Pac.Pic bank

A Pac.Pic bank holds a single compressed Amiga screen image. There are two variants:

• PACK — plain compressed image. Load it with Load and unpack with Unpack <bankno>. You must set up the destination screen yourself before calling Unpack.
• SPACK — compressed image with an embedded screen header carrying palette, resolution and screen coordinates. Use Unpack <bankno> To <screenno> to create and configure the screen automatically, including loading the palette.

Commands

# Export to JSON + PNG
portamos disasm Picture.Abk output/picture.json

# Export with IFF ILBM instead of PNG
portamos disasm --ilbm Picture.Abk output/picture.json

# Reassemble
portamos asm output/picture.json Picture.Abk

# Reassemble with palette optimizer (smaller file, slower build)
portamos asm --optimize output/picture.json Picture.Abk

JSON schema

SPACK bank — minimal (auto-fill from image)

The simplest SPACK configuration: just include an empty screen object and all fields are derived from the image file’s dimensions, colour count, and embedded CMAP palette.

{
  "type": "PacPic",
  "bankNumber": 1,
  "chipRam": false,
  "imageFile": "picture.png",
  "screen": {} // all screen fields filled in automatically from the image
}

SPACK bank — explicit screen header

Override individual fields when the image metadata is insufficient or when you need non-standard screen parameters.

{
  "type": "PacPic",
  "bankNumber": 1,
  "chipRam": false,
  "imageFile": "picture.png",
  "optimize": true, // run palette optimizer for smaller output
  "screen": {
    "width": 320,
    "height": 200,
    "displayWidth": 320,
    "displayHeight": 200,
    "numPlanes": 4,
    "numColors": 16,
    "bplCon0": 16896, // (numPlanes << 12) | 0x0200 — standard lores + COLOR flag
    "hardX": 129,     // 0x81 — standard AMOS lores screen start X
    "hardY": 50,      // 0x32 — standard AMOS lores screen start Y
    "palette": [0, 4095, 3840, 240] // Amiga 0x0RGB colour words
  }
}

PACK bank (no screen header)

{
  "type": "PacPic",
  "bankNumber": 1,
  "chipRam": false,
  "imageFile": "picture.png"
  // no "screen" key — plain PACK bank; use Unpack <bankno> after setting up the screen yourself
}

Omit "screen" entirely for a plain PACK bank.

Field reference

Top level

Field	Type	Description
type	string	Always "PacPic"
bankNumber	integer	Bank slot (1-based)
chipRam	boolean	true = chip RAM, false = fast RAM (default)
imageFile	string	Path to PNG or IFF image file relative to the JSON
optimize	boolean	Run palette optimizer before compressing (slower build, smaller output)
planes	integer	Override bit-depth (normally inferred from the image’s colour count)

screen object

Include this object for SPACK banks. An empty object {} is valid — all fields default automatically from the image.

Field	Auto-default	Description
width	image width	Width in pixels
height	image height	Height in pixels
displayWidth	image width	Displayed width
displayHeight	image height	Displayed height
numPlanes	derived from colour count	Number of bitplanes
numColors	1 << numPlanes	Number of palette entries
bplCon0	(numPlanes << 12) \| 0x0200	Amiga BPLCON0 register value
hardX	0x81 (129)	Screen start X (standard AMOS lores)
hardY	0x32 (50)	Screen start Y (standard AMOS lores)
palette	from image CMAP	Array of Amiga 12-bit colours (0x0RGB integers)

Palette optimizer

The --optimize flag (or "optimize": true in the JSON) runs a greedy pairwise palette-swap search before compressing. It does not affect image appearance but can significantly reduce compressed size, at the cost of longer assembly time.

Image	Baseline	Optimized	Saving
320×200 32-colour scene	27 416 B	18 812 B	31%
320×200 16-colour level	17 248 B	13 242 B	23%
320×256 16-colour game	12 230 B	10 226 B	16%