Every Gerber file, however complex the board, is drawn with a surprisingly small kit of tools. Those tools are called apertures, and understanding them unlocks a lot of otherwise mysterious viewer behaviour: why one pad looks crisp and another looks wrong, why a file’s statistics list shapes you never placed, and what the aperture-usage report in Gerbv is actually telling you. An aperture is best pictured as a stencil or a pen tip: the file’s drawing commands move apertures along paths to draw tracks, or stamp them at single points to make pads. This article covers the standard shapes, the two ways they are used, and how to inspect them. If terms are new, what is a Gerber file is the gentle starting point, and the Gerber viewer hub plus the best free Gerber viewers roundup cover the tools that display all of this.
Draws and flashes: the two moves
Apertures are used in exactly two ways. A draw sweeps the aperture along a line or arc, like dragging a pen: a round aperture drawn along a path makes a track with rounded ends, of the aperture’s width. A flash stamps the aperture once, at one coordinate, like a rubber stamp: that is how pads are made — the aperture’s shape becomes the pad’s shape exactly. Much of Gerber literacy is seeing a board in these two moves: tracks are draws, pads are flashes, and large copper areas are built either from many draws or from filled outlines the format calls regions. When something renders oddly, asking “is that a draw or a flash, and with what aperture” usually localises the problem.
The standard shapes
The format defines a small family of standard apertures, each described by a few parameters in the file’s aperture definitions. The circle is the workhorse — a diameter, optionally with a hole in the middle — used for round tracks ends, vias and round pads. The rectangle adds width and height and is the natural stamp for rectangular surface-mount pads, again optionally with a hole. The obround — a rectangle with semicircular ends, like a stadium outline — suits oval pads and slot-like features. A regular polygon form exists for the rarer cases that need one. These four cover the overwhelming majority of what real boards contain, and each is a complete, precise description: shape plus dimensions, defined once in the file header and then referenced by number everywhere it is used.
Macros: when standard shapes are not enough
Some shapes refuse to be a circle or a rectangle: a thermal relief pattern in a ground plane, a crosshair target, a pad with a notch. For those, the format provides aperture macros — small recipes that build a custom shape from primitive elements, with parameters the file can set per use. A macro is defined once and instantiated with different sizes, which keeps files compact and shapes consistent. In the viewer, macros are invisible as a mechanism — you simply see the resulting shape — but they explain why a statistics report may list a macro where you expected a simple pad, and why a very old or minimal tool in the chain occasionally renders a custom shape differently from a full viewer. In Gerbv the shapes render as specified; the macro machinery only becomes your concern when a shape on screen does not match the design, and its definition is the first place to look.
Reading the aperture-usage report in Gerbv
Gerbv opens report windows with statistics about the Gerber codes in use, including an aperture-usage view: which apertures a layer uses, and how often each is flashed or drawn. This is a surprisingly practical audit. A pad aperture flashed a plausible number of times agrees with your component count; an aperture used exactly once in a design with no one-off part is a question worth asking. A track-width aperture that appears in the report but not in your design rules points at an export setting. And a layer whose report lists no pad apertures at all is not a copper layer, whatever its filename claims — one of the fastest content checks available when a set arrives with cryptic names. Pair the report with the visual stack from the download page build and the identification routine in loading and aligning layers, and few layer mysteries survive. The Gerber viewer hub puts this in the overall checking flow.
Why apertures matter beyond curiosity
Apertures are where design intent meets manufacturing reality. The fab’s equipment reproduces the shapes your file defines — a pad that is the wrong aperture is the wrong pad on every board in the batch, rendered faithfully. Fine features push the other way: an aperture smaller than the process handles becomes a conversation with the fab, not a viewer problem. You cannot edit your way out of either in a viewer, and Gerbv does not pretend otherwise — but seeing the shapes, and reading the report of how they are used, is how you catch the problem while it is still a file and not a shipment.
Frequently asked questions
What is an aperture in a Gerber file?
An aperture is a named shape — defined once with its dimensions — that the file’s commands use to draw the board: swept along a path to draw a track, or flashed at a point to stamp a pad. Circles, rectangles, obrounds and custom macros are the standard kit.
What is the difference between a draw and a flash?
A draw moves the aperture along a line or arc, producing a stroke of the aperture’s width — that is how tracks are made. A flash stamps the aperture once at a coordinate, producing a shape identical to the aperture — that is how pads are made.
What does Gerbv’s aperture-usage report tell me?
It lists which apertures a file uses and how often each appears. That turns into a quick audit: pad counts that match your components, unexpected one-off shapes worth questioning, and missing aperture types that betray a mislabelled layer.