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Diffstat (limited to 'vendor/gioui.org/shader/gio/stencil.frag')
| -rw-r--r-- | vendor/gioui.org/shader/gio/stencil.frag | 81 |
1 files changed, 81 insertions, 0 deletions
diff --git a/vendor/gioui.org/shader/gio/stencil.frag b/vendor/gioui.org/shader/gio/stencil.frag new file mode 100644 index 0000000..956dae8 --- /dev/null +++ b/vendor/gioui.org/shader/gio/stencil.frag @@ -0,0 +1,81 @@ +#version 310 es + +// SPDX-License-Identifier: Unlicense OR MIT + +precision mediump float; + +layout(location=0) in highp vec2 vFrom; +layout(location=1) in highp vec2 vCtrl; +layout(location=2) in highp vec2 vTo; + +layout(location = 0) out vec4 fragCover; + +void main() { + float dx = vTo.x - vFrom.x; + // Sort from and to in increasing order so the root below + // is always the positive square root, if any. + // We need the direction of the curve below, so this can't be + // done from the vertex shader. + bool increasing = vTo.x >= vFrom.x; + vec2 left = increasing ? vFrom : vTo; + vec2 right = increasing ? vTo : vFrom; + + // The signed horizontal extent of the fragment. + vec2 extent = clamp(vec2(vFrom.x, vTo.x), -0.5, 0.5); + // Find the t where the curve crosses the middle of the + // extent, x₀. + // Given the Bézier curve with x coordinates P₀, P₁, P₂ + // where P₀ is at the origin, its x coordinate in t + // is given by: + // + // x(t) = 2(1-t)tP₁ + t²P₂ + // + // Rearranging: + // + // x(t) = (P₂ - 2P₁)t² + 2P₁t + // + // Setting x(t) = x₀ and using Muller's quadratic formula ("Citardauq") + // for robustnesss, + // + // t = 2x₀/(2P₁±√(4P₁²+4(P₂-2P₁)x₀)) + // + // which simplifies to + // + // t = x₀/(P₁±√(P₁²+(P₂-2P₁)x₀)) + // + // Setting v = P₂-P₁, + // + // t = x₀/(P₁±√(P₁²+(v-P₁)x₀)) + // + // t lie in [0; 1]; P₂ ≥ P₁ and P₁ ≥ 0 since we split curves where + // the control point lies before the start point or after the end point. + // It can then be shown that only the positive square root is valid. + float midx = mix(extent.x, extent.y, 0.5); + float x0 = midx - left.x; + vec2 p1 = vCtrl - left; + vec2 v = right - vCtrl; + float t = x0/(p1.x+sqrt(p1.x*p1.x+(v.x-p1.x)*x0)); + // Find y(t) on the curve. + float y = mix(mix(left.y, vCtrl.y, t), mix(vCtrl.y, right.y, t), t); + // And the slope. + vec2 d_half = mix(p1, v, t); + float dy = d_half.y/d_half.x; + // Together, y and dy form a line approximation. + + // Compute the fragment area above the line. + // The area is symmetric around dy = 0. Scale slope with extent width. + float width = extent.y - extent.x; + dy = abs(dy*width); + + vec4 sides = vec4(dy*+0.5 + y, dy*-0.5 + y, (+0.5-y)/dy, (-0.5-y)/dy); + sides = clamp(sides+0.5, 0.0, 1.0); + + float area = 0.5*(sides.z - sides.z*sides.y + 1.0 - sides.x+sides.x*sides.w); + area *= width; + + // Work around issue #13. + if (width == 0.0) + area = 0.0; + + fragCover.r = area; +} |