Drawing a page¶
render turns a page into pixels. It
reads with reader, rasterises with go-gfx,
and outlines text with go-opentype
— so it builds for js/wasm and a PDF can be shown in a browser tab with
nothing on the far end.
Options takes a Scale or a DPI, a background colour, and a MaxPixels
that refuses a page a file made up rather than filling memory with it.
What it draws¶
The transform stack, path construction and all ten painting operators; filling under both winding rules; stroking with the caps, joins, miter limit and dash pattern the graphics state carries; clipping, including clips that intersect and are let go again; the device, calibrated, profile-based, indexed, separation and pattern colour spaces; constant transparency; form XObjects with their own matrix, box and resources.
Images at every bit depth and colour space, with a /Decode array, as
one-bit stencils painted in the colour in force, and with either kind of
transparency. A JPEG is decoded by the standard library; a format nothing
here reads is left undrawn rather than drawn wrong.
Text from an embedded TrueType, OpenType, bare CFF or PostScript Type 1 program, from a composite font addressed by character identifier, or — for a Type 3 font, whose glyphs are little content streams — by running them. A font whose program cannot be read still advances the pen by its stated widths, so what follows stays where it belongs.
Gradients and patterns: axial and radial shadings painted on their own with
sh or used as a fill or a stroke; tiling patterns, coloured and uncoloured;
and all four kinds of PDF function, which are also what a Separation or
DeviceN tint transform is written in.
And the mesh shadings, all four kinds, which is how a plotting tool writes a surface: free-form and lattice-form triangles, each corner its own colour, and Coons and tensor patches, whose four sides are cubic curves. A patch is drawn by cutting it into a grid of little quadrilaterals — the inside of a Coons patch follows from its twelve boundary points, and a tensor patch says four more — and each triangle is filled by mixing the colours at its corners across it. A patch may carry on from the one before it, sharing an edge and two of its colours.
A pattern is placed in the space of the content that names it: the page's own space at the top level, and inside a form the form's — its matrix and the transform that drew it both count. Nearly every figure a plotting tool writes is a form, so a pattern that stayed at the page's origin would miss the shape it was asked to fill by the width of a page.
How it is checked¶
By writing a PDF whose geometry is known and looking at the pixels: a rectangle from (10,10) to (40,40) on a hundred-point page has to be ink at (25,75) and paper at (25,55), which is what says the page's coordinates — which count up from the bottom left — were mapped the right way up.
Then on the corpus: 4 108 pages drawn from 3 999 files with no panics and six blank. When anything underneath changes, every page is drawn before and after and its pixels hashed, and the biggest changes are put beside what the operating system's own renderer draws.