Draw glyphs and icons from one symbol format, by exact pixel area

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).

5 files changed, 102 insertions, 0 deletions

diff --git a/tests/check.py b/tests/check.py
index f0c495e..c3bcfe2 100644
--- a/tests/check.py
+++ b/tests/check.py
@@ -29,9 +29,15 @@
    text and row clicks, holds only the rows asked for around the focus, and
    every open page hears of each change. In headless Chromium (when node and
    chromium are there) the page itself takes keys, a click and typed text.
+10. Symbols: SVG icons (arcs, compact flags, holes, no viewBox) become
+   outlines of the right area, an icon asking for what symbols cannot hold is
+   refused by name, damaged files are refused whole, and (when DejaVu Sans is
+   there) glyphs keep their exact area and draw within a few levels of
+   FreeType's own coverage.
 """
 import argparse
 import json
+import math
 import os
 import random
 import shutil
@@ -902,11 +908,95 @@ def robustness(vega, aup, work, rounds):
     return len(examples), runs
 
 
+def symbols(tool, work):
+    """Build, read back and damage symbol files (docs/design/002-symbol-format.md)."""
+    icons = ROOT / 'tests' / 'symbols'
+    out = work / 'icons.vsym'
+    built = run(tool, 'build', '--svg', f'0xE002={icons / "rounded.svg"}', '--svg', f'0xE000={icons / "circle.svg"}',
+                '--svg', f'0xE001={icons / "frame.svg"}', '-o', out)
+    assert built.returncode == 0, built.stderr
+    info = json.loads(run(tool, 'info', out).stdout)
+    units = info['units']
+    square = (units / 16) ** 2  # an icon pixel, in square units
+    expected = {0xE000: math.pi * 49, 0xE001: 196 - 64, 0xE002: 196 - 16 + 4 * math.pi}
+    assert [s['code'] for s in info['symbols']] == sorted(expected), info
+    for symbol in info['symbols']:
+        area = abs(symbol['area']) / square
+        assert abs(area - expected[symbol['code']]) < 2e-3 * expected[symbol['code']], (symbol, expected)
+        assert symbol['bounds'] == [units // 16, units // 16, units * 15 // 16, units * 15 // 16], symbol
+        assert symbol['advance'] == units, symbol
+    refused = run(tool, 'build', '--svg', f'0xE000={icons / "turned.svg"}', '-o', work / 'turned.vsym')
+    assert refused.returncode != 0 and 'turned.svg' in refused.stderr and 'transform' in refused.stderr, refused.stderr
+    twice = run(tool, 'build', '--svg', f'0xE000={icons / "circle.svg"}', '--svg', f'0xE000={icons / "frame.svg"}',
+                '-o', work / 'twice.vsym')
+    assert twice.returncode != 0 and 'twice' in twice.stderr, twice.stderr
+
+    picture = work / 'circle.pgm'
+    assert run(tool, 'render', out, '0xE000', 32, picture).returncode == 0
+    header, pixels = picture.read_bytes().split(b'255\n', 1)
+    w, h = map(int, header.split()[1:3])
+    assert len(pixels) == w * h and abs(sum(pixels) / 255 - math.pi * 49 * 4) < 2, (w, h, sum(pixels) / 255)
+    assert run(tool, 'render', out, '0x41', 32, picture).returncode != 0  # no such symbol
+
+    good = out.read_bytes()
+    count = int.from_bytes(good[12:16], 'little')
+    records = 28
+
+    def damaged(name, data):
+        path = work / f'{name}.vsym'
+        path.write_bytes(bytes(data))
+        refused = run(tool, 'info', path)
+        assert refused.returncode != 0 and refused.stdout == '', (name, refused.stdout)
+        return refused.stderr.strip()
+
+    why = [damaged('short', good[:-1]), damaged('magic', b'VEGASYM2' + good[8:])]
+    swapped = bytearray(good)
+    swapped[records:records + 24], swapped[records + 24:records + 48] = good[records + 24:records + 48], good[records:records + 24]
+    why.append(damaged('order', swapped))
+    beyond = bytearray(good)
+    beyond[records + 8:records + 12] = (1 << 20).to_bytes(4, 'little')
+    why.append(damaged('beyond', beyond))
+    flagged = bytearray(good)
+    flagged[records + 22] = 1
+    why.append(damaged('flags', flagged))
+    counted = bytearray(good)
+    counted[12:16] = (count + 1).to_bytes(4, 'little')
+    why.append(damaged('count', counted))
+    assert len(set(why)) == len(why), why
+
+    report = {'icons': len(info['symbols'])}
+    font = Path('/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf')
+    if font.exists():
+        glyphs = work / 'ascii.vsym'
+        assert run(tool, 'build', '--font', font, '--codes', '0x20-0x7e', '-o', glyphs).returncode == 0
+        lines = run(tool, 'compare', font, glyphs, '--sizes', '12,16,32').stdout.splitlines()
+        for line in lines[1:]:
+            size = json.loads(line)
+            assert size['glyphs'] == 94 and size['mean_difference'] < 3 and size['within_16'] > 0.99, size
+        report['glyph_sizes'] = len(lines) - 1
+        try:
+            from fontTools.pens.areaPen import AreaPen
+            from fontTools.ttLib import TTFont
+        except ImportError:
+            return report
+        face = TTFont(font)
+        outlines, cmap = face.getGlyphSet(), face.getBestCmap()
+        shown = json.loads(run(tool, 'info', glyphs).stdout)
+        k = shown['units'] / face['head'].unitsPerEm
+        for symbol in shown['symbols']:
+            pen = AreaPen()
+            outlines[cmap[symbol['code']]].draw(pen)
+            assert abs(abs(pen.value) * k * k - abs(symbol['area'])) < 1, (chr(symbol['code']), pen.value, symbol)
+        report['exact_glyphs'] = len(shown['symbols'])
+    return report
+
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument('--vega', type=Path, required=True)
     parser.add_argument('--aup', type=Path, required=True)
     parser.add_argument('--serve', type=Path, required=True)
+    parser.add_argument('--symbols', type=Path)
     parser.add_argument('--seeds', type=int, default=8)
     parser.add_argument('--rounds', type=int, default=3)
     args = parser.parse_args()
@@ -937,6 +1027,8 @@ def main():
         report['stored_surface_lines'] = stored_surfaces(vega, modules['todo'], work, range(args.seeds))
         report['convention_findings'] = conventions(vega, aup, work, modules)
         report['examples'], report['robustness_runs'] = robustness(vega, aup, work, args.rounds)
+        if args.symbols:
+            report['symbols'] = symbols(args.symbols.resolve(), work)
     print(json.dumps(report))
 
 
diff --git a/tests/symbols/circle.svg b/tests/symbols/circle.svg
new file mode 100644
index 0000000..3a6debd
--- /dev/null
+++ b/tests/symbols/circle.svg
@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <path d="M 8 1 A 7 7 0 1 1 8 15 A 7 7 0 1 1 8 1 Z" fill="#2e3436"/>
+</svg>
diff --git a/tests/symbols/frame.svg b/tests/symbols/frame.svg
new file mode 100644
index 0000000..ce1cfb2
--- /dev/null
+++ b/tests/symbols/frame.svg
@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+  <path d="M1 1 H15 V15 H1 Z M4 4 V12 H12 V4 Z"/>
+</svg>
diff --git a/tests/symbols/rounded.svg b/tests/symbols/rounded.svg
new file mode 100644
index 0000000..880765d
--- /dev/null
+++ b/tests/symbols/rounded.svg
@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path d="M3 1h10a2 2 0 012 2v10a2 2 0 01-2 2H3a2 2 0 01-2-2V3a2 2 0 012-2z"/></svg>
diff --git a/tests/symbols/turned.svg b/tests/symbols/turned.svg
new file mode 100644
index 0000000..9221891
--- /dev/null
+++ b/tests/symbols/turned.svg
@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+  <path d="M1 1 H15 V15 H1 Z" transform="rotate(45 8 8)"/>
+</svg>