Adobe Illustrator

An Illustrator file — .ai, or the .eps Illustrator exports — is not one format but three, and which one you have decides how much of the document survives being opened somewhere other than Illustrator.

Illustrator arrived alongside the PostScript printer in the late 1980s, and its early files are PostScript programs. With version 9 the native format became a PDF carrying the editable Illustrator document alongside it.

Paintapp reads all three the way Illustrator itself reopens them: not by drawing the picture the file carries for printing, but by reading Illustrator's own private syntax, where the layers, groups, editable paths, gradients and gradient meshes still exist.

Capabilities

Feature	Details
Opening `.ai` files	All three containers
Layers, groups and object names	With layer visibility
Editable paths	Anchors and Bézier handles
Compound paths	Kept as single objects
Gradients	Linear and radial, with stop midpoints
Gradient meshes	Free-form shading over a lattice of colored anchor points
Document color mode	Taken from the file's own declaration
Embedded ICC profiles	Verified before use
XMP metadata	Shown in File Info
Artboards	First artboard only

An Illustrator .eps that carries no editable data is run as PostScript instead; see EPS and PostScript for what that half supports.

Limitations

Limitation	Result
Text	Type objects skipped entirely
Placed and embedded images	Recognized but not drawn
Pattern fills	Not read
Object opacity and blending	Everything opens opaque
Clipping masks	Not applied; masked artwork drawn in full
Dashes, caps, joins and miter limit	Dropped; only stroke width survives
Spot and named-swatch flat fills	Not read; process fills unaffected
Gradients with a named-color stop	Gradient dropped rather than approximated
Spot-ink gradient meshes	Refused, with the ink named
Multi-artboard documents	Only the first artboard opens
One shorthand curve form	Slightly approximated shape
Illustrator export	Not supported

Everything on this page describes reading; there is no .ai or .eps export.

The Three Containers

This is the most useful thing to know about the format, because everything else follows from it.

Variant	What the file really is	Where the editable document lives
Pre-v9 `.ai`	A PostScript program written in Illustrator's own operator set.	The whole file — the program is the document.
v9+ `.ai`	A PDF. The page renders the artwork for anything that reads PDF.	A private stream inside the PDF, deflate- or Zstandard-compressed.
Illustrator `.eps`	Both at once: printable PostScript, then a complete second copy of the same document. The artwork is in the file twice.	Behind the printable half, encoded as text and compressed.

Both compressed forms are handled either way round. A .ai states which compression it used; the EPS variant states nothing, so it is identified from the data itself.

Read It, Don't Run It

Running the PostScript gives you a picture. Reading the Illustrator syntax gives you a document. Both halves draw the same page, but Adobe's printing procedures flatten layers, groups, compound paths, live gradients and separations into anonymous marks. Only the private half still knows that this group is called "Layer 1", or that this fill is a gradient mesh rather than a few hundred small colored patches.

So wherever Illustrator's own syntax is present, it is read and the printable half is not run. The PostScript interpreter is used only when a file genuinely carries nothing else — saved with "Preserve editing capabilities" turned off. If private data is present but unreadable, Paintapp warns and falls back to drawing the printable half; an unrecognized operator is reported once, by name, rather than failing the file.

Thumbnails follow the same rule: an Illustrator EPS is thumbnailed from the editable document, not from its embedded preview, which is a dithered indexed raster and showed up blank. See Previews on the EPS page.

Page Size

A document opens at the size of its artboard, not its bounding box — the bounding box says where the marks are, not where the page is, and opening on it crops every document to its own artwork. Objects on the pasteboard keep their true position outside the page.

A file saved empty still opens at its stated artboard. Only a file that states no artboard and contains nothing falls back to a US Letter page. Recent Illustrator versions also state a 200-inch scratch canvas beside the real artboard; the two are not confused.

Color and Document Mode

Fills and strokes are read in the model the file wrote them in — gray stays gray, CMYK stays CMYK — and nothing is converted on the way in. Any conversion that is genuinely needed happens later, through a real profile (see Color Management).

A document is either RGB or CMYK, and the file declares which; Paintapp takes that declaration, so a CMYK file opens as a CMYK document. Two traps:

The list of inks is not the mode. Illustrator also records which plates the artwork uses, and that disagrees with the declared mode in both directions.
No declaration means CMYK. Such a file predates Illustrator 9, when a document had no RGB mode to be in.

The mode is not only a label: Illustrator often states a color twice, once as CMYK and once as RGB, and the document's mode decides which half is read. That is what keeps a flat fill and a gradient stop of "the same" color agreeing inside one file.

Illustrator writes its entire default swatch library — sixteen gradients, "Golden Ring", "Sky", "Summer" and the rest — into every file it saves. A file containing gradient definitions is not evidence that anything in it is painted with a gradient.