plotter/tests/test_primitives.cpp

// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

// CPU-side contract of the render layer: the staging batches that feed the
// instanced line and text pipelines, and the embedded font's metrics and atlas.
// Nothing here touches GL, so it runs on any machine that can build the tests.
#include "harness.hpp"

#include <plot/render/detail/text_scale.hpp>
#include <plot/render/embedded_mono_font.hpp>
#include <plot/render/primitives.hpp>

#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <limits>
#include <string>
#include <sys/wait.h>
#include <vector>

PLOT_TEST(line_batch_emits_two_verts_per_segment) {
  plot::render::LineBatch b;
  b.begin();
  const plot::Vec2 pts[3] = {{0, 0}, {1, 1}, {2, 0}};
  b.add_polyline(pts, 3);                 // 2 segments
  PLOT_CHECK(b.vertex_count() == 4);
  b.clear();
  PLOT_CHECK(b.vertex_count() == 0);
  PLOT_CHECK(b.data().capacity() >= 4);   // clear() must not free capacity
  b.add_segment({0, 0}, {1, 1});
  PLOT_CHECK(b.vertex_count() == 2);
}

PLOT_TEST(line_batch_stages_segment_endpoints) {
  plot::render::LineBatch b;
  b.begin();
  b.add_segment({1.0f, 2.0f}, {3.0f, 4.0f});
  PLOT_CHECK(b.data().size() == 4);       // x0,y0,x1,y1 for the one instance
  PLOT_CHECK_NEAR(b.data()[0], 1.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[1], 2.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[2], 3.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[3], 4.0, 0.0);

  // A polyline stages one instance per consecutive pair, in order.
  const plot::Vec2 pts[3] = {{0.0f, 0.0f}, {1.0f, 0.0f}, {1.0f, 1.0f}};
  b.begin();
  b.add_polyline(pts, 3);
  PLOT_CHECK(b.data().size() == 8);
  PLOT_CHECK_NEAR(b.data()[2], 1.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[4], 1.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[5], 0.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[6], 1.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[7], 1.0, 0.0);
}

// `add_polyline_xy` takes the layout `Series::gather` appends to, so this pins
// that a gathered window becomes the segments it says it does -- an off-by-one in
// the stride here would draw a diagonal where the data has a corner.
PLOT_TEST(line_batch_stages_interleaved_points) {
  plot::render::LineBatch b;
  b.begin();
  const plot::f32 xy[8] = {0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.0f};
  b.add_polyline_xy(xy, 4);  // 3 segments
  PLOT_CHECK(b.segment_count() == 3);
  PLOT_CHECK(b.vertex_count() == 6);
  PLOT_CHECK(b.data().size() == 12);
  PLOT_CHECK_NEAR(b.data()[0], 0.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[1], 1.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[2], 2.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[3], 3.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[10], 6.0, 0.0);
  PLOT_CHECK_NEAR(b.data()[11], 7.0, 0.0);
  // Fewer than two points, or none: nothing to draw, and nothing to crash on.
  b.clear();
  b.add_polyline_xy(xy, 1);
  b.add_polyline_xy(nullptr, 4);
  PLOT_CHECK(b.vertex_count() == 0);
}

PLOT_TEST(line_batch_ignores_degenerate_polylines) {
  plot::render::LineBatch b;
  b.begin();
  const plot::Vec2 one[1] = {{5.0f, 5.0f}};
  b.add_polyline(one, 1);                 // no pair -> no instance
  b.add_polyline(nullptr, 0);
  PLOT_CHECK(b.vertex_count() == 0);
  PLOT_CHECK(b.data().empty());
  b.add_polyline(nullptr, 3);             // null with a count: still no deref
  PLOT_CHECK(b.vertex_count() == 0);
}

PLOT_TEST(line_batch_reuses_capacity_across_frames) {
  plot::render::LineBatch b;
  const plot::Vec2 pts[5] = {{0, 0}, {1, 1}, {2, 0}, {3, 1}, {4, 0}};
  b.begin();
  b.add_polyline(pts, 5);                 // 4 segments -> 16 floats
  const std::size_t cap = b.data().capacity();
  const f32* store = b.data().data();
  PLOT_CHECK(cap >= 16);

  b.clear();
  PLOT_CHECK(b.vertex_count() == 0);
  PLOT_CHECK(b.data().capacity() == cap);  // clear() keeps the store

  b.begin();
  b.add_polyline(pts, 5);                  // same workload: must not reallocate
  PLOT_CHECK(b.data().size() == 16);
  PLOT_CHECK(b.data().capacity() == cap);
  PLOT_CHECK(b.data().data() == store);
}

namespace {

using plot::render::EmbeddedMonoFont;

/// The coverage byte of one texel of `slot`'s cell: 0 background, 255 full ink.
plot::u8 cell_texel(const EmbeddedMonoFont& font, plot::u32 slot, plot::u32 col, plot::u32 row) {
  const plot::Vec2 origin = EmbeddedMonoFont::atlas_origin(slot);
  const plot::u32 x = static_cast<plot::u32>(origin.x) + col;
  const plot::u32 y = static_cast<plot::u32>(origin.y) + row;
  const std::size_t index = static_cast<std::size_t>(y) * font.atlas_width() + x;
  return font.atlas_image()[index];
}

/// Total coverage of one cell: zero means the cell rasterised blank.
plot::u32 cell_ink_units(const EmbeddedMonoFont& font, plot::u32 slot) {
  plot::u32 total = 0u;
  for (plot::u32 y = 0; y < EmbeddedMonoFont::cell_height(); ++y) {
    for (plot::u32 x = 0; x < EmbeddedMonoFont::cell_width(); ++x) {
      total += cell_texel(font, slot, x, y);
    }
  }
  return total;
}

/// The fallback slot, found by the marker codepoint rather than assumed.
plot::u32 fallback_slot() {
  for (plot::u32 slot = 0; slot < EmbeddedMonoFont::glyph_count(); ++slot) {
    if (EmbeddedMonoFont::codepoint_of_slot(slot) == EmbeddedMonoFont::kFallbackCodepoint) {
      return slot;
    }
  }
  return EmbeddedMonoFont::glyph_count();
}

/// The plot library's own source root, so the generator test can run the committed
/// script against the committed output. Injected by tests/CMakeLists.txt.
constexpr const char* kSourceDir = PLOT_SOURCE_DIR;

}  // namespace

// The embedded atlas' metrics: a line taller than its ascent (descenders need the
// room), a non-empty advance, and -- the point of the face -- a fixed advance, so
// a label's width is its length and nothing else. A text backend with variable
// advances is the tile the plot library deliberately does not lay out (see the
// spec's non-goals), so `i` and `W` measure the same and a caller cannot be
// surprised by a label's reserved strip.
PLOT_TEST(embedded_font_metrics_are_sane) {
  const EmbeddedMonoFont f;
  PLOT_CHECK(f.height() > f.ascent());
  PLOT_CHECK(f.ascent() > 0.0f);
  PLOT_CHECK_NEAR(f.measure(""), 0.0, 1e-6);
  PLOT_CHECK_NEAR(f.measure(nullptr), 0.0, 1e-6);
  // One advance per codepoint, whatever the glyph: this is `i` and `W` alike.
  PLOT_CHECK_NEAR(f.measure("i"), f.measure("W"), 1e-6);
  PLOT_CHECK_NEAR(f.measure("i"), EmbeddedMonoFont::advance_codepoint('i'), 1e-6);
  // Monotone in length, and exactly the sum of the advances it stages.
  PLOT_CHECK(f.measure("ab") < f.measure("abc"));
  PLOT_CHECK(f.measure("abc") < f.measure("abcd"));
  PLOT_CHECK_NEAR(f.measure("12.5 ms"),
                  (f.glyph('1')->advance + f.glyph('2')->advance + f.glyph('.')->advance +
                   f.glyph('5')->advance + f.glyph(' ')->advance + f.glyph('m')->advance +
                   f.glyph('s')->advance),
                  1e-6);
  // The em is the size the table was baked at, and the scale the renderer draws
  // with is a ratio of it -- so it must be a positive number and not `height()`.
  PLOT_CHECK_NEAR(f.em_px(), static_cast<plot::f32>(EmbeddedMonoFont::kEmPx), 1e-6);
  PLOT_CHECK(f.em_px() > 0.0f);
}

// Every codepoint the atlas claims to cover has its own cell, the cell is inside
// the atlas and is the declared cell, and its ink is non-empty -- the generator
// refuses to emit a blank cell for a covered codepoint, and this pins that the
// committed atlas is the generator's output and not a hand edit. U+0020 is the
// one documented blank: a space has no ink by definition and still advances.
PLOT_TEST(embedded_atlas_covers_every_expected_codepoint) {
  const EmbeddedMonoFont f;
  const plot::Vec2 cell = f.cell_uv();
  PLOT_CHECK_NEAR(cell.x, static_cast<plot::f32>(EmbeddedMonoFont::cell_width()) /
                             static_cast<plot::f32>(f.atlas_width()),
                  1e-9);
  PLOT_CHECK_NEAR(cell.y, static_cast<plot::f32>(EmbeddedMonoFont::cell_height()) /
                             static_cast<plot::f32>(f.atlas_height()),
                  1e-9);
  PLOT_CHECK(f.atlas_pixels() != nullptr);

  std::vector<plot::u32> expected;
  for (plot::u32 cp = 0x20u; cp <= 0x7Eu; ++cp) expected.push_back(cp);
  for (plot::u32 cp : {0x00B0u, 0x00B5u, 0x03A9u, 0x2013u, 0x2014u}) expected.push_back(cp);
  for (plot::u32 cp : expected) {
    const plot::u32 slot = EmbeddedMonoFont::glyph_index(cp);
    PLOT_CHECK(slot < EmbeddedMonoFont::glyph_count());
    PLOT_CHECK(EmbeddedMonoFont::codepoint_of_slot(slot) == cp);
    const plot::render::Glyph* g = f.glyph(cp);
    PLOT_CHECK(g != nullptr);
    // The rect is the atlas cell, and it lies inside the atlas.
    PLOT_CHECK(plot::render::glyph_is_cell(*g, cell));
    PLOT_CHECK(g->u0 >= 0.0f && g->u1 <= 1.0f && g->v0 >= 0.0f && g->v1 <= 1.0f);
    if (cp == 0x20u) {
      PLOT_CHECK(cell_ink_units(f, slot) == 0u);  // a space is blank ...
      PLOT_CHECK(g->advance > 0.0f);              // ... and still advances
    } else {
      PLOT_CHECK(cell_ink_units(f, slot) > 0u);   // a covered glyph has ink
    }
  }

  // The fallback box is a real cell with real ink, at a fixed slot, and every
  // codepoint outside the coverage set draws it -- so a label the atlas cannot
  // spell is visible rather than silently blank.
  const plot::u32 fallback = fallback_slot();
  PLOT_CHECK(fallback < EmbeddedMonoFont::glyph_count());
  PLOT_CHECK(cell_ink_units(f, fallback) > 0u);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x4E00u) == fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0xFFFFu) == fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(plot::render::kInvalidCodepoint) == fallback);
  PLOT_CHECK(f.glyph(0x4E00u)->advance > 0.0f);
}

// The *shipped table* -- not the generator, and not a lookup -- is what the
// renderer uploads, so the coverage and quality contract is asserted on it
// directly rather than only on the codepoint lookups above. Slot by slot, the
// atlas holds exactly the documented codepoints in order, every cell is the one
// declared size and lies inside the image, every cell but the space carries ink,
// and the fallback box is a real inked glyph at its fixed final slot.
//
// It is the case the drift guard beside it cannot replace: `--check` compares the
// committed bytes to a bake made with whatever coverage list the generator
// currently holds, so dropping a codepoint from that list keeps it green. These
// assertions are what make a dropped codepoint (say U+00B5) or a blank cell fail
// loudly, on the data that actually ships.
PLOT_TEST(embedded_atlas_matches_its_documented_coverage) {
  const EmbeddedMonoFont f;
  const plot::Vec2 cell = f.cell_uv();

  // The documented coverage set, in the documented slot order: printable ASCII,
  // the three codepoints `plot::format_si` emits plus the two label dashes, then
  // the fallback box appended last.
  std::vector<plot::u32> documented;
  for (plot::u32 cp = 0x20u; cp <= 0x7Eu; ++cp) documented.push_back(cp);
  for (plot::u32 cp : {0x00B0u, 0x00B5u, 0x03A9u, 0x2013u, 0x2014u}) documented.push_back(cp);
  PLOT_CHECK(EmbeddedMonoFont::glyph_count() == documented.size() + 1u);

  const plot::u8* atlas = f.atlas_pixels();
  const plot::u32 atlas_w = f.atlas_width();
  const plot::u32 atlas_h = f.atlas_height();
  const std::size_t atlas_texels = static_cast<std::size_t>(atlas_w) * atlas_h;
  plot::u32 inked_cells = 0u;

  for (plot::u32 slot = 0; slot < EmbeddedMonoFont::glyph_count(); ++slot) {
    const plot::u32 expected =
        slot < documented.size() ? documented[slot] : EmbeddedMonoFont::kFallbackCodepoint;
    const plot::u32 cp = EmbeddedMonoFont::codepoint_of_slot(slot);
    PLOT_CHECK(cp == expected);  // the table is the documented one, in order
    if (cp != EmbeddedMonoFont::kFallbackCodepoint) {
      PLOT_CHECK(EmbeddedMonoFont::glyph_index(cp) == slot);
    }

    // Its rect is the one declared cell, and it lies inside the image ...
    const plot::render::Glyph* g = f.glyph(cp);
    PLOT_CHECK(g != nullptr);
    PLOT_CHECK(plot::render::glyph_is_cell(*g, cell));
    PLOT_CHECK(g->u0 >= 0.0f && g->u1 <= 1.0f && g->v0 >= 0.0f && g->v1 <= 1.0f);

    // ... and the cell's texels are all inside the image with ink in them, bar
    // the one documented blank. The bounds check is what a table claiming more
    // bytes than it ships fails on.
    const plot::Vec2 origin = EmbeddedMonoFont::atlas_origin(slot);
    const plot::u32 ox = static_cast<plot::u32>(origin.x);
    const plot::u32 oy = static_cast<plot::u32>(origin.y);
    PLOT_CHECK(ox + EmbeddedMonoFont::cell_width() <= atlas_w);
    PLOT_CHECK(oy + EmbeddedMonoFont::cell_height() <= atlas_h);
    plot::u32 ink = 0u;
    for (plot::u32 y = 0; y < EmbeddedMonoFont::cell_height(); ++y) {
      for (plot::u32 x = 0; x < EmbeddedMonoFont::cell_width(); ++x) {
        const std::size_t index = static_cast<std::size_t>(oy + y) * atlas_w + (ox + x);
        PLOT_CHECK(index < atlas_texels);
        ink += atlas[index];
      }
    }
    if (cp == 0x20u) {
      PLOT_CHECK(ink == 0u);  // the space is the one blank cell
    } else {
      PLOT_CHECK(ink > 0u);  // every covered glyph, and the box, has ink
      ++inked_cells;
    }
  }
  PLOT_CHECK(inked_cells == EmbeddedMonoFont::glyph_count() - 1u);

  // The fallback sits at the fixed final slot, as documented: a regeneration that
  // inserted or dropped a glyph would move it, and this catches that where the
  // lookups alone would not.
  PLOT_CHECK(fallback_slot() == EmbeddedMonoFont::glyph_count() - 1u);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x10FFFFu) == fallback_slot());
}

// The atlas carries more than one coverage value: an anti-aliased raster has grey
// edges, and a 1-bit table could only ever be 0 or 255. This is the data-level
// half of the anti-aliasing evidence (the frame-level half is in the render
// smoke test): a table that passed through a threshold would fail it.
PLOT_TEST(embedded_atlas_holds_antialiased_coverage) {
  const EmbeddedMonoFont f;
  const plot::u8* atlas = f.atlas_pixels();
  const std::size_t size = static_cast<std::size_t>(f.atlas_width()) * f.atlas_height();
  plot::u32 partial = 0u;
  plot::u32 full = 0u;
  plot::u32 blank = 0u;
  for (std::size_t i = 0; i < size; ++i) {
    if (atlas[i] == 0u) ++blank;
    else if (atlas[i] == 255u) ++full;
    else ++partial;
  }
  PLOT_CHECK(full > 0u);
  PLOT_CHECK(blank > 0u);
  // Every glyph's stroke edge contributes: the partial population is a large
  // fraction of the inked texels, not a handful of rounding artefacts.
  PLOT_CHECK(partial > full / 2u);
}

// The face is monospaced, so every codepoint's advance is the same -- but it is
// the font's own `hmtx` advance carried through the bake, not a constant invented
// here: `advance_codepoint` and the per-glyph `advance` agree, and the whitespace
// glyphs that must never be zero are not.
PLOT_TEST(embedded_font_advance_matches_the_font_units) {
  const EmbeddedMonoFont f;
  const plot::f32 advance = EmbeddedMonoFont::advance_codepoint('A');
  PLOT_CHECK(advance > 0.0f);
  PLOT_CHECK(EmbeddedMonoFont::advance_codepoint(' ') > 0.0f);
  const char* samples = "iW.1_m ";
  for (const char* c = samples; *c != '\0'; ++c) {
    PLOT_CHECK_NEAR(f.glyph(static_cast<plot::u32>(*c))->advance, advance, 1e-6);
  }
  // `advance_codepoint` is a lookup, not a walk: it agrees with `measure` for a
  // single decoded codepoint, and disagrees with nothing in the table.
  PLOT_CHECK_NEAR(f.measure("W"), EmbeddedMonoFont::advance_codepoint('W'), 1e-6);
  for (plot::u32 cp = 0x20u; cp <= 0x7Eu; ++cp) {
    PLOT_CHECK_NEAR(f.glyph(cp)->advance, EmbeddedMonoFont::advance_codepoint(cp), 1e-6);
  }
}

// Decoding and the fallback, on the codepoints `plot::format_si` and a plotting
// label actually emit: the three SI symbols are real glyphs, everything else is
// the visible fallback box, and a malformed byte is not measured as zero.
PLOT_TEST(embedded_font_decodes_si_units_and_falls_back_visibly) {
  const EmbeddedMonoFont f;
  const plot::u32 fallback = fallback_slot();
  const f32 micro = f.measure("\xC2\xB5s");
  const f32 degree = f.measure("\xC2\xB0");
  const f32 ohm = f.measure("\xCE\xA9");
  PLOT_CHECK(micro > 0.0f);
  PLOT_CHECK(degree > 0.0f);
  PLOT_CHECK(ohm > 0.0f);
  PLOT_CHECK_NEAR(micro, EmbeddedMonoFont::advance_codepoint(0x00B5u) + f.glyph('s')->advance, 1e-6);
  // Each maps to its own cell, not to the box.
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x00B5u) != fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x00B0u) != fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x03A9u) != fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x00B5u) != EmbeddedMonoFont::glyph_index(0x00B0u));
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x00B0u) != EmbeddedMonoFont::glyph_index(0x03A9u));
  // The two dashes the coverage adds are real cells too, not the box.
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x2013u) != fallback);
  PLOT_CHECK(EmbeddedMonoFont::glyph_index(0x2014u) != fallback);
  // A CJK codepoint and a bare malformed byte both draw the box, and both
  // advance: a missing glyph is visible and never a zero-width hole.
  PLOT_CHECK_NEAR(f.measure("\xE4\xB8\x80"), EmbeddedMonoFont::advance_codepoint(0x4E00u), 1e-6);
  PLOT_CHECK_NEAR(f.measure("\x80"), EmbeddedMonoFont::advance_codepoint(0x4E00u), 1e-6);
}

// The atlas must be left-to-right in absolute terms. The coverage bytes are what
// the GPU samples, so a generator that rasterised with the opposite x sense -- or
// an atlas packed with the cells reversed -- would keep every rect check green
// while drawing mirrored text. 'A' cannot catch that (it is symmetric about its
// vertical axis); 'L', 'b' and '/' can, and they are asserted against absolute
// texel coordinates rather than against the table itself.
PLOT_TEST(embedded_atlas_orientation_runs_left_to_right) {
  const EmbeddedMonoFont f;
  using plot::render::EmbeddedMonoFont;
  auto ink = [&f](char c, plot::u32 col, plot::u32 row) {
    const plot::u32 slot = EmbeddedMonoFont::glyph_index(static_cast<plot::u32>(c));
    return cell_texel(f, slot, col, row) != 0u;
  };
  // 'L': the stem runs down the left, and the foot runs right along the bottom --
  // so at cap height there is ink on the left and none on the right, and the
  // bottom row has ink to the right of where the stem stopped.
  PLOT_CHECK(ink('L', 1u, 6u));
  PLOT_CHECK(!ink('L', 6u, 6u));
  PLOT_CHECK(ink('L', 5u, 11u));
  PLOT_CHECK(!ink('L', 5u, 3u));
  // 'b': the stem is the left column of the upper rows, the bowl the right.
  PLOT_CHECK(ink('b', 1u, 4u));
  PLOT_CHECK(!ink('b', 6u, 4u));
  PLOT_CHECK(ink('b', 6u, 8u));
  // '/': starts at the top right and ends at the bottom left -- the opposite of
  // '\'. This is the pair that pins the diagonal's direction.
  PLOT_CHECK(ink('/', 5u, 2u));
  PLOT_CHECK(!ink('/', 2u, 2u));
  PLOT_CHECK(ink('/', 2u, 12u));
  PLOT_CHECK(!ink('/', 5u, 12u));
}

// The atlas is a fixed-cell grid of one texel format: a whole number of cells
// across, the cell dividing the height, and every glyph's rect the declared cell
// -- so the pipeline's single per-draw rect size is the right one for every glyph.
PLOT_TEST(embedded_atlas_is_uploadable) {
  using plot::render::EmbeddedMonoFont;
  PLOT_CHECK(EmbeddedMonoFont::cell_width() > 0u);
  PLOT_CHECK(EmbeddedMonoFont::cell_height() > 0u);
  PLOT_CHECK(EmbeddedMonoFont::glyph_count() == 95u + 5u + 1u);  // ASCII, SI/dashes, box

  const EmbeddedMonoFont f;
  PLOT_CHECK(f.atlas_width() == EmbeddedMonoFont::cell_width() * EmbeddedMonoFont::glyph_count());
  PLOT_CHECK(f.atlas_height() == EmbeddedMonoFont::cell_height());
  PLOT_CHECK(f.atlas_pixels() != nullptr);
  PLOT_CHECK(f.height() > 0.0f);
  PLOT_CHECK(f.ascent() > 0.0f);
  PLOT_CHECK(f.ascent() < f.height());

  // Every cell, the fallback included, is the declared cell and holds at least
  // one non-zero byte (bar the space), and the ASCII codepoints are a direct
  // index into the table.
  for (plot::u32 slot = 0; slot < EmbeddedMonoFont::glyph_count(); ++slot) {
    const plot::u32 cp = EmbeddedMonoFont::codepoint_of_slot(slot);
    if (cp != EmbeddedMonoFont::kFallbackCodepoint) {
      PLOT_CHECK(EmbeddedMonoFont::glyph_index(cp) == slot);
    }
    const plot::render::Glyph* g = f.glyph(cp);
    PLOT_CHECK(plot::render::glyph_is_cell(*g, f.cell_uv()));
  }
  for (plot::u32 cp = 0x20u; cp <= 0x7Eu; ++cp) {
    PLOT_CHECK(EmbeddedMonoFont::glyph_index(cp) == cp - 0x20u);
  }
  // A single-row atlas: the cell spans the whole height, so v is 0..1.
  PLOT_CHECK_NEAR(f.cell_uv().y, 1.0, 1e-9);
  PLOT_CHECK_NEAR(EmbeddedMonoFont::atlas_origin(0u).x, 0.0, 1e-9);
  PLOT_CHECK_NEAR(EmbeddedMonoFont::atlas_origin(1u).x,
                  static_cast<plot::f32>(EmbeddedMonoFont::cell_width()), 1e-9);
}

// The size the renderer draws a run by is a plain ratio, `requested_px / em_px`,
// and not a whole multiple of the backend's cell. This is the rule the whole
// anti-aliasing change rests on: a style asking for 12 px from a 13 px bake is
// drawn at 12/13, where the old 1-bit font had to snap to a multiple of its 8 px
// cell (16 px here). `detail::text_scale` is that rule, so it is asserted without
// a GL context; the frame-level half is in the render smoke test.
PLOT_TEST(text_scale_is_a_ratio_not_a_whole_multiple) {
  using plot::render::detail::text_scale;
  const plot::f32 em = 13.0f;  // what the committed atlas is baked at

  // 12/13 = 0.9231, so the 17-px cell is drawn 15.69 px tall -- not the 16 px the
  // old rule would have chosen, and not the 17 px of a whole multiple of it.
  PLOT_CHECK_NEAR(text_scale(12.0f, em), 12.0f / 13.0f, 1e-6);
  PLOT_CHECK(std::fabs(text_scale(12.0f, em) * 17.0f - 16.0f) > 0.2f);
  PLOT_CHECK(std::fabs(text_scale(12.0f, em) * 17.0f - 17.0f) > 0.2f);

  // A ratio that is not a multiple is not rounded to one ...
  PLOT_CHECK_NEAR(text_scale(20.0f, em), 20.0f / 13.0f, 1e-6);
  PLOT_CHECK(text_scale(20.0f, em) > 1.5f && text_scale(20.0f, em) < 1.6f);

  // ... and an exact multiple is exactly that multiple, so nothing is lost when
  // the request does line up with the bake.
  PLOT_CHECK_NEAR(text_scale(13.0f, em), 1.0f, 1e-6);
  PLOT_CHECK_NEAR(text_scale(26.0f, em), 2.0f, 1e-6);
  PLOT_CHECK_NEAR(text_scale(39.0f, em), 3.0f, 1e-6);
}

// A backend that cannot state an em must not take the frame with it: the scale
// falls back to 1:1 rather than dividing by zero or propagating a NaN into every
// vertex, and the same holds for a nonsensical request. No shipped backend can
// reach this -- `EmbeddedMonoFont::em_px()` is a positive constant -- which is
// exactly why the rule is a function with its own case instead of a bare
// division inside `Renderer::AxisFont::reset`.
PLOT_TEST(text_scale_guards_a_backend_without_an_em) {
  using plot::render::detail::text_scale;
  const plot::f32 nan = std::numeric_limits<plot::f32>::quiet_NaN();

  PLOT_CHECK_NEAR(text_scale(12.0f, 0.0f), 1.0f, 1e-6);    // no em at all
  PLOT_CHECK_NEAR(text_scale(12.0f, -13.0f), 1.0f, 1e-6);  // nonsense em
  PLOT_CHECK_NEAR(text_scale(12.0f, nan), 1.0f, 1e-6);     // NaN em
  PLOT_CHECK_NEAR(text_scale(nan, 13.0f), 1.0f, 1e-6);     // NaN request
  PLOT_CHECK_NEAR(text_scale(0.0f, 13.0f), 1.0f, 1e-6);    // zero request
  PLOT_CHECK_NEAR(text_scale(-12.0f, 13.0f), 1.0f, 1e-6);  // negative request

  // The guard does not fire for a healthy pair: the ratio is still exact.
  PLOT_CHECK_NEAR(text_scale(26.0f, 13.0f), 2.0f, 1e-6);
  PLOT_CHECK_NEAR(text_scale(12.0f, 13.0f), 12.0f / 13.0f, 1e-6);
}

// Three exit codes say something about the *machine* rather than about the
// committed bytes, and none of them can tell a drifted table from a good one:
// `gen_atlas.py`'s EXIT_NO_FONTTOOLS (fontTools not installed, 3) and
// EXIT_BAD_FONT (no source face -- the adwaita-fonts package -- or an
// unparseable one, 2), and the shell's "command not found" for a missing
// `python3` (127). Each is *reported* with what is missing and skipped, rather
// than failed, so an environment difference does not look like atlas drift --
// a machine with fontTools from pip but no distro font package would otherwise
// fail here and send someone hunting for a corruption that is not there. Only a
// real mismatch between the committed atlas and a fresh bake (1) fails. The
// data cases above still assert the shipped table on such a machine, so the
// only thing lost is the drift comparison itself.
PLOT_TEST(generator_is_deterministic_and_current) {
  // Both paths are quoted: `kSourceDir` is a build-time value, and a source tree
  // whose path contains a space would otherwise split into extra arguments.
  const std::string command = "python3 \"" + std::string(kSourceDir) +
                              "/tools/fonts/gen_atlas.py\" --check --out \"" + kSourceDir +
                              "/src/render/embedded_mono_font.cpp\"";
  const int status = std::system(command.c_str());
  PLOT_CHECK(status != -1);
  PLOT_CHECK(WIFEXITED(status));
  const int code = WEXITSTATUS(status);
  const char* missing = code == 3     ? "fontTools is not installed"
                        : code == 2   ? "the Adwaita Mono face is not installed (adwaita-fonts)"
                        : code == 127 ? "python3 is not on PATH"
                                      : nullptr;
  if (missing != nullptr) {
    std::printf(
        "SKIPPED: generator_is_deterministic_and_current -- %s, so the committed "
        "atlas could not be re-baked\n",
        missing);
    return;
  }
  PLOT_CHECK(code == 0);
}

PLOT_TEST(text_batch_stages_one_quad_per_glyph) {
  plot::render::EmbeddedMonoFont f;
  plot::render::TextBatch b;
  b.begin();
  const plot::Vec2 end = b.add_text("12.5 ms", {0.0f, 0.0f}, 1.0f, f);
  PLOT_CHECK(b.glyph_count() == 7);
  PLOT_CHECK(b.data().size() == 42);                 // x,y,u,v,ax,ay per glyph
  PLOT_CHECK_NEAR(end.x, f.measure("12.5 ms"), 1e-6);
  PLOT_CHECK_NEAR(end.y, 0.0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[0], 0.0, 1e-6);           // first cell at the pen
  PLOT_CHECK_NEAR(b.data()[1], 0.0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[6], f.glyph('1')->advance, 1e-6);  // pen advanced per glyph
  // The staged cell's UV is the glyph's own atlas rect (see `Glyph`), and the
  // basis of an unrotated run is the screen's +x. '.' is the string's third
  // glyph, so its instance is the third: floats 12..17.
  const plot::render::Glyph* dot = f.glyph(static_cast<plot::u32>('.'));
  PLOT_CHECK(dot != nullptr);
  PLOT_CHECK_NEAR(b.data()[14], dot->u0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[15], dot->v0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[16], 1.0, 1e-6);          // ax
  PLOT_CHECK_NEAR(b.data()[17], 0.0, 1e-6);          // ay
  // Every glyph a batch stages is read through one cell, and the batch publishes
  // it for the draw (`u_glyph_uv`).
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, dot->u1 - dot->u0, 1e-6);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, dot->v1 - dot->v0, 1e-6);

  // Non-ASCII runs stage one quad per codepoint, and preserve capacity on reuse.
  const std::size_t cap = b.data().capacity();
  b.begin();
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, 0.0, 1e-6);   // begin() clears the cell too
  b.add_text("1.5\xC2\xB5s", {0.0f, 0.0f}, 1.0f, f);  // 5 codepoints, 6 bytes
  PLOT_CHECK(b.glyph_count() == 5);
  PLOT_CHECK(b.data().capacity() == cap);

  // A run drawn at twice the backend's size advances twice as far, so the staged
  // label still ends where the scaled measurement says it does.
  b.begin();
  const plot::Vec2 scaled = b.add_text("AW", {10.0f, 4.0f}, 2.0f, f);
  PLOT_CHECK(b.glyph_count() == 2);
  PLOT_CHECK_NEAR(scaled.x, 10.0f + 2.0f * f.measure("AW"), 1e-6);
  PLOT_CHECK_NEAR(scaled.y, 4.0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[6], 10.0f + 2.0f * f.glyph('A')->advance, 1e-6);  // second cell's pen
}

// The pen follows the basis, not the screen axes. A run staged with `axis` has to
// be exactly as long as the plot layer measured it *in that direction* -- that is
// what reserves the room for a rotated y label -- and its cells have to march along
// the same direction, or the ink lands beside the pen instead of up from it.
PLOT_TEST(text_batch_advances_along_its_basis) {
  plot::render::EmbeddedMonoFont f;
  plot::render::TextBatch b;
  const plot::Vec2 pen{100.0f, 40.0f};
  const f32 width = f.measure("12.5 ms");

  // Unrotated, for the shape of the assertion: the run is `measure()` long and
  // does not move in y.
  b.begin();
  const plot::Vec2 flat = b.add_text("12.5 ms", pen, 1.0f, f);
  PLOT_CHECK_NEAR((flat - pen).length(), width, 1e-4);
  PLOT_CHECK_NEAR(flat.y, pen.y, 1e-6);

  // Up the screen: the same length, measured upward, with a unit basis on every
  // staged cell.
  b.begin();
  const plot::Vec2 up = b.add_text("12.5 ms", pen, 1.0f, f, {0.0f, -1.0f});
  PLOT_CHECK_NEAR((up - pen).length(), width, 1e-4);
  PLOT_CHECK_NEAR(up.x, pen.x, 1e-6);
  PLOT_CHECK_NEAR(up.y, pen.y - width, 1e-4);
  for (plot::u32 i = 0; i < b.glyph_count(); ++i) {
    PLOT_CHECK_NEAR(b.data()[6u * i + 4u], 0.0, 1e-6);
    PLOT_CHECK_NEAR(b.data()[6u * i + 5u], -1.0, 1e-6);
  }
  // The second cell is one advance further up than the pen, which is the same rule
  // the unrotated case follows in x.
  PLOT_CHECK_NEAR(b.data()[6], pen.x, 1e-6);
  PLOT_CHECK_NEAR(b.data()[7], pen.y - f.glyph('1')->advance, 1e-6);

  // A diagonal direction is normalised, so a caller can hand over any direction
  // and the basis the shader receives is still unit.
  b.begin();
  const plot::Vec2 diag = b.add_text("12.5 ms", pen, 1.0f, f, {3.0f, 4.0f});
  PLOT_CHECK_NEAR((diag - pen).length(), width, 1e-4);
  PLOT_CHECK_NEAR(b.data()[4], 0.6, 1e-5);
  PLOT_CHECK_NEAR(b.data()[5], 0.8, 1e-5);
}

/// A backend whose atlas is *not* a fixed-cell grid: it declares the cell its
/// pipeline needs and then publishes one glyph trimmed to its ink, which is what
/// a naive TrueType packing produces (`stbtt_GetCodepointBitmap` hands back an ink
/// box, not a cell). The pipeline cannot draw that glyph correctly -- a staged
/// instance carries the rect's origin, and the draw carries one size for the whole
/// run -- so the staging path asserts `glyph_is_cell()` for every glyph it stages.
class TrimmedCellBackend final : public plot::render::TextBackend {
 public:
  plot::f32 measure(const char*) const override { return 8.0f; }
  plot::f32 height() const override { return 8.0f; }
  plot::f32 ascent() const override { return 8.0f; }
  plot::f32 em_px() const override { return 8.0f; }
  plot::Vec2 cell_uv() const override { return plot::Vec2{0.5f, 0.25f}; }
  plot::u32 revision() const override { return 1u; }  // baked once, never re-baked
  const plot::render::Glyph* glyph(plot::u32 codepoint) const override {
    // 'i' is trimmed to half a cell wide, as an ink box would be.
    return codepoint == static_cast<plot::u32>('i') ? &trimmed_ : &cell_;
  }
  const plot::u8* atlas_pixels() const override { return atlas_; }
  plot::u32 atlas_width() const override { return 8u; }
  plot::u32 atlas_height() const override { return 4u; }

 private:
  // Two cells of a 2x2 grid of 0.5 x 0.25, one of them trimmed to the ink.
  const plot::render::Glyph cell_{0.0f, 0.0f, 0.5f, 0.25f, 0.0f, 0.0f, 4.0f};
  const plot::render::Glyph trimmed_{0.5f, 0.0f, 0.6875f, 0.25f, 0.0f, 0.0f, 4.0f};
  const plot::u8 atlas_[8 * 4] = {};
};

// The fixed-cell contract, and the check that enforces it. A backend that trims
// each atlas rect to its ink -- the shape a TrueType packing produces, and the
// reason the contract is stated on `TextBackend::cell_uv()` -- publishes glyphs
// the pipeline would draw at the wrong size: the instance carries the rect's
// *origin* only, so a 3/8-cell rect would be read as a half cell, past its own
// rows.
//
// This drives the predicate the staging path asserts, rather than staging the
// offending glyph, because `add_text()` with this backend aborts the test binary
// in a debug build -- which is the point of the assertion.
PLOT_TEST(a_trimmed_glyph_is_not_the_atlas_cell) {
  TrimmedCellBackend backend;
  const plot::Vec2 cell = backend.cell_uv();
  PLOT_CHECK_NEAR(cell.x, 0.5, 1e-6);
  PLOT_CHECK_NEAR(cell.y, 0.25, 1e-6);

  // The glyph that occupies a whole cell passes ...
  PLOT_CHECK(plot::render::glyph_is_cell(*backend.glyph(static_cast<plot::u32>('A')), cell));
  // ... and the one trimmed to its ink does not: this is what the staging path
  // asserts in a debug build.
  PLOT_CHECK(!plot::render::glyph_is_cell(*backend.glyph(static_cast<plot::u32>('i')), cell));

  // The check is per axis and per glyph, not a single "is it square" test: a rect
  // that matches on one axis and not the other is still not the cell.
  plot::render::Glyph wide = *backend.glyph(static_cast<plot::u32>('A'));
  wide.u1 = wide.u0 + 0.75f;
  PLOT_CHECK(!plot::render::glyph_is_cell(wide, cell));
  plot::render::Glyph tall = *backend.glyph(static_cast<plot::u32>('A'));
  tall.v1 = tall.v0 + 0.5f;
  PLOT_CHECK(!plot::render::glyph_is_cell(tall, cell));

  // Rounding a correct cell through the atlas arithmetic twice is not a failure:
  // a rect is a subtraction of two rounded fractions, and the tolerance is two
  // orders of magnitude above that slop and below one texel.
  plot::render::Glyph rounded = *backend.glyph(static_cast<plot::u32>('A'));
  rounded.u1 = rounded.u0 + cell.x + 1e-7f;
  PLOT_CHECK(plot::render::glyph_is_cell(rounded, cell));

  // A real font's grid holds for every glyph it publishes, not just the first.
  plot::render::EmbeddedMonoFont font;
  for (plot::u32 codepoint = 32u; codepoint < 127u; ++codepoint) {
    const plot::render::Glyph* g = font.glyph(codepoint);
    PLOT_CHECK(g != nullptr);
    PLOT_CHECK(plot::render::glyph_is_cell(*g, font.cell_uv()));
  }
  PLOT_CHECK_NEAR(font.cell_uv().x,
                  static_cast<plot::f32>(plot::render::EmbeddedMonoFont::cell_width()) /
                      static_cast<plot::f32>(font.atlas_width()),
                  1e-9);
  PLOT_CHECK_NEAR(font.cell_uv().y,
                  static_cast<plot::f32>(plot::render::EmbeddedMonoFont::cell_height()) /
                      static_cast<plot::f32>(font.atlas_height()),
                  1e-9);
}

/// A backend whose cell is nothing like the bitmap font's -- half by a quarter of
/// a small atlas -- so anything a batch reports that equals it came from the
/// backend and not from a glyph of some other font.
class HalfCellBackend final : public plot::render::TextBackend {
 public:
  plot::f32 measure(const char*) const override { return 4.0f; }
  plot::f32 height() const override { return 8.0f; }
  plot::f32 ascent() const override { return 6.0f; }
  plot::f32 em_px() const override { return 8.0f; }
  plot::Vec2 cell_uv() const override { return plot::Vec2{0.5f, 0.25f}; }
  plot::u32 revision() const override { return 1u; }
  const plot::render::Glyph* glyph(plot::u32) const override { return &cell_; }
  const plot::u8* atlas_pixels() const override { return atlas_; }
  plot::u32 atlas_width() const override { return 8u; }
  plot::u32 atlas_height() const override { return 4u; }

 private:
  // One cell, at the second slot of a 2x2 grid. Its rect is the declared cell to
  // within the atlas tolerance (5e-7) but not bit for bit -- which is what any
  // backend's rounded arithmetic looks like, and what makes "the backend's
  // number" and "this glyph's rect size" two distinguishable values.
  const plot::render::Glyph cell_{0.5f, 0.0f, 1.0000005f, 0.2500005f, 0.0f, 0.0f, 4.0f};
  const plot::u8 atlas_[8 * 4] = {};
};

// The draw's one rect size is the *backend's* `cell_uv()`, and it is known before
// a glyph is staged: a batch that took it from whatever glyph it happened to
// stage first could not answer the first of these, and would take another font's
// grid the moment a caller staged two backends into one batch. `add_text()`
// declares it from the backend it is handed, and `set_cell()` is how a caller
// staging glyphs one at a time -- the renderer's clipped walk -- says it.
PLOT_TEST(text_batch_takes_its_cell_from_the_backend) {
  HalfCellBackend backend;
  plot::render::TextBatch b;

  b.begin();
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, 0.0, 1e-6);   // nothing declared, nothing staged
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, 0.0, 1e-6);
  b.set_cell(backend.cell_uv());
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, 0.5, 1e-6);   // declared, still nothing staged
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, 0.25, 1e-6);

  // A staged run keeps it -- and keeps the *backend's* number, not the staged
  // glyph's: this backend's rects are 5e-7 off its declared cell (inside the
  // tolerance, so it is a legal grid), and an implementation that recorded
  // whatever glyph it staged first would land off these by exactly that.
  b.begin();
  const plot::Vec2 end = b.add_text("AB", {0.0f, 0.0f}, 1.0f, backend);
  PLOT_CHECK(b.glyph_count() == 2);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, backend.cell_uv().x, 1e-12);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, backend.cell_uv().y, 1e-12);
  PLOT_CHECK(backend.cell_uv().x != backend.glyph(0u)->u1 - backend.glyph(0u)->u0);
  PLOT_CHECK(plot::render::glyph_is_cell(*backend.glyph(0u), b.glyph_cell_uv()));
  PLOT_CHECK_NEAR(end.x, 2.0f * backend.glyph(static_cast<plot::u32>('A'))->advance, 1e-6);
  // The instances carry the cell's *origin* in the atlas; the size rides along
  // once, above, which is the whole reason a glyph fits in six floats.
  PLOT_CHECK_NEAR(b.data()[2], backend.glyph(0u)->u0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[3], backend.glyph(0u)->v0, 1e-6);
  PLOT_CHECK_NEAR(b.data()[4], 1.0, 1e-6);           // unrotated basis

  // And the real font's grid, staged the same way, reports the font's cell -- the
  // two never mix silently, and the batch's value tracks the backend it was given.
  plot::render::EmbeddedMonoFont font;
  b.begin();
  b.add_text("A", {0.0f, 0.0f}, 1.0f, font);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, font.cell_uv().x, 1e-9);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, font.cell_uv().y, 1e-9);
  PLOT_CHECK(b.glyph_cell_uv().x != backend.cell_uv().x);

  // begin() forgets the declaration along with the instances, so a frame cannot
  // inherit the previous one's cell.
  b.begin();
  PLOT_CHECK_NEAR(b.glyph_cell_uv().x, 0.0, 1e-6);
  PLOT_CHECK_NEAR(b.glyph_cell_uv().y, 0.0, 1e-6);
}

/// A backend with a hole in its artwork: it can draw 'A' and 'B' and publishes no
/// glyph at all for anything else, which is what a font whose coverage falls short
/// of a label looks like. `measure()` counts only what it can draw, so "what a
/// string is worth" and "what it stages" stay the same thing -- the pipeline draws
/// nothing for the missing codepoint and moves the pen nowhere, rather than
/// inventing a box or a width.
class HoleInCoverageBackend final : public plot::render::TextBackend {
 public:
  plot::f32 measure(const char* utf8) const override {
    plot::f32 width = 0.0f;
    for (const char* p = utf8; p != nullptr && *p != '\0';) {
      const plot::render::Glyph* g = glyph(plot::render::next_codepoint(p));
      if (g != nullptr) width += g->advance;
    }
    return width;
  }
  plot::f32 height() const override { return 8.0f; }
  plot::f32 ascent() const override { return 6.0f; }
  plot::f32 em_px() const override { return 8.0f; }
  plot::Vec2 cell_uv() const override { return plot::Vec2{0.5f, 1.0f}; }
  plot::u32 revision() const override { return 1u; }
  const plot::render::Glyph* glyph(plot::u32 codepoint) const override {
    if (codepoint == static_cast<plot::u32>('A')) return &a_;
    if (codepoint == static_cast<plot::u32>('B')) return &b_;
    return nullptr;  // no artwork at all: not even a fallback box
  }
  const plot::u8* atlas_pixels() const override { return nullptr; }
  plot::u32 atlas_width() const override { return 0u; }
  plot::u32 atlas_height() const override { return 0u; }

 private:
  const plot::render::Glyph a_{0.0f, 0.0f, 0.5f, 1.0f, 0.0f, 0.0f, 6.0f};
  const plot::render::Glyph b_{0.5f, 0.0f, 1.0f, 1.0f, 0.0f, 0.0f, 4.0f};
};

// A backend that publishes no glyph for a codepoint: `add_text` stages nothing for
// it and does not advance the pen, so a run stays exactly as long as the same
// backend measured it and the glyphs it *can* draw keep their places. A backend
// that would rather show something publishes a fallback box as that codepoint's
// glyph (which is what `EmbeddedMonoFont` does); this is the other legal answer, and the
// staging path has to honour it rather than draw whatever was staged last or
// advance by a width nobody published.
PLOT_TEST(add_text_skips_a_codepoint_the_backend_cannot_draw) {
  HoleInCoverageBackend backend;
  const plot::render::Glyph* a = backend.glyph(static_cast<plot::u32>('A'));
  const plot::render::Glyph* b = backend.glyph(static_cast<plot::u32>('B'));
  PLOT_CHECK(a != nullptr);
  PLOT_CHECK(b != nullptr);
  PLOT_CHECK(backend.glyph(static_cast<plot::u32>('?')) == nullptr);

  const plot::Vec2 pen{10.0f, 4.0f};
  plot::render::TextBatch batch;
  batch.begin();
  const plot::Vec2 end = batch.add_text("A?B", pen, 1.0f, backend);

  // Two glyphs staged for three codepoints, and the pen moved by exactly the two
  // advances the backend published.
  PLOT_CHECK(batch.glyph_count() == 2);
  PLOT_CHECK_NEAR(end.x, pen.x + a->advance + b->advance, 1e-6);
  PLOT_CHECK_NEAR(end.y, pen.y, 1e-6);
  // Which is the length the backend measured for the same string, so a label's
  // reserved space still covers the run drawn into it.
  PLOT_CHECK_NEAR((end - pen).length(), backend.measure("A?B"), 1e-6);
  PLOT_CHECK_NEAR(backend.measure("A?B"), a->advance + b->advance, 1e-6);
  // 'B' starts where 'A' ended -- the skipped codepoint left no gap and drew no
  // glyph of its own, which the second instance's atlas rect says outright.
  PLOT_CHECK_NEAR(batch.data()[0], pen.x, 1e-6);
  PLOT_CHECK_NEAR(batch.data()[2], a->u0, 1e-6);
  PLOT_CHECK_NEAR(batch.data()[6], pen.x + a->advance, 1e-6);
  PLOT_CHECK_NEAR(batch.data()[8], b->u0, 1e-6);

  // A run of nothing but undrawable codepoints stages nothing at all and leaves
  // the pen where it was, rather than drawing at the pen.
  batch.begin();
  const plot::Vec2 same = batch.add_text("??", pen, 1.0f, backend);
  PLOT_CHECK(batch.glyph_count() == 0);
  PLOT_CHECK(batch.data().empty());
  PLOT_CHECK_NEAR(same.x, pen.x, 1e-6);
  PLOT_CHECK_NEAR(same.y, pen.y, 1e-6);
  PLOT_CHECK_NEAR(backend.measure("??"), 0.0, 1e-6);

  // A real font never hits this path (every codepoint maps to its box), so the
  // same string through it stages one glyph per codepoint: the two answers to
  // "no glyph" are the backend's choice, not the pipeline's.
  plot::render::EmbeddedMonoFont font;
  batch.begin();
  batch.add_text("A?B", pen, 1.0f, font);
  PLOT_CHECK(batch.glyph_count() == 3);
}