Module Hugin_vg_raster

Draws renderables as pixels: an RGBA tensor with render, a PNG file with png. A density, in device pixels per point, fixes the size of a pixel on the page: at density 2., a page of 360 by 240 points is 720 by 480 pixels, each pixel half a point wide.

Accuracy

Each pixel approximates the average over its square of what the picture paints, as Semantics defines it:

• Fills, glyphs and clips are antialiased by exact coverage: a pixel takes the fraction of the colour equal to the fraction of its square that the area covers. Curves are first flattened to within a tenth of a pixel, and glyphs are drawn from their outlines, without hinting.
• A clipped primitive takes in each pixel the product of its coverage and the clip's. One whose box, as Hugin_vg.Picture.bounds takes it, does not meet the boxes of the paths of the clips above it, or only touches them, to within rounding, paints nothing.
• A stroke's outline is covered as the union of the pieces it is made of, one per segment, join and cap, with round ones flattened likewise. A pixel that the edges of two overlapping pieces cross takes the sum of their coverages, up to the whole pixel. A point of a subpath within a twentieth of a pixel, along both axes, of the point kept before it is dropped, so an open subpath or a dash shorter than that is drawn as its caps alone. Dashes are cut from the points a subpath keeps, so their lengths leave out the arc through dropped points, a loss of second order on a smooth curve, and a dash's end within that distance of a kept point moves to it. A solid closed subpath that would keep fewer than three points keeps all of them but exact repeats instead, so that it has the extent of its pen at any size. A dash pattern whose period is below a thousandth of a pixel is drawn solid.
• Primitives are composited one after another, each against what the previous left. Two shapes that abut along an edge thus each cover part of the pixels on the edge, and let a little of the backdrop through there, as in browsers and PDF viewers.
• Compositing is source-over on the encoded components, premultiplied by alpha, in single-precision floating point. Components are rounded to 8 bits once, when the image is read out, so that translucent primitives drawn over one another reach the colour they composite to: up to a thousand fills at any opacity from 0.02 land within a level of it.
• An image paints a pixel with the image pixel under the pixel's centre, so its edges are not antialiased; a centre on the edge of the image, to within rounding, is under it. Where an image is shown smaller than its pixels, a pixel averages the image pixels under k by k points spread evenly over its square, k being the ratio of image pixels to device pixels rounded up, at most image_samples.
• An opacity is drawn into a layer of its own, then composited as one primitive.
• The instances of a stamp are placed at their positions rounded to the nearest quarter of a pixel in each direction, an error of at most an eighth of a pixel.
• A picture under transforms whose composition, with the density, has no inverse or overflows the range of floats paints nothing.
• Tags are ignored.

Rendering