plotter/include/plot/render/style.hpp

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

#pragma once
#include <plot/core/color.hpp>

namespace plot::render {

/// Every colour and metric the renderer draws with. Defaults are the dark theme
/// the plan's render smoke test asserts against; a caller swaps a field and the
/// next frame picks it up, so nothing here is baked into a shader.
///
/// ## Units: every metric below is **logical**, and `device()` converts it
///
/// A metric a user perceives as a *size* -- a font, a stroke width, a padding, a
/// marker, a swatch, the minimum spacing between ticks -- is written in logical
/// units, so a 12 px label is the same physical size on a 1x panel and a 2x one
/// and a 1 px grid is the same hairline on both. The host states what its display
/// is with `ui_scale` (device pixels per logical unit) and every consumer converts
/// exactly once, through `device()`: `Plot::layout()` when it sizes the label
/// strips, the title strips and the legend's chrome, the renderer when it converts
/// widths and font px.
///
/// The one exception is `dense_threshold_spp`, which is a **density** rather than a
/// size: samples per *rasterised* pixel. It is compared against a samples-per-pixel
/// ratio computed from the framebuffer's own width, so scaling it would make the
/// chosen LOD level depend on the display and not on the data. It is used raw.
///
/// The same rule classifies the metrics that live outside this struct, which are
/// logical too and converted by the same call:
///
///   * `Legend`'s pad, swatch, gap and margin;
///   * `Trace`'s (and `TraceDesc`'s) `width` -- a stroke like `line_width_px`, and
///     the one a plot draws most of, so leaving it raw would invert the
///     trace/axis stroke hierarchy at any scale above 1;
///   * the renderer's marker arm lengths and text offsets.
struct Style {
  // --- colours: neither logical nor device, so nothing to scale ------------

  Color background = Color::hex(0x101418);
  /// The labelled grid. Brighter than the subdivisions and still well under `axis_line`
  /// (0x8A94A6): the grid is structure the eye can read, the axis is the frame, and the
  /// hierarchy between the three is what keeps a dense plot legible. It is already fully
  /// opaque, so "more prominent" means a brighter colour and nothing else.
  Color grid_major = Color::hex(0x39414C);
  /// The minor grid's colour, **with an alpha** so it blends into the background rather than
  /// sitting on it: a subdivision is a reading aid, and at full opacity a dense minor grid
  /// reads as a second set of ticks competing with the labelled one. The line shader applies
  /// `u_color.a` (see `src/render/shaders.cpp`), so this one blends and the major -- opaque at
  /// 0x39414C -- stays the structure the eye picks out. Together with `grid_minor_width` and
  /// the dash below, the defaults put a minor line at an order of magnitude less ink than a
  /// major one.
  Color grid_minor = Color::hex(0x1C2126, 0.5f);
  Color axis_line = Color::hex(0x8A94A6);
  Color axis_text = Color::hex(0xC8D0DC);
  Color cursor = Color::hex(0xFFB020);
  Color measure_fill = Color::hex(0xFFB020, 0.15f);
  /// The box-zoom selection's frame, drawn while a modified left drag is choosing a range: a light
  /// neutral near full alpha for the border and the same neutral at a low one for the fill. Neutral
  /// and not a trace colour, on `invalid_fill`'s argument -- the frame says "this is the area", and a
  /// tint a trace could have would read as data. The fill's alpha is chosen against the same floor:
  /// low enough that what is under the frame stays readable, because the point is to show what is
  /// being selected, not to hide it.
  Color selection_fill = Color::hex(0xC8D0DC, 0.12f);
  Color selection_edge = Color::hex(0xC8D0DC, 0.85f);
  /// The shading over a span the host has declared invalid (`Plot::set_invalid_spans()`): the
  /// axis furniture's own neutral grey at a low alpha, on the same model as `measure_fill`
  /// above. Neutral and not a trace colour on purpose -- a band is the *absence* of data, and
  /// one in a hue a trace could have would read as a signal rather than as a gap. The alpha is
  /// low enough that the grid under it stays legible, which is what makes the band read as a
  /// region of the plot rather than as a panel laid over it.
  Color invalid_fill = Color::hex(0x8A94A6, 0.18f);
  /// The same shading for a span whose samples have since arrived and been written in, at well
  /// under half the alpha. The gap is still reported -- the window was drawn without that data --
  /// but a reader can tell a hole that is still empty from one that has been filled. Fainter by
  /// alpha alone and not by hue, so the two read as two depths of one mark rather than as two
  /// different kinds of thing; and still above the threshold where the difference from *no* band
  /// is visible, which is the floor this alpha is chosen against.
  Color invalid_repaired_fill = Color::hex(0x8A94A6, 0.08f);
  /// The tint a plot's **phosphor density** is drawn in (`Plot::set_density()`): the grid's
  /// luminance comes from the accumulated counts, and this is the monochrome hue it is multiplied
  /// into -- the density says *how often*, so it must not also say *which signal*, and a trace
  /// colour here would make a ghost read as a trace that is not there. A near-white neutral, one
  /// step brighter than `invalid_fill`'s grey, because this is ink the plot *has* and a hole is
  /// ink it is missing. The alpha is the whole brightness scale's ceiling -- the shader multiplies
  /// it by each cell's own value -- so under 1 it puts a lid on how bright the display can get.
  Color density = Color::hex(0xDFF2FF, 0.85f);
  /// The crosshair's colour: its two lines and the readout panel's border (spec §15). Markers are
  /// deliberately *not* this colour -- they carry each trace's own, because a marker says *which*
  /// trace and the line says *where*.
  ///
  /// **Hue-neutral by design**, and that is the whole choice: a saturated colour here would be
  /// confusable with a trace, and the library's own default trace colour is a saturated cyan
  /// (`Trace`'s default, 0x40C0FF) that a hue-based reading cannot tell from a cyan crosshair --
  /// only lightness would separate them, which no reader can rely on. A near-white can never be a
  /// saturated trace of *any* hue, and it is still plainly not `cursor`'s orange, which is what
  /// the spec asks for.
  ///
  /// The stroke is thin and dashed by default -- see `crosshair_width`, `crosshair_dash_on` and
  /// `crosshair_dash_off` below for the values: thinner than a trace so the instrument reads as
  /// chrome rather than data, and dashed so it is distinguishable by *form* as well as by
  /// lightness.
  Color crosshair = Color::hex(0xE6EBF2);

  /// The crosshair's stroke and dash, in logical pixels (spec §17). The crosshair is a
  /// *passive* hover instrument drawn over the data, so its default **recedes**: a
  /// thinner stroke than a trace's (`Trace::width`'s 1.5) so it reads as chrome rather
  /// than as data, and a dash so it is distinguishable from the grid and the traces at
  /// a glance.
  ///
  /// `crosshair_dash_on` is the ink length and `crosshair_dash_off` the gap; **`off == 0`
  /// means solid**, which is the value every *other* line in the library is drawn at, so
  /// the one pipeline serves them unchanged (the dash is a fragment-shader discard on
  /// `v_along`, so no geometry is added). A `Crosshair` may override all three for one
  /// plot without touching this global style.
  f32 crosshair_width = 0.75f;
  f32 crosshair_dash_on = 4.0f;
  f32 crosshair_dash_off = 4.0f;
  /// The full extent, in logical pixels, of the little cross drawn at each trace's readout
  /// point (`CrosshairRow::marker`), in that trace's own colour.
  ///
  /// **Deliberately larger than `marker_px`**, which is what a trace's own samples are dotted
  /// with at a deep zoom. In the default `CrosshairValue::NearestSample` the readout's mark is
  /// *snapped to a sample*, so it lands among those dots -- at the same size it would read as
  /// one more sample instead of as "this is the one the panel is quoting". The lines are chrome
  /// and recede; this mark is the answer to the question the hover asks, and it is the one
  /// crosshair element that should not.
  ///
  /// Its stroke is `crosshair_marker_width`, thicker than the lines' for the same reason. `0`
  /// across draws no mark at all, which is how a caller that wants the panel without the marks
  /// says so.
  f32 crosshair_marker_px = 9.0f;
  f32 crosshair_marker_width = 1.5f;

  // --- the scale ----------------------------------------------------------

  /// Device pixels per logical unit. 1.0 is the 1x display every default here was
  /// chosen on. Supplied by the host (`Renderer::style()` is what an adapter
  /// writes), and handed to the plot by `Renderer::draw()` before it lays out, so
  /// layout and drawing cannot disagree about it.
  f32 ui_scale = 1.0f;

  // --- logical metrics: a size a user perceives, scaled by `device()` ------

  f32 axis_font_px = 12.0f;        ///< logical em the axis text is drawn at.
  f32 grid_label_pad = 4.0f;       ///< logical gap between a tick label and the axes,
                                   ///< and the inset inside a title strip.
  /// The two grid classes, each with its own width and its own switch. The grid's
  /// *positions* come from the tick set alone -- `Grid::build()` tags every segment
  /// with the class of the tick it came from -- so these four decide only how a
  /// class is drawn. A class switched off stages no geometry at all: the renderer
  /// skips it before the first segment is staged, so a dense plot's minor grid
  /// costs nothing rather than being drawn transparent (spec §11). The colours
  /// above are already per class.
  /// When a trace's window holds at most this many raw samples, each one is also drawn as a dot
  /// `marker_px` logical pixels across, in the trace's own colour. `0` draws none. Zoomed in far
  /// enough to see individual samples -- a handful of them across a whole panel -- the polyline
  /// *between* them stops being the shape, and the samples themselves are what a reader is looking
  /// at; a marker makes each one a place rather than a bend in a line.
  f32 marker_below_samples = 50.0f;
  f32 marker_px = 3.0f;
  f32 grid_major_width = 1.0f;     ///< logical width of a major (labelled-tick) line.
  /// Logical width of a minor (subdivision) line: **thinner than the major hairline**, and
  /// sub-pixel on purpose. At 1x that is expressed as *coverage* -- the anti-aliased ramp
  /// (`line_aa_px`) lights one column at roughly three quarters of the ink -- and on a HiDPI
  /// display as a line that is literally narrower. The point is hierarchy: the labelled grid is
  /// structure, the subdivisions are texture.
  f32 grid_minor_width = 0.75f;
  bool grid_major_visible = true;  ///< stage the major grid at all.
  bool grid_minor_visible = true;  ///< stage the minor grid at all.
  /// The minor grid's dash, in logical pixels: ink length and gap. **`off == 0` draws it
  /// solid** -- the same convention as the crosshair's and a trace's dash -- which is what a
  /// caller that wants the old solid subdivision states. Dashed is the default because an
  /// unlabelled line at an unlabelled position should never be mistaken for a labelled one, and
  /// because a broken line carries less weight over data than a continuous one. The dash is a
  /// fragment discard, never extra geometry, so a dense minor grid costs no more vertices for
  /// being dashed.
  f32 grid_minor_dash_on = 3.0f;
  f32 grid_minor_dash_off = 3.0f;
  f32 line_width_px = 1.5f;        ///< logical width of the strokes framing the data
                                   ///< (axis lines, cursor markers).
  /// Logical half-width of the anti-aliased fringe the line shader blends at
  /// every stroke edge (the smoothstep ramp runs over `|dist| in [w - aa,
  /// w + aa]`). Clamped to at least 0.25 device px at use: below that the
  /// ramp is thinner than a fragment and edges alias again. 1.0 is the value
  /// every default was chosen on.
  f32 line_aa_px = 1.0f;
  /// Logical width of a *draggable handle* drawn highlighted: the tab a hovered or
  /// grabbed cursor marker is widened to, and the tab a region's hot bound is drawn
  /// as -- the latter sized by `Plot::layout()` with this same metric, so the two
  /// kinds of handle are one width. Wider than `line_width_px` on purpose: the cue
  /// for "this marker will follow your drag" has to be visible on a marker whose
  /// own colour may already be the accent.
  f32 handle_width_px = 5.0f;
  f32 tick_min_px = 60.0f;         ///< logical minimum spacing between major ticks, so
                                   ///< a 2x display gets the same tick *count*.

  // --- device metric: a density, never scaled ------------------------------

  f32 dense_threshold_spp = 1.5f;  ///< spec §4.3: samples per **device** pixel at or
                                   ///< above which a trace is gathered from the LOD
                                   ///< pyramid instead of from its raw samples. A
                                   ///< density, not a size: it is deliberately not
                                   ///< scaled by `ui_scale`, or the chosen level
                                   ///< would move with the display's DPI. It selects
                                   ///< the gather level, never a primitive: both
                                   ///< sides draw through the one instanced
                                   ///< anti-aliased quad path.

  // --- intensity grading: counts and ratios, never sizes -------------------

  /// Draw a dense trace's pixel columns **graded by how long the signal dwells at
  /// each height**, the way a digital-phosphor scope does, instead of as one flat
  /// band per column: bright where samples pile up (a flat top, a peak), faint
  /// where they only pass through (an edge, a noise excursion).
  ///
  /// Off by default, because it is a change of *look* and not a correction: the
  /// flat band is the honest answer to "what did this column reach", and a host
  /// that wants that -- or that draws few enough samples never to take the column
  /// path at all -- should not have its traces restyled by an upgrade. A scope-like
  /// host turns it on.
  ///
  /// It is not derivable from the flat band. A column's min and max are the same
  /// whether its samples fill the span or sit piled at its two ends, which is
  /// exactly the difference between a ramp and a pulse -- so grading needs the
  /// gather asked at a *finer* level than the pixel grid (`intensity_sub_columns`)
  /// and reads the distribution from those sub-buckets.
  ///
  /// The measure is *dwell per row*, and the weighting is the whole of it: each
  /// sub-column segment deposits **one unit spread over the rows it spans**, so a
  /// tall crossing leaves a thin smear and a short one at a turning point leaves a
  /// bright mark. Counting a row at +1 per segment that touches it instead -- the
  /// obvious thing -- grades a sine's fast middle as brightly as its rails, which
  /// is the pattern inverted.
  ///
  /// Below `dense_threshold_spp`, where there are no columns to reduce, this grades
  /// the **polyline** by each segment's own slope instead (`FrameStats::slope_traces`
  /// against `graded_traces`) -- the same measure for a sparse window, where a
  /// segment *is* the sub-column. One switch covers both because the alternative was
  /// a defect: gated on the reduction, the model switched off as a window narrowed
  /// past the threshold, and an edge drawn at a third of a flat top's ink jumped to
  /// all of it in one pixel of zoom.
  bool intensity_grading = false;

  /// Sub-buckets per pixel column the gather is asked for when grading: the level
  /// is chosen for `samples_per_pixel / intensity_sub_columns`, so a column's
  /// density is read from this many envelopes instead of from one.
  ///
  /// It buys resolution in *dwell*, linearly, for the same factor in gathered
  /// points -- and the points stay bounded by the screen, which is the property
  /// that matters (8 of them over a 1000 px axis is 16 000 points whatever the
  /// window holds). Below about 4 a pulse's flat levels and its edge are too few
  /// samples apart to separate.
  ///
  /// It is a *want*, not a promise: the renderer floors the level it asks for at the
  /// pyramid's coarsest bucket level, so a window too sparse to hold this many buckets
  /// a column is graded from however many it does hold. Without that floor `level_for`
  /// answers 0 -- raw samples -- the moment a sub-column falls under one bucket, which
  /// turns a frame the flat path draws off the pyramid into an O(visible) gather. So
  /// raising this past about 8 costs resolution nothing and buys it nothing either: the
  /// bucket size is the limit, not this number.
  u32 intensity_sub_columns = 8;

  /// Quantisation of the grading, in alpha steps. Each step is one draw call (the
  /// line pipeline's colour is a uniform, so a per-segment alpha would mean a
  /// per-instance one), and the segments are bounded by the columns, so the cost
  /// of a step is a draw call over a few hundred instances.
  ///
  /// Four is enough to read a waveform's structure and few enough that a step is a
  /// visible tier rather than a gradient -- the point is to separate "dwelt here"
  /// from "passed through", not to measure.
  u32 intensity_steps = 4;

  /// Alpha the faintest graded step is drawn at, as a fraction of the trace's own.
  ///
  /// Not 0: a column the signal merely passed through is still a column the signal
  /// *reached*, and a riser dimmed out of existence is a square wave drawn as two
  /// disconnected lines. This is the floor that keeps the trace continuous.
  f32 intensity_floor = 0.3f;

  /// Exponent the normalised dwell is raised to before it is quantised. Below 1 it
  /// lifts the faint end, which is what makes a fast edge visible beside a flat top
  /// whose dwell is three orders of magnitude greater.
  ///
  /// The dwell is normalised **against the column's own peak**, not against a scale
  /// for the whole frame, so the steps spend their range on the structure *inside*
  /// each column. One scale for the frame needs the whole `w * h` density image
  /// (33 MB at 4K, which is why the accumulator is one column wide) and reads
  /// worse: a column the signal flew straight through holds little dwell at any
  /// height, so it would grade uniformly faint -- a trace that fades out exactly
  /// where it moves fastest.
  f32 intensity_gamma = 0.45f;

  /// `logical` in device pixels: the single conversion point for every logical
  /// metric above, so the arithmetic appears once per consumer instead of a `*
  /// ui_scale` scattered over the renderer and the layout.
  ///
  /// A scale that is not positive is not a scale -- a host that has not measured
  /// its display, a field nobody filled in -- and converts 1:1 rather than
  /// collapsing every strip, font and stroke to nothing.
  f32 device(f32 logical) const noexcept {
    return ui_scale > 0.0f ? logical * ui_scale : logical;
  }
};

}  // namespace plot::render