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vega pixels runs the same scripts as tui --dump and writes each frame as
an image: the view's cells (only those on screen are evaluated) become
glyphs, each drawn once per size with the exact area of its curves, in its
tone's colour on its row's background. No page, no scene in between. A
1920x1350 grid takes 12 ms a frame here, writing the image included.
Symbol files carry the font's line now (format 3: ascent and descent where
format 2 kept zeros), the cell height and baseline; and the build tool takes
fonts in order, later ones filling the codes earlier ones lack: a CJK font
alone drew ambiguous-width arrows two cells wide in a one-cell grid, which
the new check caught, so a monospace Latin font comes first and Hangul after.
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Each size's symbols are drawn on the CPU into one 8-bit atlas, every pixel
the exact area its curves cover, and text is a quad per glyph reading it.
A 1080p page of 11,382 glyphs composes in 1.07 ms at 16 px (it took 525 ms
computing coverage per fragment), and the composed pixels are the CPU
raster's exactly (worst difference 0 for DejaVu's ASCII, 1 for Noto's
11,172 Hangul syllables).
Computing coverage on the GPU lost on both counts: a compute pass filling
the atlas took 5-13x longer than one Cortex-A76 core (35 against 2.7 ms at
16 px), and its faster sorting-network form computed whole rows wrongly
under this Mesa V3D. Coverage now has one implementation, in C, and the
shaders only compose. vega_symbol_raster takes a row stride to draw into
the atlas in place.
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src/ held two parts, the surfaces that show modules to people and the
symbol format with its CPU and GPU drawing. As in Pure (design 009), top
level directories are now the parts, and the README says what each builds.
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OpenGL ES 3.1: the file's points go up as one storage buffer, each symbol
gets 16 row bands listing the curves they reach (cut by integer division of
doubled coordinates, so CPU and shader agree), and every pixel of a symbol's
box sums the exact area of its bands' curves, as vega_coverage does.
vega-symbols gpu draws every symbol against the CPU reference and times
1080p pages.
On the Pi 5's V3D it matches the reference (mean 0.001-0.1 levels, worst
4) but takes 142-757 ms a page, 83% of it in the area function. A sorting
network in registers made it 2.3x faster yet computes wrong rows in the
fragment stage with this driver, while the same code is exact in a compute
shader; so drawing each glyph size once into an atlas by compute comes next.
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Format 2 (VEGASYM2) writes each contour as a chain of 4-byte points, on or
off the curve as in TrueType: endpoints are shared, lines have no control
point, and two off points imply the on point between them. A curve is still
had from one point and its two neighbours, so a GPU can point at curves by
point index and upload the table as it is. Cubics (CFF, SVG curves and arcs)
become quadratic splines the cu2qu way, joins left implied, within a unit.
Files are about half of format 1: 1.3-1.4x a TrueType font's glyf (the
whole subset font's size), 2.0-2.9x CFF, 2.6x smaller than the SVG icons.
Opening checks each symbol's first and last point instead of every curve:
0.34 ms rather than 19 ms for 11,172 Hangul syllables. Points decode
without branching on their flags (9.7 ns a point), still 1.2-10x cheaper
than FreeType's outline.
Glyph advances were half their width since outlines are doubled for
FreeType's implied points; they now match hmtx, and the check says so.
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Symbol files (VEGASYM1) hold glyphs and icons as quadratic curves, verified
once per symbol and then read in place. vega-symbols builds them from fonts
(through FreeType, losslessly: twice the font's units keeps TrueType's
implied points whole) and from solid SVG paths (arcs included), refusing by
name what an icon asks for that symbols cannot hold.
The reference rasterizer computes each pixel's covered area exactly: every
curve's integral of clamp(1 - x) dy inside the pixel, split where it crosses
the pixel's sides. The Slug-style two-ray average first tried missed FreeType
by 15.4 levels on average at 12 px; exact area is within 1.2-3.5 at 8-16 px,
and where it differs most it matches 64x64 supersampling, not FreeType.
Reaching an outline costs 4.6-29x less than through FreeType, but the files
are 2.1-6.6x larger than glyf or CFF: design 002 has the numbers and the next
question (points shared along contours, lines without control points).
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