The part is id & 15, here a face: GLH.box() stores four vertices per face, so gl_VertexID / 4 is the face number in an indexed draw (a real model would carry a part attribute). The older trick of writing the part number into the visible canvas's alpha limits a scene to 255 parts and makes the canvas translucent unless it is created with alpha: false.
| Face (gl_VertexID / 4) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Box side | +x | -x | +y | -y | +z | -z |
| Book part | fore-edge | spine | top | bottom | front cover | back cover |
<!doctype html>
<style>
body { margin: 0; font: 12px Georgia, serif; background: #f7f4ee; color: #333; }
.wrap { position: relative; width: 100%; max-width: 600px; }
canvas { display: block; width: 100%; cursor: grab; }
.names { display: flex; max-width: 600px; text-align: center; font: 11px monospace; }
.names div { flex: 1; padding: 4px 2px; }
#card { position: absolute; right: 10px; bottom: 10px; padding: 6px 10px; background: #fffdf7; border: 1px solid #c9b99a; border-radius: 6px; }
#log { position: absolute; left: 10px; top: 8px; font: 11px monospace; }
</style>
<div class="wrap">
<canvas id="c" width="1200" height="480"></canvas>
<div id="card"></div>
<code id="log"></code>
</div>
<div class="names"><div>face 0 +x fore-edge</div><div>1 -x spine</div><div>2 +y top</div><div>3 -y bottom</div><div>4 +z front cover</div><div>5 -z back cover</div></div>
<script>
const m4 = {
perspective(fovy, aspect, near, far) {
const f = 1 / Math.tan(fovy * Math.PI / 360), d = near - far;
return [f / aspect, 0, 0, 0, 0, f, 0, 0, 0, 0, (far + near) / d, -1, 0, 0, 2 * far * near / d, 0];
},
lookAt(eye, target, up = [0, 1, 0]) {
const cross = (a, b) => [a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0]];
const unit = (v) => v.map((c) => c / Math.hypot(...v));
const dot = (a, b) => a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
const z = unit(eye.map((c, i) => c - target[i])), x = unit(cross(up, z)), y = cross(z, x);
return [x[0], y[0], z[0], 0, x[1], y[1], z[1], 0, x[2], y[2], z[2], 0, -dot(x, eye), -dot(y, eye), -dot(z, eye), 1];
},
multiply(a, b) {
const out = new Array(16).fill(0);
for (let c = 0; c < 4; c++) for (let r = 0; r < 4; r++) for (let k = 0; k < 4; k++) out[c * 4 + r] += a[k * 4 + r] * b[c * 4 + k];
return out;
},
model(x, y, z, sx, sy, sz, turn = 0) { // T * Ry(turn) * S
const c = Math.cos(turn), s = Math.sin(turn);
return [c * sx, 0, -s * sx, 0, 0, sy, 0, 0, s * sz, 0, c * sz, 0, x, y, z, 1];
},
normal(m) {
const [a, b, c, , d, e, f, , g, h, i] = m;
const n = [e * i - f * h, f * g - d * i, d * h - e * g, c * h - b * i, a * i - c * g, b * g - a * h, b * f - c * e, c * d - a * f, a * e - b * d];
const det = a * n[0] + b * n[1] + c * n[2];
return n.map((v) => v / det);
},
};
function program(vs, fs) {
const p = gl.createProgram();
for (const [type, src] of [[gl.VERTEX_SHADER, vs], [gl.FRAGMENT_SHADER, fs]]) {
const s = gl.createShader(type); gl.shaderSource(s, src); gl.compileShader(s);
if (!gl.getShaderParameter(s, gl.COMPILE_STATUS)) throw new Error(gl.getShaderInfoLog(s));
gl.attachShader(p, s);
}
gl.linkProgram(p);
return p;
}
const canvas = document.getElementById('c');
const gl = canvas.getContext('webgl2');
const u = (p, n) => gl.getUniformLocation(p, n);
// A box whose faces come in book order (6 vertices each, so face = gl_VertexID / 6):
// +x fore-edge, -x spine, +y top, -y bottom, +z front cover, -z back cover.
const FACES = ['fore-edge', 'spine', 'top', 'bottom', 'front cover', 'back cover'];
const cube = [];
for (const [n, a1, b1] of [[[1,0,0],[0,0,-1],[0,1,0]], [[-1,0,0],[0,0,1],[0,1,0]], [[0,1,0],[1,0,0],[0,0,-1]],
[[0,-1,0],[1,0,0],[0,0,1]], [[0,0,1],[1,0,0],[0,1,0]], [[0,0,-1],[-1,0,0],[0,1,0]]])
for (const [a, b] of [[-1,-1],[1,-1],[1,1],[-1,-1],[1,1],[-1,1]])
cube.push(...[0, 1, 2].map(i => (n[i] + a * a1[i] + b * b1[i]) / 2), ...n, (a + 1) / 2, (b + 1) / 2);
const sceneVAO = gl.createVertexArray();
gl.bindVertexArray(sceneVAO);
gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(cube), gl.STATIC_DRAW);
gl.vertexAttribPointer(0, 3, gl.FLOAT, false, 32, 0); gl.enableVertexAttribArray(0);
gl.vertexAttribPointer(1, 3, gl.FLOAT, false, 32, 12); gl.enableVertexAttribArray(1);
gl.vertexAttribPointer(2, 2, gl.FLOAT, false, 32, 24); gl.enableVertexAttribArray(2);
// BookNest's catalog, its covers (drawn on 2D canvases) and the shelf.
const books = [['The Quiet Harbor', 'Mara Ellison', 14.99, 4.6, '#1f5f8b'], ['Patterns of the Deep Web', 'Tomas Reyes', 39.5, 4.3, '#5b3f99'],
['Salt and Saffron', 'Priya Nair', 24, 4.8, '#e09a10'], ['Small Steps to Big Summits', 'Jonas Berg', 18.75, 4.1, '#3f7d3a'],
["The Clockmaker's Paradox", 'Elena Sokolova', 16.2, 4.5, '#b5452f'], ['Gardens in Glass', 'Aiko Tanaka', 21.3, 4.4, '#2a9d8f']]
.map(([title, author, price, rating, color]) => ({ title, author, price, rating, color }));
const rgb = (hex) => [1, 3, 5].map((k) => parseInt(hex.slice(k, k + 2), 16) / 255);
const covers = books.map(({ title, color }) => {
const c = Object.assign(document.createElement('canvas'), { width: 128, height: 192 }), g = c.getContext('2d');
g.fillStyle = color; g.fillRect(0, 0, 128, 192);
g.strokeStyle = '#f2ead8'; g.lineWidth = 3; g.strokeRect(8, 8, 112, 176);
g.fillStyle = '#f2ead8'; g.font = 'bold 15px Georgia'; g.textAlign = 'center';
title.split(' ').reduce((lines, w) => { const l = lines[lines.length - 1]; if ((l + ' ' + w).length > 11) lines.push(w); else lines[lines.length - 1] = (l + ' ' + w).trim(); return lines; }, [''])
.forEach((line, i) => g.fillText(line, 64, 90 + i * 20));
const t = gl.createTexture(); gl.bindTexture(gl.TEXTURE_2D, t);
gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, true);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA8, gl.RGBA, gl.UNSIGNED_BYTE, c); gl.generateMipmap(gl.TEXTURE_2D);
return t;
});
const items = books.map(({ color }, i) => ({ color: rgb(color), model: m4.model(-1.3 + i * 0.52, 0.5, 0, 0.42, 0.98, 0.12, (30 - i * 12) * Math.PI / 180) }));
items.push({ color: rgb('#8a6240'), model: m4.model(0, -0.03, 0, 3.4, 0.06, 0.7) }); // the plank: object 7
// The visible pass, with the selection as uniforms: brighten the book, frame the face in gold.
const lit = program(`#version 300 es
layout(location = 0) in vec3 aPosition; layout(location = 1) in vec3 aNormal; layout(location = 2) in vec2 aUV;
uniform mat4 uViewProjection, uModel; uniform mat3 uNormalMatrix;
out vec3 vPosition, vNormal; out vec2 vUV; flat out int vFace; // integers must be flat
void main() { vec4 w = uModel * vec4(aPosition, 1.0); vPosition = w.xyz; vNormal = uNormalMatrix * aNormal;
vUV = aUV; vFace = gl_VertexID / 6; gl_Position = uViewProjection * w; }`, `#version 300 es
precision highp float;
in vec3 vPosition, vNormal; in vec2 vUV; flat in int vFace; out vec4 outColor;
uniform sampler2D uCover; uniform vec3 uColor, uLamp;
uniform int uPart; uniform float uTextured, uSelected; // selected face; 1 if this book is selected
void main() {
float light = 0.4 + 0.75 * max(dot(normalize(vNormal), normalize(uLamp - vPosition)), 0.0);
vec3 base = vFace == 4 && uTextured > 0.5 ? texture(uCover, vUV).rgb : uColor;
vec2 edge = min(vUV, 1.0 - vUV);
bool frame = uSelected > 0.5 && vFace == uPart && min(edge.x, edge.y) < 0.04;
outColor = vec4(frame ? vec3(1.0, 0.75, 0.15) : base * light * (1.0 + 0.3 * uSelected), 1.0);
}`);
// The ID pass: an R32UI target the size of the canvas; each fragment writes object * 16 + face.
const idProgram = program(`#version 300 es
layout(location = 0) in vec3 aPosition; uniform mat4 uViewProjection, uModel; flat out uint vFace;
void main() { vFace = uint(gl_VertexID / 6); gl_Position = uViewProjection * uModel * vec4(aPosition, 1.0); }`,
`#version 300 es
flat in uint vFace; uniform uint uObject; out uint id;
void main() { id = uObject * 16u + vFace; }`);
const idTexture = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, idTexture);
gl.texStorage2D(gl.TEXTURE_2D, 1, gl.R32UI, canvas.width, canvas.height);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
const idDepth = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, idDepth);
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT24, canvas.width, canvas.height);
const idFBO = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, idFBO);
gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, idTexture, 0);
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, idDepth);
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
function renderIDs(viewProjection, w = canvas.width, h = canvas.height) {
gl.bindFramebuffer(gl.FRAMEBUFFER, idFBO);
gl.viewport(0, 0, w, h);
gl.clearBufferuiv(gl.COLOR, 0, [0, 0, 0, 0]); // clear() is invalid on integer targets
gl.clearBufferfv(gl.DEPTH, 0, [1]);
gl.enable(gl.DEPTH_TEST); gl.enable(gl.CULL_FACE);
gl.useProgram(idProgram);
gl.bindVertexArray(sceneVAO);
gl.uniformMatrix4fv(u(idProgram, 'uViewProjection'), false, viewProjection);
items.forEach(({ model }, i) => {
gl.uniform1ui(u(idProgram, 'uObject'), i + 1); // 0 means nothing
gl.uniformMatrix4fv(u(idProgram, 'uModel'), false, model);
gl.drawArrays(gl.TRIANGLES, 0, 36);
});
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
}
function readID([x, y]) {
const id = new Uint32Array(4); // RGBA_INTEGER always returns four
gl.bindFramebuffer(gl.FRAMEBUFFER, idFBO);
gl.readPixels(x, y, 1, 1, gl.RGBA_INTEGER, gl.UNSIGNED_INT, id);
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
return id[0];
}
function drawScene(viewProjection, selected = 0, part = -1) {
gl.enable(gl.DEPTH_TEST); gl.enable(gl.CULL_FACE);
gl.useProgram(lit);
gl.bindVertexArray(sceneVAO);
gl.uniformMatrix4fv(u(lit, 'uViewProjection'), false, viewProjection);
gl.uniform3f(u(lit, 'uLamp'), -0.6, 1.8, 2.0);
gl.uniform1i(u(lit, 'uPart'), part);
items.forEach(({ color, model }, i) => {
gl.bindTexture(gl.TEXTURE_2D, covers[i] || null);
gl.uniform1f(u(lit, 'uTextured'), i < 6 ? 1 : 0);
gl.uniform1f(u(lit, 'uSelected'), i + 1 === selected ? 1 : 0);
gl.uniformMatrix4fv(u(lit, 'uModel'), false, model);
gl.uniformMatrix3fv(u(lit, 'uNormalMatrix'), false, m4.normal(model));
gl.uniform3fv(u(lit, 'uColor'), color);
gl.drawArrays(gl.TRIANGLES, 0, 36);
});
}
// Pointer input: drags turn an orbit camera (with inertia), a press and release that barely moved is a click.
function locate(e) { // CSS pixels -> buffer pixels (y up) and clip space
const r = canvas.getBoundingClientRect();
const x = (e.clientX - r.left) * canvas.width / r.width, y = (e.clientY - r.top) * canvas.height / r.height;
return { pixel: [Math.floor(x), canvas.height - 1 - Math.floor(y)], clip: [2 * x / canvas.width - 1, 1 - 2 * y / canvas.height] };
}
// Scripted input for the preview: synthetic PointerEvents play the user.
const fire = (type, x, y) => {
const r = canvas.getBoundingClientRect();
canvas.dispatchEvent(new PointerEvent(type, { clientX: r.left + x * r.width, clientY: r.top + y * r.height, pointerId: 1, bubbles: true }));
};
// A camera from above and to the right, so tops, fore-edges and covers are all visible.
const camera = m4.multiply(m4.perspective(38, canvas.width / canvas.height, 0.1, 20), m4.lookAt([2.4, 2.2, 2.6], [0, 0.35, 0]));
renderIDs(camera);
let selected = 0, part = -1;
function select(pixel) {
const id = readID(pixel), object = id >> 4, face = id & 15; // the low 4 bits: up to 16 parts per object
[selected, part] = object >= 1 && object <= 6 ? [object, face] : [0, -1];
document.getElementById('log').textContent = `id ${id} = ${object} x 16 + ${face}`;
document.getElementById('card').innerHTML = selected ? `<b>${books[object - 1].title}</b><br>part: ${FACES[face]}` : 'nothing selectable';
}
let userClicked = false, downAt = null;
canvas.addEventListener('pointerdown', (e) => { downAt = [e.clientX, e.clientY]; if (e.isTrusted) userClicked = true; });
canvas.addEventListener('pointerup', (e) => {
if (downAt && Math.hypot(e.clientX - downAt[0], e.clientY - downAt[1]) < 4) select(locate(e).pixel);
});
// Demo clicks on different parts: [book, point in the book's unit box].
const probes = [[2, [0, 0.2, 0.5]], [2, [0, 0.5, 0]], [4, [0.5, 0, 0]], [0, [0, 0.5, 0]], [5, [0.5, 0.1, 0]], [3, [0, -0.2, 0.5]]];
function cssOf([index, [x, y, z]]) {
const m = items[index].model, w = [0, 1, 2].map(r => m[r] * x + m[4 + r] * y + m[8 + r] * z + m[12 + r]);
const c = [0, 1, 3].map(r => camera[r] * w[0] + camera[4 + r] * w[1] + camera[8 + r] * w[2] + camera[12 + r]);
return [(c[0] / c[2] + 1) / 2, (1 - c[1] / c[2]) / 2];
}
let next = -1, step = 0;
function frame(ms) {
if (next < 0) next = ms; // the first demo click comes at once
if (!userClicked && ms >= next) {
const [x, y] = cssOf(probes[step++ % probes.length]);
fire('pointerdown', x, y); fire('pointerup', x, y);
next += 1200;
}
gl.viewport(0, 0, canvas.width, canvas.height);
gl.clearColor(0.93, 0.91, 0.87, 1); gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
drawScene(camera, selected, part);
requestAnimationFrame(frame);
}
requestAnimationFrame(frame);
</script>