plotter/src/render/shaders.cpp

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

#include <plot/render/shaders.hpp>

// Both pipelines are `#version 330 core` and stay untouched by the upload
// strategy: the 4.3+ path changes how vertex data reaches the GPU, never the
// GLSL, so one program pair serves every context the library supports. The
// declarations here are the contract the render layer's attribute layouts and
// uniform setters mirror (see program.hpp, vertex_array.hpp, shaders.hpp).

namespace plot::render {

const char* const kLineVS = R"(#version 330 core
layout(location = 0) in vec2 a_p0;  // per-instance segment start, data space
layout(location = 1) in vec2 a_p1;  // per-instance segment end

uniform vec2 u_scale;     // data -> pixels
uniform vec2 u_offset;    // pixels
uniform vec2 u_viewport;  // render target size, pixels
uniform float u_width;    // stroke width, pixels
uniform float u_aa;       // AA fringe half-width, pixels

out float v_dist;   // signed distance to the segment axis, pixels
// Distance travelled from a_p0, pixels -- and the dash's phase anchor. Both of the
// crosshair's lines are staged from an *axes edge* (its vertical from `y_range.min`,
// the axes' bottom; its horizontal from `x_range.min`, the axes' left), so a dash measured
// from here is measured from a fixed edge of the rect: it does not crawl as the view pans
// or zooms, and a stationary crosshair renders identically frame to frame.
out float v_along;

void main() {
  vec2 p0 = a_p0 * u_scale + u_offset;
  vec2 p1 = a_p1 * u_scale + u_offset;
  vec2 d = p1 - p0;
  float len = length(d);
  float half_w = u_width * 0.5 + u_aa;  // + the AA fringe
  // A degenerate segment has no direction: pin the normal, and give the quad
  // area by stretching its two ends apart along x. Collapsing all four corners
  // onto p0 would rasterise nothing, and a flat trace staged as p == p (the
  // dense path's min == max buckets) would then silently vanish. When
  // len > 1e-6 the two ends are p0 and p1 exactly as before.
  vec2 n = len > 1e-6 ? vec2(-d.y, d.x) / len : vec2(0.0, 1.0);
  vec2 e0 = len > 1e-6 ? p0 : p0 - vec2(half_w, 0.0);
  vec2 e1 = len > 1e-6 ? p1 : p0 + vec2(half_w, 0.0);

  // gl_VertexID picks the corner: 0,1 are the sides of the segment's start and
  // 2,3 of its end, which is the order GL_TRIANGLE_STRIP expects.
  float side = (gl_VertexID & 1) == 0 ? -1.0 : 1.0;
  vec2 base = (gl_VertexID & 2) == 0 ? e0 : e1;
  vec2 px = base + n * (side * half_w);

  // Rect is top-left origin, GL is bottom-left: flip y once, here, and never
  // again in the fragment shader or in a caller.
  vec2 ndc = vec2(px.x / u_viewport.x * 2.0 - 1.0, 1.0 - px.y / u_viewport.y * 2.0);
  gl_Position = vec4(ndc, 0.0, 1.0);

  v_dist = side * half_w;
  v_along = (gl_VertexID & 2) == 0 ? 0.0 : len;
}
)";

const char* const kLineFS = R"(#version 330 core
in float v_dist;   // signed distance to the segment axis, pixels
in float v_along;  // distance travelled from a_p0, pixels

uniform float u_width;  // stroke width, pixels
uniform float u_aa;     // AA fringe half-width, pixels
uniform vec4 u_color;
uniform vec2 u_dash;    // dash pattern in pixels: x = ink, y = gap; y <= 0 is solid

out vec4 o_color;

void main() {
  // The dash (spec §17): a fragment-shader discard, never new geometry -- the vertices
  // stay in data space and the pattern is one per-draw uniform. `v_along` interpolates the
  // distance from `a_p0` across the quad in pixels, so `mod` measures the pattern along the
  // line rather than in screen space, which is what keeps it anchored to the segment while
  // the view pans. `y <= 0` is solid, so every other stroke in the library -- grid, traces,
  // axis lines, regions, cursors, the legend border -- draws unchanged by this uniform's
  // default.
  if (u_dash.y > 0.0) {
    float period = u_dash.x + u_dash.y;
    if (period > 0.0 && mod(v_along, period) >= u_dash.x) {
      discard;  // in the gap of the pattern
    }
  }
  float half_w = u_width * 0.5;
  float d = abs(v_dist);
  if (d > half_w + u_aa) {
    discard;  // outside the fringe; nothing to blend
  }
  // Ramp over the fringe so a thin stroke keeps its weight instead of aliasing
  // into a dashed line: the ramp is centred on `half_w` and spans `u_aa`, so
  // `u_aa == 1.0` -- the style default -- reproduces the historical single-pixel
  // ramp exactly. `u_aa` is the style's `line_aa_px` in device pixels, clamped
  // at 0.25 by the renderer.
  float alpha = 1.0 - smoothstep(half_w - 0.5 * u_aa, half_w + 0.5 * u_aa, d);
  o_color = vec4(u_color.rgb, u_color.a * alpha);
}
)";

const char* const kTextVS = R"(#version 330 core
layout(location = 0) in vec2 a_pos;   // per-instance glyph cell top-left, pixels
layout(location = 1) in vec2 a_uv;    // per-instance glyph rect top-left, normalised
layout(location = 2) in vec2 a_axis;  // per-instance cell +x axis, unit, pixels
layout(location = 3) in vec4 a_colour;  // per-instance tint, straight alpha

uniform vec2 u_viewport;  // render target size, pixels
uniform vec2 u_px_range;  // glyph cell on screen, pixels (width, height)
uniform vec2 u_glyph_uv;  // glyph cell in the atlas, normalised (width, height)

out vec2 v_uv;      // normalised atlas coordinate of this fragment
out vec4 v_colour;  // the instance's tint

void main() {
  // The cell's two axes: the run direction the instance carries and its
  // perpendicular, so the cell follows the basis instead of the screen axes. The
  // perpendicular is the cell's own +v axis -- with an unrotated basis
  // (1.0, 0.0) it is (0.0, 1.0), i.e. down the screen, which is the direction v
  // grows in the atlas -- and building it this way keeps the quad's orientation:
  // a mirrored perpendicular would draw every glyph flipped about its diagonal.
  vec2 ux = a_axis;
  vec2 uy = vec2(-ux.y, ux.x);
  vec2 corner = vec2(float(gl_VertexID & 1), float((gl_VertexID >> 1) & 1));
  vec2 px = a_pos + corner.x * u_px_range.x * ux + corner.y * u_px_range.y * uy;
  // Rect is top-left origin, GL is bottom-left: the one and only y flip.
  vec2 ndc = vec2(px.x / u_viewport.x * 2.0 - 1.0, 1.0 - px.y / u_viewport.y * 2.0);
  gl_Position = vec4(ndc, 0.0, 1.0);
  // The same handedness as the cell, so the glyph's rect lands on it the right
  // way round: u runs along the basis, v down the glyph.
  v_uv = a_uv + corner * u_glyph_uv;
  v_colour = a_colour;
}
)";

const char* const kTextFS = R"(#version 330 core
in vec2 v_uv;      // normalised atlas coordinate of this fragment
in vec4 v_colour;  // the instance's tint

uniform sampler2D u_atlas;  // GL_R8 glyph atlas, GL_NEAREST
uniform vec2 u_tex_size;    // atlas size in texels

out vec4 o_color;

void main() {
  // Sample the centre of the texel the fragment lands in. GL_NEAREST alone
  // would tie at a texel boundary, where the driver may pick the neighbouring
  // glyph cell; rounding to a centre removes the tie by construction.
  vec2 texel = clamp(floor(v_uv * u_tex_size), vec2(0.0), u_tex_size - vec2(1.0));
  float coverage = texture(u_atlas, (texel + vec2(0.5)) / u_tex_size).r;
  o_color = vec4(v_colour.rgb, v_colour.a * coverage);
}
)";

}  // namespace plot::render