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feat(render): GPU analytical rendering for circles, arcs, and ellipses

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- Render planar circles, circular arcs, ellipses, and elliptical arcs analytically via GPU-instanced screen-space quads and WGSL shaders.
- Implement exact arc sweep clamping with round end caps matching the CAD wire shader.
- Implement robust Newton-Raphson closest-point solver for ellipses with isotropic screen-space derivative anti-aliasing.
- Integrate into wire pipeline and PersistentWireArena upload paths.
- Remove unused mesh_volume_centroid function in doc_api.
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Karim Jerbi committed 2026-09-07 13:50:12 +01:00
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commit 0425f979f9
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src/shaders/circle.wgsl
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// GPU-instanced analytical circle shader — renders 2D/planar circles as screen/plane-aligned quads.
// Topology: TriangleList, 6 vertices drawn per INSTANCE.
//
// One instance = one complete circle. The six vertex IDs map to the corners of a
// two-triangle quad covering the circle's bounding box (+ lineweight margin).
//
// The fragment shader analytically computes the exact distance to the circle perimeter:
// d_world = abs(length(local_xy) - radius)
// and uses screen-space derivatives (fwidth) to achieve perfect sub-pixel anti-aliasing
// at any zoom level, without any polygon faceting or CPU re-tessellation.
struct Uniforms {
viewport_size: vec2<f32>,
world_per_pixel: f32,
lwdisplay_enable: f32,
flat_shade: f32,
transparency_enable: f32,
linetype_scale: f32,
lineweight_scale: f32,
// Relative-to-eye (double-single)
view_rot: mat4x4<f32>,
eye_high: vec3<f32>,
_pad_eh: f32,
eye_low: vec3<f32>,
_pad_el: f32,
}
@group(0) @binding(0) var<uniform> u: Uniforms;
const DRAW_ORDER_BIAS: f32 = 0.001;
const MODEL_LINEWEIGHT_BOOST: f32 = 2.0;
const MODEL_LINEWEIGHT_MAX_PX: f32 = 10.0;
const TAU: f32 = 6.283185307179586;
struct InstanceIn {
@location(0) center_high: vec4<f32>, // xyz = center_high, w = unused
@location(1) center_low: vec4<f32>, // xyz = center_low, w = radius
@location(2) axis_x: vec4<f32>, // xyz = axis_x, w = start_angle
@location(3) axis_y: vec4<f32>, // xyz = axis_y, w = end_angle
@location(4) color: vec4<f32>, // rgba
@location(5) params: vec4<f32>, // half_width_px, pattern_length, draw_depth, unused
@location(6) pat0: vec4<f32>,
@location(7) pat1: vec4<f32>,
}
struct VertexOut {
@builtin(position) clip_pos: vec4<f32>,
@location(0) local_pos: vec2<f32>,
@location(1) radius: f32,
@location(2) hw_px: f32,
@location(3) color: vec4<f32>,
@location(4) pattern_length: f32,
@location(5) pat0: vec4<f32>,
@location(6) pat1: vec4<f32>,
@location(7) @interpolate(flat) min_elem: f32,
@location(8) @interpolate(flat) start_angle: f32,
@location(9) @interpolate(flat) end_angle: f32,
}
fn resolve_hw(px_hw: f32) -> f32 {
var display_hw = max(px_hw * u.lineweight_scale, 0.5);
if u.lineweight_scale < 0.0 {
let scale = -u.lineweight_scale;
let base_hw = select(
min(px_hw * MODEL_LINEWEIGHT_BOOST, MODEL_LINEWEIGHT_MAX_PX * 0.5),
0.5,
px_hw <= 0.5,
);
display_hw = max(base_hw * scale, 0.5);
}
return select(0.5, display_hw, u.lwdisplay_enable > 0.5);
}
@vertex fn vs_main(@builtin(vertex_index) vid: u32, in: InstanceIn) -> VertexOut {
// Two-triangle unit quad corner table:
// vid 0,1,2 = (-1,-1) ( 1,-1) ( 1, 1)
// vid 3,4,5 = (-1,-1) ( 1, 1) (-1, 1)
let u_arr = array<f32, 6>(-1.0, 1.0, 1.0, -1.0, 1.0, -1.0);
let v_arr = array<f32, 6>(-1.0, -1.0, 1.0, -1.0, 1.0, 1.0);
let u_val = u_arr[vid];
let v_val = v_arr[vid];
let center_high = in.center_high.xyz;
let center_low = in.center_low.xyz;
let radius = in.center_low.w;
let axis_x = in.axis_x.xyz;
let axis_y = in.axis_y.xyz;
let color = in.color;
let hw_px = resolve_hw(in.params.x);
let pattern_length = in.params.y;
let draw_depth = in.params.z;
// Expand bounding quad so anti-aliasing margin and lineweight never clip
let margin_world = (hw_px + 2.0) * max(u.world_per_pixel, 1e-6);
let ext = radius + margin_world;
let center_rel = (center_high - u.eye_high) + (center_low - u.eye_low);
let world_pos_rel = center_rel + (u_val * axis_x + v_val * axis_y) * ext;
var clip_pos = u.view_rot * vec4<f32>(world_pos_rel, 1.0);
clip_pos.z = clip_pos.z - draw_depth * DRAW_ORDER_BIAS * clip_pos.w;
var out: VertexOut;
out.clip_pos = clip_pos;
out.local_pos = vec2<f32>(u_val * ext, v_val * ext);
out.radius = radius;
out.hw_px = hw_px;
out.color = color;
out.pattern_length = pattern_length;
out.pat0 = in.pat0;
out.pat1 = in.pat1;
// Compute smallest non-zero pattern element for LOD short-circuiting
let elems = array<f32, 8>(in.pat0.x, in.pat0.y, in.pat0.z, in.pat0.w, in.pat1.x, in.pat1.y, in.pat1.z, in.pat1.w);
var min_elem = 1e9;
for (var i = 0u; i < 8u; i++) {
let e = abs(elems[i]);
if e > 0.0 && e < min_elem {
min_elem = e;
}
}
out.min_elem = select(0.0, min_elem, min_elem < 1e8);
out.start_angle = in.axis_x.w;
out.end_angle = in.axis_y.w;
return out;
}
fn mod_tau(val: f32) -> f32 {
let m = val % TAU;
return select(m, m + TAU, m < 0.0);
}
fn in_dash(dist: f32, pat_len: f32, p0: vec4<f32>, p1: vec4<f32>) -> bool {
let elems = array<f32, 8>(p0.x, p0.y, p0.z, p0.w, p1.x, p1.y, p1.z, p1.w);
var count = 0u;
for (var i = 0u; i < 8u; i++) {
if elems[i] != 0.0 { count = i + 1u; }
}
if count == 0u {
return true;
}
let d = ((dist % pat_len) + pat_len) % pat_len;
var pos = 0.0f;
let dot_half = u.world_per_pixel * 0.75;
for (var i = 0u; i < count; i++) {
let elem = elems[i];
if elem == 0.0 {
let dd = abs(d - pos);
if min(dd, pat_len - dd) <= dot_half { return true; }
} else if elem > 0.0 {
if d >= pos && d < pos + elem { return true; }
pos += elem;
} else {
pos += -elem;
}
}
return false;
}
@fragment fn fs_main(in: VertexOut) -> @location(0) vec4<f32> {
let r = length(in.local_pos);
// Screen-space derivative anti-aliasing:
// Use the gradient of the monotonic radial distance `r` instead of `abs(r - radius)`.
// Taking derivatives across the cusp of abs() cancels out at scanline crossings,
// which previously collapsed fw to near-zero and punched periodic 1-2px holes (false dashes).
let grad = vec2<f32>(dpdx(r), dpdy(r));
let grad_len = length(grad);
let fw = max(select(u.world_per_pixel, grad_len, grad_len > 1e-6), 1e-6);
let sa = in.start_angle;
let ea = in.end_angle;
var sweep = mod_tau(ea - sa);
if sweep <= 1e-5 && abs(ea - sa) > 1e-5 {
sweep = TAU;
}
let angle = atan2(in.local_pos.y, in.local_pos.x);
let theta = select(angle, angle + TAU, angle < 0.0);
let d_theta = mod_tau(theta - sa);
var d_world: f32;
var arc_dist: f32;
if sweep >= TAU - 1e-5 || d_theta <= sweep {
// Inside arc sweep or full circle
d_world = abs(r - in.radius);
arc_dist = in.radius * d_theta;
} else {
// Outside sweep: measure distance to start and end endpoints for round end caps
let p_start = in.radius * vec2<f32>(cos(sa), sin(sa));
let p_end = in.radius * vec2<f32>(cos(ea), sin(ea));
let d_start = length(in.local_pos - p_start);
let d_end = length(in.local_pos - p_end);
if d_start < d_end {
d_world = d_start;
arc_dist = 0.0;
} else {
d_world = d_end;
arc_dist = in.radius * sweep;
}
}
let d_px = d_world / fw;
let alpha_cov = clamp(0.5 + in.hw_px - d_px, 0.0, 1.0);
if alpha_cov <= 0.0 {
discard;
}
// Linetype dash pattern test
if in.pattern_length > 0.0 {
let pat_len = in.pattern_length * u.linetype_scale;
if in.min_elem * u.linetype_scale >= u.world_per_pixel && pat_len > 0.0 {
if !in_dash(arc_dist, pat_len, in.pat0, in.pat1) {
discard;
}
}
}
let alpha = select(1.0, in.color.a, u.transparency_enable > 0.5) * alpha_cov;
return vec4<f32>(in.color.rgb, alpha);
}