. Resizable.
The composited atlas texture. Updates automatically when any layout, sprite, or algorithm setting changes.
Timestamped messages from pack operations. Resizable.

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Managing sprites

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Adding sprites

There are multiple ways to add sprites to your project:
1

Using the toolbar

Click the Add Sprites button to open a folder picker.
2

Using the menu

Select File > Add Sprites… to open a folder picker.
3

Drag and drop

Drop a folder directly onto the window to add it as a source directory.
Dropping an image file adds its parent folder as a source directory.

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Sprite list panel

The top section lists configured source directories. Use the Add and Remove buttons to manage entries. The bottom section shows the packed frames from the most recent pack. Click a frame to select it. The selection highlights the corresponding region in the atlas preview and opens a detail view with:
  • Frame and source coordinates
  • Nine-patch editor: checkbox to enable, four inputs for top/right/bottom/left border widths, preview showing the boundary lines over the sprite
  • Pivot editor: checkbox for a custom pivot, X/Y sliders scaled 0.0–1.0, interactive preview where dragging the crosshair updates both sliders
  • Add Override button to persist the nine-patch and pivot values into the project
Frames marked as aliases show the name of the canonical sprite.

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Atlas preview panel

The atlas texture fills the center panel with interactive controls:
  • Scroll to zoom (0.05× to 64×)
  • Click and drag to pan
  • Double-click to reset zoom and pan
The current zoom percentage and atlas dimensions are shown in the top-left corner. A golden rectangle outlines the selected frame during a selection. The bottom-right corner shows sprite count, alias count, and overflow count from the last pack.
A blue overlay with “Drop folders or .fpsheet here” appears when files are dragged over the window.

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Settings panel

The settings panel has four collapsible sections:

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Texture settings

Controls output path (text field + Browse button), name prefix, texture format (PNG/JPEG/WebP), pixel format, data format (JSON Hash/JSON Array/Phaser 3/PixiJS), quality, premultiply alpha, and texture path prefix.
These settings affect export output only and do not trigger a repack.

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Layout settings

Controls:
  • Max width/height
  • Optional fixed width/height
  • Size constraint (Any/Power of 2/Multiple of 4/Word aligned)
  • Force square toggle
  • Allow rotation toggle
  • Border padding
  • Shape padding
  • Algorithm (Grid/Basic/MaxRects/Polygon)
  • MaxRects heuristic
  • Pack mode (Fast/Good/Best)

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Sprite settings

Controls:
  • Trim mode (None/Trim/Crop/CropKeepPos/Polygon)
  • Trim margin
  • Trim threshold
  • Extrude
  • Common divisors
  • Alias detection toggle

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Variants

Lists scale variants. Each variant has:
  • Scale factor
  • Scale mode (Smooth/Fast/Scale2x/Scale3x/HQ2x)
  • Filename suffix
Add and remove variants with the + and − buttons.
Pixel art modes (Scale2x/Scale3x/HQ2x) are planned but not yet implemented.

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Auto-repack

Changing any layout, sprite, or algorithm setting triggers an immediate repack. The atlas preview updates in place without needing to click a button.
Output-only settings (texture format, data format, quality, output directory, name) do not repack.

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Exporting

1

Set output directory

Configure the output directory in the Texture settings first.
2

Export

Click the Export button in the toolbar or select Atlas > Export from the menu.
3

Review output

File sizes are logged in the output panel.
Export compresses the atlas image using the configured texture format (PNG/JPEG/WebP) and writes a data file in the selected data format alongside it.

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Output log panel

Messages appear with a timestamp prefix:
  • Info messages are grey
  • Warnings are orange
  • Errors are red
The log scrolls automatically to the latest entry. A Clear button empties it.

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Keyboard shortcuts

Ctrl+N   New project
Ctrl+O   Open project file
Ctrl+S   Save project file
Ctrl+P   Export atlas to disk
P        Open/close animation preview (when two or more sprites are selected)
Shortcuts work both through the menus and directly while any panel has focus.
All shortcuts can be rebound in Edit → Preferences → Keybinds.

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Drag and drop

FastPack supports drag and drop for quick operations:
  • Dropping a .fpsheet file opens it as the current project
  • Dropping a folder adds it as a source directory and triggers an automatic pack
  • Dropping an image file adds its parent folder as a source directory

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Project files

Open and save with File > Open/Save or the corresponding keyboard shortcut. The window title shows the project name with an asterisk when there are unsaved changes. The project file is TOML with a .fpsheet extension. See the project file guide for the full schema.

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Theme

The toolbar’s Light/Dark button toggles between the custom dark theme (default) and a standard light theme. The choice persists in app state but is not saved to the project file.

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TexturePacker compatibility

The GUI layout and settings mirror TexturePacker’s settings panel. All core layout fields (max size, padding, rotation, size constraints, trim mode) correspond directly to TexturePacker options. The data format selector covers the same output targets. Settings are stored in the project file rather than a separate history file.

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