.AI File Format – Prolog, setup and trailer

Document grammar

An Illustrator file is a PostScript document that follows the Document Structuring Conventions (see the DSC comments): a prolog and a script. Illustrator’s own operators are defined by procedure sets (procsets) that the prolog names; Illustrator’s native files do not contain the procsets themselves, which Adobe licensed separately and left largely undocumented. The native format is not an official EPS file; Illustrator EPS adds what EPS requires.

<document>  ::= <prolog> <script>
<prolog>    ::= %!PS-Adobe-M EPSF-N   (or %!PS-Adobe-M)
                <header comments>
                %%EndComments
                %%BeginProlog
                {<procset>}*
                %%EndProlog
<procset>   ::= %%IncludeResource: procset <name>
              | %%BeginResource: … %%EndResource
<script>    ::= <setup>
                {<layer>}* | {<object>}*
                {<page trailer>}
                <document trailer>
                %%EOF
<setup>     ::= %%BeginSetup
                {%%IncludeFont: font}*
                {<procset init>}*
                <font encoding>
                <gradient defs>
                <color palette>
                <pattern defs>
                <gradient defs>
                %%EndSetup
<procset init>     ::= <dict name>+ /initialize get exec
<page trailer>     ::= %%PageTrailer
                       gsave annotatepage grestore showpage
<document trailer> ::= %%Trailer
                       {<proc set termination>}*

The first line gives the DSC version, and optionally the EPSF version, that the file was written to, for example %!PS-Adobe-3.0 EPSF-3.0.

Kinds of comment

From version 5, new operators starting with X extend the format in a backward-compatible way: newer applications and the interpreter understand them; older applications ignore them.

Revisable and final form. Illustrator files, including Illustrator EPS, are revisable: they hold information needed for editing but not printing, such as unused gradient definitions. A final-form file holds only what printing needs; it can still be read, but relationships between objects may be lost.

Header comments

Most are optional. “Required” and “as necessary” are the specification’s own marks.

%%Creator: Adobe Illustrator(r) 6.0
%%For: (John Doe) (John Doe Design)
%%Title: (LayeredGradients.ai)
%%CreationDate: (5/9/96) (3:57 PM)
%%BoundingBox: 224 631 268 661
%%HiResBoundingBox: 224.065 631 268 660.5208
%%DocumentProcessColors: Cyan Magenta Yellow
%%DocumentCustomColors: (PANTONE 156 CV)
%%CMYKCustomColor: 0.5 0.5 0.5 0.5 (PANTONE 156 CV)
%%DocumentFonts: CooperBlack
%%+ Minion-Regular
%%DocumentNeededResources: procset Adobe_level2_AI5 1.0 0
%%+ procset Adobe_typography_AI5 1.0 0
%%+ procset Adobe_Illustrator_AI5 1.0 0
%AI5_FileFormat 2.0
%AI3_ColorUsage: Color
%AI3_TemplateBox: 306 396 306 396
%AI3_TileBox: 31 31 583 761
%AI3_DocumentPreview: None
%AI5_ArtSize: 612 792
%AI5_RulerUnits: 2
%AI5_ArtFlags: 1 0 0 1 0 0 0 1 0
%AI5_TargetResolution: 800
%AI5_NumLayers: 2
%AI5_OpenToView: -78 780 1 826 581 58 0 1 2 40
%AI5_OpenViewLayers: 77
%%EndComments

%AI5_ArtFlags

%AI5_OpenToView

%AI5_OpenToView: ulx uly zoom w h view_style ruler tiling ul_monx ul_mony grid snap_grid

The specification calls these twelve parameters, though the example it gives has ten values.

%AI7_GridSettings

%AI7_GridSettings: horiz_space horiz_subdiv vert_space vert_subdiv back style gridcolorR gridcolorG gridcolorB subdivcolorR subdivcolorG subdivcolorB

Colour separation set (6.0)

%%AI6_ColorSeparationSet: version magicNumber setname; version 1 (other versions are ignored), magic number ignored, set name reserved. Subcomments follow on %%+ lines as %%+ name: version magicNumber arguments…; a newer version of a subcomment may follow an older one and overrides it in programs that know it.

The specification’s introduction to the options subcomment writes it %%++ Options and its examples %%+ Options.

Ruler origin

Artwork, the bounding box, template box and tile box are written relative to the ruler origin, x to the right, y up, bounds in the order left, bottom, right, top. The template box is always centred on the artboard (a degenerate box at its centre when there is no template), so a reader can recover the ruler origin from the artboard size, in Macintosh coordinates with y down:

x = (artboard width  − templateBox.left − templateBox.right) / 2
y = (artboard height + templateBox.top  + templateBox.bottom) / 2

A program that does not care about Illustrator’s ruler can pick any convenient origin, such as a corner of the bounding box.

Setup section

Fonts and encodings

Illustrator re-encodes every font it uses. [encodingPairs TE sets the platform’s standard encoding (there is no closing bracket). Then each re-encoded font is defined:

%AI3_BeginEncoding: _Times-Roman Times-Roman
[ 39/foo 96/bar 128/bzaa/bazb
/_Times-Roman/Times-Roman 0 0 1 TZ
%AI3_EndEncoding AdobeType

• encodingPairs is code/name/name… runs: each code (0 to 255) is the first index to replace, and the following names fill successive indices.
• TZ forms: [ newFontName oldFontName direction fontScript useDefault TZ, the same with encodingPairs first, and both again with a final Multiple Master weight vector [w0 w1 … wn].
• direction: 0 horizontal, 1 vertical writing; present only in 3.x and the Japanese edition. fontScript: 0 Roman, 1 Japanese, 2 traditional Chinese, 3 Korean, 25 simplified Chinese. useDefault: 1 use the TE encoding, 0 not (for pi, non-Roman and TrueType fonts).
• The font type after %AI3_EndEncoding is AdobeType or TrueType. %%IncludeFont: names fonts Illustrator should look for; it substitutes another if missing.

Patterns

%AI3_BeginPattern: (patternname)
(patternname) llx lly urx ury [ <pattern layers> ] E
%AI3_EndPattern

Each pattern layer is (colordefinition) @ followed by (tiledefinition) &. The colour definition is a string holding overprint (O, R) and colour operators (g G k K x X); the tile definition is a string holding ordinary path objects, so a layer is limited to 64 KB. The first layer is the tile background; the _ operator fills the tile rectangle with the fill colour given to @. Patterns may hold paths and text but not placed files or graphs. Version 6 files bracket each tile layer’s contents with %AI6_BeginPatternLayer and %AI6_EndPatternLayer, and the specification’s example has an %AI3_Tile line before the first layer. (“Layers” here are paint order, not document layers.)

Gradients (5.0)

2 Bn
%AI5_BeginGradient: (Red & Yellow)
(Red & Yellow) 0 2 Bd
[
0 1 1 0 1 50 0 %_Bs
0 0 1 0 1 50 100 %_Bs
BD
%AI5_EndGradient

• nGradients Bn: number of gradients; required only when there are any.
• name type nColors Bd: type 0 linear, 1 radial.
• colorSpec colorStyle midPoint rampPoint %_Bs, one per colour (at least two). rampPoint is a percentage along the vector, strictly increasing; midPoint is 13 to 87, ignored for the last stop. Colour styles: 0 grey (1 value), 1 CMYK (4), 2 RGB (7: c m y k r g b), 3 CMYK custom (6: c m y k name tint), 4 RGB custom (10: c m y k r g b name tint type, type always 1).
• rampSpec… rampType %_Br, written for imaging (printing, EPS), after a [: types 0 grey (1 string), 1 CMYK (4), 2 one CMYK custom colour (5), 3 two CMYK custom colours (6), 4 RGB (7), 5 one RGB custom colour (8), 6 two RGB custom colours (9). Each is a hex string for image; a constant channel is written as one integer (01010101 as 1, FFFFFFFF as 255). Types 4 to 6 are new in 7.0.
• BD ends the definition.

Gradient instances, which paint objects, are on the operators page.

Colour palette (5.0)

Between %AI5_BeginPalette and %AI5_EndPalette: topLeftCellIndex selectedIndex Pb, then the cells, each started by Pc and followed by its paint (grey, CMYK, custom colour or a gradient instance), or Pn for a cell painted “none”; a final Pc, then PB. Cells are numbered from the upper left. Gradient origins in palette cells (such as -4014 4716) are meaningless and recomputed on use.

Global objects (7.0)

Patterns and gradients are global objects: named, defined in the setup, kept even when unused, and written to a print or EPS stream only when used. The name shown to the user is the part before a \n in the stored name. When a file brings a global object with the same name as one already open, the newer definition replaces the older.

Trailer

After %%Trailer, each procset initialised in the setup is terminated in reverse order, for example Adobe_Illustrator /terminate get exec, then %%EOF. A page trailer, when present, is %%PageTrailer followed by gsave annotatepage grestore showpage.

Macintosh resources

Save options: no preview without EPSF header; no preview with EPSF header; black-and-white PICT; colour PICT; IBM PC (TIFF), a binary file with a pointer to TIFF data (the DOS EPS header). The last four write the same data fork.

Illustrator 4 for Windows

Uses a subset of the header: %%Creator: Adobe Illustrator(TM) version, %%For, %%Title, %%CreationDate, %%DocumentProcSets: Adobe_Illustrator_version level revision, %%DocumentSuppliedProcSets:, %%DocumentFonts, %%BoundingBox, %%TemplateBox, plus four comments that replace the Macintosh resources:

In some versions text items in comments are PostScript strings; in others the last item runs to the end of the line.

Grid (Windows 4 and NeXT 3). %AI3_Grid.version hzSpace vtSpace gridSnap R G B visibility, just before %%EndProlog: version 0; spacing in display pixels from the page origin; gridSnap bits 0 horizontal snap, 1 vertical snap, 2 snapping enabled (values 0 to 7); RGB 0.0 to 1.0; visibility bit 0 grid in front, bit 1 grid visible (values 0 to 3).

Clipboard (AICB)

From Illustrator 5.0 and Illustrator 4 for Windows, art on the clipboard is a complete ASCII Illustrator file: clipboard type AICB on the Macintosh, and the registered clipboard format name ADOBE AI3 on Windows.

Source

• Adobe Illustrator File Format Specification – Adobe Technical Note LPS5007, 23 February 1998 (archived copy of Adobe’s partners site)