Where vector quietly becomes pixels

The effects that are not vector at all

Some things in a vector application are geometry. Some are pictures generated on the fly. The second group has a resolution, and if you do not set it, the software picks one for you, often badly.

Raster by nature, in every program:

• Drop shadow, outer glow, inner glow
• Gaussian blur and feather
• Most texture, grain and distortion filters
• Gradient mesh (Illustrator) and mesh fills (Inkscape, CorelDRAW)
• Anything under Illustrator’s Effect ▸ Photoshop menu

Compositing, which is not raster but needs flattening in older workflows:

• Any object below 100% opacity
• Blend modes: multiply, screen, overlay, soft light
• Transparency inside a placed PDF or PSD

The 72 ppi trap

Illustrator has a document setting called Raster Effects Resolution. For years the default on a new document was Screen (72 ppi). Every drop shadow, every feather, every blur in that document is generated at 72 ppi.

You will not see it. On screen at 100% it looks perfect, because your screen is roughly 72 to 110 ppi. It prints as a visibly chunky, stepped shadow.

Fix it on day one: Effect ▸ Document Raster Effects Settings ▸ 300 ppi for print, and check it again whenever you open somebody else’s file. Inkscape’s equivalent is in Document Properties (default export dpi and filter quality); CorelDRAW sets it per-effect in the bitmap resolution dropdown.

What flattening actually does

A press workflow that cannot handle live transparency (PDF/X-1a, EPS, PostScript, older RIPs) must flatten it. The flattener does something surprising:

1. It finds every region where a transparent object overlaps something.
2. It divides the artwork into atomic regions: areas that can each be described with a single opaque appearance.
3. Regions that can be expressed as vectors stay vector. Regions that cannot (because a gradient is blending through a blur, say) are rasterised at the flattener’s chosen resolution.
4. It reassembles the page from those pieces.

Two well-known artefacts come straight out of step 3 and 4.

The grey or white box behind a logo. The rasterised region is rectangular, it carries the background it was composited against, and if the background in the file is not the background on the paper, the rectangle shows.

The hairline seam. Two adjacent atomic regions share an edge. One is drawn as vector, one as a raster tile. Their anti-aliasing does not agree exactly along that edge, so a light line one pixel wide appears across your artwork: visible on screen at some zoom levels and, worse, sometimes on the printed sheet.

Illustrator’s Flattener Preview panel (Window ▸ Flattener Preview) shows you which regions will rasterise before you export. Acrobat Pro’s Output Preview has a transparency flattening view. Both are worth a look before a big job.

How to avoid the whole problem

• Export PDF/X-4 where the shop can take it. X-4 keeps transparency live all the way to the RIP, which does the flattening once with full knowledge of the device. No atomic regions in your file, no seams.
• Ask. “Do you want X-1a or X-4?” is one line in an email and it settles this.
• Build flat where you can. A shadow made from a solid shape at a lower tint is not transparency and never flattens. A “gradient” built as five flat steps in a screen-printed job is a deliberate decision, not a limitation.
• Rasterise on purpose. If an element needs a blur, rasterise it yourself at 300 ppi with a known background, and place it. You now control the resolution and the edge rather than discovering them later.

The limitation

Resolution independence is a property of geometry, not of the file extension. An .ai file whose hero image is a gradient mesh with a Gaussian blur over it is, for practical purposes, a raster file in a vector wrapper. It will not enlarge cleanly and it will choke a plotter.

The test is not what the file is called. It is what is inside: select everything, look at the Document Info or Objects panel, and see whether the count is paths or effects.