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Edited
Jan 31, 2021
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Drawing indexed mesh data as screen-space normals without duplicating data
Also listed in…
WebGL
createREGL = require('regl')
regl = createREGL({
canvas: canvas,
extensions: ['ANGLE_instanced_arrays']
})
mesh = ({
vertices: regl.buffer(bunnyVertices),
faces: regl.elements(bunnyFaces),
normals: regl.buffer(bunnyNormals),
faceCount: bunnyFaces.length,
vertexCount: bunnyVertices.length,
})
drawMesh = regl({
vert: `
precision highp float;
attribute vec3 aVertex, aNormal;
uniform mat4 uProjectionView;
varying vec3 vNormal;
void main () {
vNormal = aNormal;
gl_Position = uProjectionView * vec4(aVertex, 1);
}
`,
frag: `
precision highp float;
varying vec3 vNormal;
void main () {
gl_FragColor = vec4(0.5 + 0.5 * vNormal, 1);
}
`,
attributes: {
aVertex: regl.prop('vertices'),
aNormal: regl.prop('normals'),
},
elements: regl.prop('faces'),
});
drawNormals = {
return regl({
vert: `
precision highp float;
uniform mat4 uProjectionView, uView, uProjection;
uniform float uTailWidth, uAspect, uScale, uForeshortening;
uniform vec2 uArrowheadShape;
attribute vec3 aVertex, aNormal;
attribute vec4 aArrow;
void main () {
vec4 vp = uView * vec4(aVertex, 1);
vec4 vpn = uView * vec4(aVertex + uScale * aNormal, 1);
// Project the vertex into homogeneous coordinates
vec4 p = uProjection * vp;
// Project the vertex + normal into homogeneous coordinates
vec4 pn = uProjection * vpn;
float foreshortening = max(0.1, sqrt(1.0 - uForeshortening * abs((vp - vpn).z) / length((vp - vpn).xyz)));
// Use the y component of aArrow to select either p or pn
gl_Position = mix(p, pn, aArrow.y);
// Compute a screen-space vector parallel to the arrow.
// This step includes "perspective division" to convert homogeneous
// 4D coordinates into screen space coordinates.
// NB: it also includes an aspect ratio to scale x and y equally. This could be
// done more cleanly.
vec2 unitVector = normalize((pn.xy / pn.w - p.xy / p.w) * vec2(uAspect, 1));
// Rotate 90 degrees to get a perpendicular vector
vec2 perpUnitVector = vec2(-unitVector.y, unitVector.x);
// Perturb the point according to the aArrow instance data
gl_Position.xy += (
// Offset perpendicular to the length of the arrow:
perpUnitVector * (aArrow.x * uTailWidth + aArrow.w * uArrowheadShape.y) +
// and parallel to the length of the arrow:
+ unitVector * aArrow.z * uArrowheadShape.x * foreshortening
// This final step is just a bit tricky, but we need to pull the aspect
// ratio back out and then multiply by w to get the arrow scaled correctly
) / vec2(uAspect, 1) * gl_Position.w;
}
`,
frag: `
precision highp float;
uniform vec4 uColor;
void main () {
gl_FragColor = uColor;
}
`,
attributes: {
aVertex: {
buffer: regl.prop('vertices'),
divisor: 1, // Advance the mesh vertex once per instance
stride: 12 // each instance advances 3 floats (= 12 bytes)
},
aNormal: {
buffer: regl.prop('normals'),
divisor: 1,
stride: 12
},
// prettier-ignore
aArrow: new Float32Array([
// The per-instance triangles are defined in terms of four pieces of data which tell where
// on the arrow we are, using the mesh vertex and mesh normal as inputs. The components are:
// x: selects the position perpendicular to the length of the arrow in screen space
// y: selects either the (vertex) or (vertex + normal) in 3D space
// z: selects the arrowhead length-wise offset in screen space
// w: selects the arrowhead width-wise offset in screen space
// The first triangle of the tail:
-1, 0, 0, 0,
1, 0, 0, 0,
1, 1, -1, 0,
// The second triangle of the tail:
-1, 0, 0, 0,
1, 1, -1, 0,
-1, 1,-1, 0,
// The arrowhead:
0, 1, -1, -1,
0, 1, -1, 1,
0, 1, 0, 0
])
},
uniforms: {
// Define the screen-space line width in terms of a property but also
// scaled by the pixel ratio so that it remains constant at different
// pixel ratios. (The framebuffer height is just for proper screen-space
// scaling)
uTailWidth: (ctx, props) =>
(props.arrowTailWidth / ctx.framebufferHeight) * ctx.pixelRatio,
// Define the shape of the arrowhead. This just the scale factor
// for the ones and zeros above.
uArrowheadShape: (ctx, props) => [
(props.arrowheadLength / ctx.framebufferHeight) * ctx.pixelRatio * 2.0,
(props.arrowheadWidth / ctx.framebufferHeight) * ctx.pixelRatio
],
// The aspect ratio affects computation of offsets for the screen-space
// lines.
uAspect: ctx => ctx.framebufferWidth / ctx.framebufferHeight,
uColor: regl.prop('arrowColor'),
uForeshortening: (ctx, props) => (props.foreshortening ? 1 : 0),
// Not really necessary, but an overall scale factor for the normals
uScale: regl.prop('arrowScale')
},
primitive: 'triangles',
instances: (ctx, props) => props.vertexCount, // One instance per vertex
count: 9 // Nine vertices per instance
});
}
mutable raf = null
meshWithProps = {
// This triggers a redraw whenever the view properties change
camera.taint();
return Object.assign({}, mesh, {
arrowTailWidth,
arrowheadWidth,
arrowheadLength,
arrowScale,
foreshortening,
arrowColor: hexToRgb(arrowColor)
});
}
{
if (mutable raf !== null) {
mutable raf.cancel()
mutable raf = null;
}
// whenever we modify this, trigger a redraw:
camera.taint()
mutable raf = regl.frame(() => {
camera((state) => {
if (!state.dirty) return;
regl.clear({color: [1, 1, 1, 1]})
// This is the crux of it all! We don't need to do anything special
// with the data to render it as either a mesh or as screen-space
// normal arrows.
drawMesh(meshWithProps)
drawNormals(meshWithProps)
})
})
}
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