feat(render): Renderer with LOD selection, 3.3 and 4.3+ upload paths, headless smoke test

include/plot/render/renderer.hpp

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+#pragma once
+#include <span>
+#include <vector>
+
+#include <plot/core/color.hpp>
+#include <plot/core/rect.hpp>
+#include <plot/core/status.hpp>
+#include <plot/core/types.hpp>
+#include <plot/gl/handle.hpp>
+#include <plot/math/vec2.hpp>
+#include <plot/plot/plot.hpp>
+#include <plot/render/bitmap_font.hpp>
+#include <plot/render/buffer.hpp>
+#include <plot/render/primitives.hpp>
+#include <plot/render/program.hpp>
+#include <plot/render/style.hpp>
+#include <plot/render/vertex_array.hpp>
+
+namespace plot {
+
+/// What one frame staged and drew. It exists because pixels alone cannot say
+/// which of the two rasterisation paths a trace took: a 1M-sample series drawn
+/// from the LOD pyramid and one drawn from raw samples can cover the same
+/// columns, so the smoke test asserts the path here and the pixels there.
+struct FrameStats {
+  u32 raw_traces = 0;        ///< traces drawn from raw samples (level 0)
+  u32 lod_traces = 0;        ///< traces drawn from a min/max pyramid level
+  u64 visible_samples = 0;   ///< samples the gathered windows covered, summed
+  u32 staged_segments = 0;   ///< line segments staged, all colour groups
+  u32 draw_calls = 0;        ///< draw calls issued, line and text
+};
+
+/// Draws a `Plot` into whatever framebuffer is bound.
+///
+/// The renderer never creates, binds or resizes a framebuffer: `begin_frame()`
+/// takes the framebuffer size for the projection and a `Rect` for the region the
+/// plot occupies, clears exactly that region (`GL_SCISSOR_TEST` around the clear,
+/// so a host that has already painted the rest of its surface keeps it) and sets
+/// `glViewport` to the flipped rect. That is the whole embedding contract of the
+/// adapters (Qt/GTK/ImGui widgets).
+///
+/// Two properties drive the rest of the design:
+///
+/// - **Only the visible window is gathered.** A trace's x range is mapped onto
+///   sample indices and `Series::gather` is asked for that window -- never the
+///   whole series -- at `pyramid().level_for(n, samples_per_pixel)` whenever a
+///   pixel covers `dense_threshold_spp` samples or more. A 100M-sample series
+///   therefore stages O(pixels) vertices, and `FrameStats` records which way each
+///   trace went.
+/// - **Vertex data stays in data space.** Segments are staged in data
+///   coordinates and mapped to pixels by the line program's `u_scale`/`u_offset`
+///   uniforms, so nothing is ever baked into screen space per primitive; a log
+///   axis is staged in the transform's pre-affine coordinate (`log10`), which is
+///   what the affine part expects. Pan and zoom are uniform updates.
+///
+/// The vertex transfer itself is one of two strategies, chosen once in `init()`
+/// from `plot::gl::caps()` and invisible to every caller afterwards: a
+/// persistent-mapped, fence-guarded ring on 4.3+/ARB_buffer_storage, and the
+/// orphan-and-refill baseline on 3.3. Setting `PLOT_FORCE_GL33_UPLOAD=1` in the
+/// environment forces the baseline path, which is how both are exercised on one
+/// machine.
+///
+/// Not thread-safe, and confined to the thread that owns the current context.
+class Renderer {
+ public:
+  Renderer() noexcept = default;
+  ~Renderer() = default;
+  Renderer(const Renderer&) = delete;
+  Renderer& operator=(const Renderer&) = delete;
+
+  /// Compiles both programs, builds the glyph atlas and allocates the vertex
+  /// buffers. The calling thread must have the context current and
+  /// `plot::gl::load()` must have succeeded, otherwise the returned `Status` is
+  /// `Unsupported`; anything the driver refuses is `GLFailure` plus its log.
+  bool init(Status* st = nullptr) noexcept;
+
+  /// Starts a frame: records `fb_w` x `fb_h` for the projection, flips
+  /// `viewport_px` (top-left origin, like every `Rect` in this library) into GL's
+  /// bottom-left origin once, clears that region to `style().background` and
+  /// empties the staging batches.
+  void begin_frame(u32 fb_w, u32 fb_h, Rect viewport_px) noexcept;
+
+  /// Draws `plot`, refreshing its layout, ticks and grid first.
+  ///
+  /// The renderer owns the presentation style: `draw()` copies `style()` into the
+  /// plot, so the strips and tick densities the plot lays out with are the values
+  /// the renderer draws with -- set colours and metrics here, not on the plot.
+  /// The plot also borrows this renderer's font (when it has no backend of its
+  /// own), so the renderer must outlive the plot's use of it.
+  void draw(Plot& plot);
+
+  /// Ends the frame, draining `glGetError()` in a loop so a stale error cannot be
+  /// attributed to a later frame (see `last_frame_had_gl_error()`).
+  ///
+  /// The GL state it leaves behind, so a host can embed the plot without guessing:
+  /// vertex array 0, program 0, `GL_ARRAY_BUFFER` 0, texture unit 0 active and
+  /// nothing bound to `GL_TEXTURE_2D` on it, blending off, scissoring off, and the
+  /// viewport left at the frame's rect (the caller sets its own for the next pass).
+  /// The framebuffer binding is never touched -- it is the caller's, and so is the
+  /// pixel-pack alignment if `dump_frame_ppm()` ran.
+  void end_frame() noexcept;
+
+  /// The frame's colours and metrics; `draw()` hands them to the plot as well, so
+  /// the layout it computes matches what is drawn. `line_width_px` is the width of
+  /// the strokes framing the data (axis lines, cursor markers) -- grid lines stay
+  /// hairlines and every trace carries its own width.
+  render::Style& style() noexcept { return style_; }
+  const render::Style& style() const noexcept { return style_; }
+
+  /// True when the last `end_frame()` found a GL error. Errors are drained in a
+  /// loop, so a stale one cannot be attributed to a later frame.
+  bool last_frame_had_gl_error() const noexcept { return gl_error_; }
+
+  /// Statistics of the most recently drawn frame.
+  const FrameStats& stats() const noexcept { return stats_; }
+
+  bool ready() const noexcept { return ready_; }
+  /// True when `init()` selected the persistent-mapped transfer (4.3+ with
+  /// ARB_buffer_storage, and `PLOT_FORCE_GL33_UPLOAD` unset).
+  bool using_persistent_upload() const noexcept { return persistent_; }
+  /// The axis text size the renderer actually draws with: `style().axis_font_px`
+  /// rounded to the nearest whole multiple of `render::BitmapFont::kGlyphPx`,
+  /// because a 1-bit glyph only scales cleanly by whole factors.
+  f32 axis_font_px() const noexcept { return font_backend_.cell_px(); }
+
+  /// Writes the framebuffer's viewport region as a binary PPM (P6, 8 bits per
+  /// channel, rows top-down) so a headless render can be looked at without a
+  /// window. Reads the framebuffer that is bound now, which is the one the frame
+  /// was drawn into. Returns false, with `st` filled in, on a bad argument or an
+  /// unreadable pixel.
+  bool dump_frame_ppm(const char* path, Status* st = nullptr) const noexcept;
+
+ private:
+  /// The bitmap font as the plot layer sees it, at the size the text pipeline
+  /// draws it: `measure`/`height`/`ascent` scale with the glyph cell, so a label
+  /// can never be laid out for one size and drawn at another. The 8x8 font scales
+  /// by whole multiples of its cell only -- anything else lands a 1-bit glyph's
+  /// strokes unevenly -- so `reset()` picks the multiple nearest the requested
+  /// size.
+  class AxisFont final : public render::TextBackend {
+   public:
+    void reset(const render::BitmapFont& font, f32 requested_px) noexcept;
+    f32 measure(const char* utf8) const override;
+    f32 height() const override;
+    f32 ascent() const override;
+    /// On-screen size of one glyph cell, in pixels.
+    f32 cell_px() const noexcept { return cell_px_; }
+
+   private:
+    const render::BitmapFont* font_ = nullptr;
+    f32 cell_px_ = 1.0f;
+    f32 scale_ = 1.0f;
+  };
+
+  /// One axis pair's data -> pixel mapping, already shifted so the viewport's
+  /// top-left corner is the origin (the shaders emit NDC relative to the GL
+  /// viewport, not the framebuffer).
+  struct Mapping {
+    Vec2 scale{1.0f, 1.0f};
+    Vec2 offset{0.0f, 0.0f};
+  };
+
+  Mapping mapping(const Plot& plot, u32 axis_x, u32 axis_y) const noexcept;
+  /// The identity mapping for geometry already staged in viewport pixels.
+  Mapping pixel_mapping() const noexcept;
+  /// Shifts one framebuffer-space pixel position into viewport space.
+  Vec2 to_viewport(Vec2 px) const noexcept;
+
+  /// The single transfer point of the line pipeline: the staged `LineBatch`
+  /// payload (four floats per instance) goes to whichever strategy `init()`
+  /// selected. Callers of this helper never see which one is live.
+  void upload_vertices(std::span<const f32> interleaved) noexcept;
+  /// Same, for the text pipeline's four-float glyph instances.
+  void upload_text_vertices(std::span<const f32> interleaved) noexcept;
+
+  /// Uploads the staged line batch, maps it with (`scale`, `offset`), rasterises
+  /// it in one colour and empties the batch. A no-op for an empty batch, so the
+  /// caller can flush unconditionally.
+  void flush_lines(const Mapping& map, Color color, f32 width) noexcept;
+  /// Draws every staged glyph in `style().axis_text` and empties the batch. Text
+  /// is texture-bound, so this runs once per frame, after all line geometry.
+  void flush_text() noexcept;
+
+  /// Stages one data-space segment, taking the two axes' log flags into account.
+  void stage_segment(f64 x0, f64 y0, f64 x1, f64 y1, bool log_x, bool log_y);
+  /// Stages `n` interleaved `x,y` data-space points (a gathered window).
+  void stage_xy(const f32* xy, u32 n, bool log_x, bool log_y);
+  /// Lays one run of text out at a framebuffer position and stages it, shifting
+  /// it into viewport space. Returns the pen past the run.
+  Vec2 stage_text(Vec2 pos_px, const char* utf8);
+
+  void draw_grid(const Plot& plot);
+  void draw_axis_lines(const Plot& plot);
+  void draw_tick_labels(const Plot& plot);
+  void draw_titles(const Plot& plot);
+  void draw_traces(const Plot& plot);
+  void draw_cursors(const Plot& plot);
+  void draw_measures(const Plot& plot);
+  void draw_annotations(const Plot& plot);
+
+  render::BitmapFont font_;
+  AxisFont font_backend_;
+  render::Style style_;
+
+  gl::Texture atlas_;
+  render::LineProgram line_program_;
+  render::TextProgram text_program_;
+  render::LineVertexArray line_vao_;
+  render::TextVertexArray text_vao_;
+
+  render::LineBatch lines_;
+  render::TextBatch glyphs_;
+  std::vector<f32> gather_;   ///< scratch for one gathered window, re-used
+  mutable std::vector<u8> readback_;  ///< dump_frame_ppm's row buffer
+
+  Rect viewport_;      ///< the frame's region, top-left origin, framebuffer px
+  Rect gl_viewport_;   ///< the same rect flipped for GL, and for readback
+  u32 fb_w_ = 0;
+  u32 fb_h_ = 0;
+  bool ready_ = false;
+  bool persistent_ = false;
+  bool gl_error_ = false;
+  FrameStats stats_{};
+};
+
+}  // namespace plot