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WebGL Picking

This article is about how to use WebGL to let the user pick or select things.

If you've read the other articles on this site you have hopefully realized that WebGL itself is just a rasterization library. It draws triangles, lines, and points into the canvas so it has no concept of "objects to be selected". It just outputs pixels via shaders you supply. That means any concept of "picking" something has to come from your code. You need to define what these things you're letting the user select are. That means while this article can cover general concepts you'll need to decide for yourself how to translate what you see here into usable concepts in your own application.

Clicking on an Object

One of the easiest ways figure out which thing a user clicked on is we come up with a numeric id for each object, we can then draw all of the objects using their id as their color with no lighting and no textures. This will give us an image of the silhouettes of each object. The depth buffer will handle sorting for us. We can then read the color of the pixel under the mouse which will give us the id of the object that was rendered there.

To implement this technique we'll need to combine several previous articles. The first is the article on drawing multiple objects which we'll use because given it draws multiple things we can try to pick them.

On top of that we generally want to render these ids off screen by rendering to a texture so we'll add in that code as well.

So, let's start with the last example from the article on drawing multiple things that draws 200 things.

To it let's add a framebuffer with attached texture and depth buffer from the last example in the article on rendering to a texture.

// Create a texture to render to
const targetTexture = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, targetTexture);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);

// create a depth renderbuffer
const depthBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);

function setFramebufferAttachmentSizes(width, height) {
  gl.bindTexture(gl.TEXTURE_2D, targetTexture);
  // define size and format of level 0
  const level = 0;
  const internalFormat = gl.RGBA;
  const border = 0;
  const format = gl.RGBA;
  const type = gl.UNSIGNED_BYTE;
  const data = null;
  gl.texImage2D(gl.TEXTURE_2D, level, internalFormat,
                width, height, border,
                format, type, data);

  gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);
  gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, width, height);
}

// Create and bind the framebuffer
const fb = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, fb);

// attach the texture as the first color attachment
const attachmentPoint = gl.COLOR_ATTACHMENT0;
const level = 0;
gl.framebufferTexture2D(gl.FRAMEBUFFER, attachmentPoint, gl.TEXTURE_2D, targetTexture, level);

// make a depth buffer and the same size as the targetTexture
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);

We put the code to set the sizes of the texture and the depth renderbuffer into a function so we can call it to resize them to match the size of the canvas.

In our rendering code if the canvas changes size we'll adjust the texture and renderbuffer to match.

function drawScene(time) {
  time *= 0.0005;

-  webglUtils.resizeCanvasToDisplaySize(gl.canvas);
+  if (webglUtils.resizeCanvasToDisplaySize(gl.canvas)) {
+    // the canvas was resized, make the framebuffer attachments match
+    setFramebufferAttachmentSizes(gl.canvas.width, gl.canvas.height);
+  }

...

Next we need a second shader. The shader in the sample renders using vertex colors but we need one we can set to a solid color to render with ids. So first here is our second shader

const pickingVS = `#version 300 es
  in vec4 a_position;

  uniform mat4 u_matrix;

  void main() {
    // Multiply the position by the matrix.
    gl_Position = u_matrix * a_position;
  }
`;

const pickingFS = `#version 300 es
  precision highp float;

  uniform vec4 u_id;

  out vec4 outColor;

  void main() {
     outColor = u_id;
  }
`;

And we need to compile, link and look up the locations using our helpers.

// setup GLSL program
// note: we need the attribute positions to match across programs
// so that we only need one vertex array per shape
const options = {
  attribLocations: {
    a_position: 0,
    a_color: 1,
  },
};
const programInfo = twgl.createProgramInfo(gl, [vs, fs], options);
const pickingProgramInfo = twgl.createProgramInfo(gl, [pickingVS, pickingFS], options);

One difference above from most samples on this site, this is one of the few times we've needed to draw the same data with 2 different shaders. Because of that we need the attributes locations to match across shaders. We can do that in 2 ways. One way is to set them manually in the GLSL

layout (location = 0) in vec4 a_position;
layout (location = 1) in vec4 a_color;

The other is to call gl.bindAttribLocation before linking a shader program

gl.bindAttribLocation(someProgram, 0, 'a_position');
gl.bindAttribLocation(someProgram, 1, 'a_color');
gl.linkProgram(someProgram);

This latter style is uncommon but it's more D.R.Y.. Our helper library will call gl.bindAttribLocation for us if we pass in the attribute names and the location we want which is what is happening above.

This we'll mean we can guarantee the a_position attribute uses location 0 in both programs so we can use the same vertex array with both programs.

Next we need to be able to render all the objects twice. Once with whatever shader we assigned to them and again with the shader we just wrote so let's extract the code that currently renders all the objects into a function.

function drawObjects(objectsToDraw, overrideProgramInfo) {
  objectsToDraw.forEach(function(object) {
    const programInfo = overrideProgramInfo || object.programInfo;
    const bufferInfo = object.bufferInfo;
    const vertexArray = object.vertexArray;

    gl.useProgram(programInfo.program);

    // Setup all the needed attributes.
    gl.bindVertexArray(vertexArray);

    // Set the uniforms.
    twgl.setUniforms(programInfo, object.uniforms);

    // Draw (calls gl.drawArrays or gl.drawElements)
    twgl.drawBufferInfo(gl, object.bufferInfo);
  });
}

drawObjects takes an optional overrideProgramInfo we can pass in to use our picking shader instead of the object's assigned shader.

Let's call it, once to draw into the texture with ids and again to draw the scene to the canvas.

// Draw the scene.
function drawScene(time) {
  time *= 0.0005;

  ...

  // Compute the matrices for each object.
  objects.forEach(function(object) {
    object.uniforms.u_matrix = computeMatrix(
        viewProjectionMatrix,
        object.translation,
        object.xRotationSpeed * time,
        object.yRotationSpeed * time);
  });

+  // ------ Draw the objects to the texture --------
+
+  gl.bindFramebuffer(gl.FRAMEBUFFER, fb);
+  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);
+
+  gl.enable(gl.CULL_FACE);
+  gl.enable(gl.DEPTH_TEST);
+
+  // Clear the canvas AND the depth buffer.
+  gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+
+  drawObjects(objectsToDraw, pickingProgramInfo);
+
+  // ------ Draw the objects to the canvas
+
+  gl.bindFramebuffer(gl.FRAMEBUFFER, null);
+  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);
+
+  drawObjects(objectsToDraw);

  requestAnimationFrame(drawScene);
}

Our picking shader needs u_id set to an id so let's add that to our uniform data where we setup our objects.

// Make infos for each object for each object.
const baseHue = rand(0, 360);
const numObjects = 200;
for (let ii = 0; ii < numObjects; ++ii) {
+  const id = ii + 1;

  // pick a shape
  const shape = shapes[rand(shapes.length) | 0];

  const object = {
    uniforms: {
      u_colorMult: chroma.hsv(eMod(baseHue + rand(0, 120), 360), rand(0.5, 1), rand(0.5, 1)).gl(),
      u_matrix: m4.identity(),
+      u_id: [
+        ((id >>  0) & 0xFF) / 0xFF,
+        ((id >>  8) & 0xFF) / 0xFF,
+        ((id >> 16) & 0xFF) / 0xFF,
+        ((id >> 24) & 0xFF) / 0xFF,
+      ],
    },
    translation: [rand(-100, 100), rand(-100, 100), rand(-150, -50)],
    xRotationSpeed: rand(0.8, 1.2),
    yRotationSpeed: rand(0.8, 1.2),
  };
  objects.push(object);

  // Add it to the list of things to draw.
  objectsToDraw.push({
    programInfo: programInfo,
    bufferInfo: shape.bufferInfo,
    vertexArray: shape.vertexArray,
    uniforms: object.uniforms,
  });
}

This will work because our helper library handles applying uniforms for us.

We had to split ids across R, G, B, and A. Because our texture's format/type is gl.RGBA, gl.UNSIGNED_BYTE we get 8 bits per channel. 8 bits only represent 256 values but by splitting the id across 4 channels we get 32bits total which is > 4 billion values.

We add 1 to the id because we'll use 0 for meaning "nothing under the mouse".

Now let's highlight the object under the mouse.

First we need some code to get a canvas relative mouse position.

// mouseX and mouseY are in CSS display space relative to canvas
let mouseX = -1;
let mouseY = -1;

...

gl.canvas.addEventListener('mousemove', (e) => {
   const rect = canvas.getBoundingClientRect();
   mouseX = e.clientX - rect.left;
   mouseY = e.clientY - rect.top;
});

Note that with the code above mouseX and mouseY are in CSS pixels in display space. That means they are in the space the canvas is displayed, not the space of how many pixels are in the canvas. In other words if you had a canvas like this

<canvas width="11" height="22" style="width:33px; height:44px;"></canvas>

then mouseX will go from 0 to 33 across the canvas and mouseY will go from 0 to 44 across the canvas. See this for more info.

Now that we have a mouse position let's add some code look up the pixel under the mouse

const pixelX = mouseX * gl.canvas.width / gl.canvas.clientWidth;
const pixelY = gl.canvas.height - mouseY * gl.canvas.height / gl.canvas.clientHeight - 1;
const data = new Uint8Array(4);
gl.readPixels(
    pixelX,            // x
    pixelY,            // y
    1,                 // width
    1,                 // height
    gl.RGBA,           // format
    gl.UNSIGNED_BYTE,  // type
    data);             // typed array to hold result
const id = data[0] + (data[1] << 8) + (data[2] << 16) + (data[3] << 24);

The code above that is computing pixelX and pixelY is converting from mouseX and mouseY in display space to pixel in the the canvas space. In other words, given the example above where mouseX went from 0 to 33 and mouseY went from 0 to 44. pixelX will go from 0 to 11 and pixelY will go from 0 to 22.

In our actual code we're using our utility function resizeCanvasToDisplaySize and we're making our texture the same size as the canvas so the display size and the canvas size match but at least we're prepared for the case where they do not match.

Now that we have an id, to actually highlight the selected object let's change the color we're using to render it to the canvas. The shader we were using has a u_colorMult uniform we can use so if an object is under the mouse we'll look it up, save off its u_colorMult value, replace it with a selection color, and restore it.

// mouseX and mouseY are in CSS display space relative to canvas
let mouseX = -1;
let mouseY = -1;
+let oldPickNdx = -1;
+let oldPickColor;
+let frameCount = 0;

// Draw the scene.
function drawScene(time) {
  time *= 0.0005;
+  ++frameCount;

  // ------ Draw the objects to the texture --------

  ...

  // ------ Figure out what pixel is under the mouse and read it

  const pixelX = mouseX * gl.canvas.width / gl.canvas.clientWidth;
  const pixelY = gl.canvas.height - mouseY * gl.canvas.height / gl.canvas.clientHeight - 1;
  const data = new Uint8Array(4);
  gl.readPixels(
      pixelX,            // x
      pixelY,            // y
      1,                 // width
      1,                 // height
      gl.RGBA,           // format
      gl.UNSIGNED_BYTE,  // type
      data);             // typed array to hold result
  const id = data[0] + (data[1] << 8) + (data[2] << 16) + (data[3] << 24);

  // restore the object's color
  if (oldPickNdx >= 0) {
    const object = objects[oldPickNdx];
    object.uniforms.u_colorMult = oldPickColor;
    oldPickNdx = -1;
  }

  // highlight object under mouse
  if (id > 0) {
    const pickNdx = id - 1;
    oldPickNdx = pickNdx;
    const object = objects[pickNdx];
    oldPickColor = object.uniforms.u_colorMult;
    object.uniforms.u_colorMult = (frameCount & 0x8) ? [1, 0, 0, 1] : [1, 1, 0, 1];
  }

  // ------ Draw the objects to the canvas

And with that we should be able to move the mouse over the scene and the object under the mouse will flash

One optimization we can make, we're rendering the ids to a texture that's the same size as the canvas. This is conceptually the easiest thing to do.

But, we could instead just render the pixel under the mouse. To do this we use a frustum who's math will cover just the space for that 1 pixel.

Until now, for 3D we've been using a function called perspective that takes as input a field of view, an aspect, and a near and far value for the z-planes and makes a perspective projection matrix that converts from the frustum defined by those values to clip space.

Most 3D math libraries have another function called frustum that takes 6 values, the left, right, top, and bottom values for the near z-plane and then the z-near and z-far values for the z-planes and generates a perspective matrix defined by those values.

Using that we can generate a perspective matrix for the one pixel under the mouse

First we compute the edges and size of what our near plane would be if we were to use the perspective function

// compute the rectangle the near plane of our frustum covers
const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
const top = Math.tan(fieldOfViewRadians * 0.5) * near;
const bottom = -top;
const left = aspect * bottom;
const right = aspect * top;
const width = Math.abs(right - left);
const height = Math.abs(top - bottom);

So left, right, width, and height are the size and position of the near plane. Now on that plane we can compute the size and position of the one pixel under the mouse and pass that to the frustum function to generate a projection matrix that covers just that 1 pixel

// compute the portion of the near plane covers the 1 pixel
// under the mouse.
const pixelX = mouseX * gl.canvas.width / gl.canvas.clientWidth;
const pixelY = gl.canvas.height - mouseY * gl.canvas.height / gl.canvas.clientHeight - 1;

const subLeft = left + pixelX * width / gl.canvas.width;
const subBottom = bottom + pixelY * height / gl.canvas.height;
const subWidth = 1 / gl.canvas.width;
const subHeight = 1 / gl.canvas.height;

// make a frustum for that 1 pixel
const projectionMatrix = m4.frustum(
    subLeft,
    subLeft + subWidth,
    subBottom,
    subBottom + subHeight,
    near,
    far);

To use this we need to make some changes. As it now our shader just takes u_matrix which means in order to draw with a different projection matrix we'd need to recompute the matrices for every object twice each frame, once with our normal projection matrix for drawing to the canvas and again for this 1 pixel projection matrix.

We can remove that responsibility from JavaScript by moving that multiplication to the vertex shaders.

const vs = `#version 300 es

in vec4 a_position;
in vec4 a_color;

-uniform mat4 u_matrix;
+uniform mat4 u_viewProjection;
+uniform mat4 u_world;

out vec4 v_color;

void main() {
  // Multiply the position by the matrix.
-  gl_Position = u_matrix * a_position;
+  gl_Position = u_viewProjection * u_world * a_position;

  // Pass the color to the fragment shader.
  v_color = a_color;
}
`;

...

const pickingVS = `#version 300 es
  in vec4 a_position;

-  uniform mat4 u_matrix;
+  uniform mat4 u_viewProjection;
+  uniform mat4 u_world;

  void main() {
    // Multiply the position by the matrix.
-   gl_Position = u_matrix * a_position;
+    gl_Position = u_viewProjection * u_world * a_position;
  }
`;

Then we can make our JavaScript viewProjectionMatrix shared among all the objects.

const objectsToDraw = [];
const objects = [];
+const viewProjectionMatrix = m4.identity();

// Make infos for each object for each object.
const baseHue = rand(0, 360);
const numObjects = 200;
for (let ii = 0; ii < numObjects; ++ii) {
  const id = ii + 1;

  // pick a shape
  const shape = shapes[rand(shapes.length) | 0];

  const object = {
    uniforms: {
      u_colorMult: chroma.hsv(eMod(baseHue + rand(0, 120), 360), rand(0.5, 1), rand(0.5, 1)).gl(),
-      u_matrix: m4.identity(),
+      u_world: m4.identity(),
+      u_viewProjection: viewProjectionMatrix,
      u_id: [
        ((id >>  0) & 0xFF) / 0xFF,
        ((id >>  8) & 0xFF) / 0xFF,
        ((id >> 16) & 0xFF) / 0xFF,
        ((id >> 24) & 0xFF) / 0xFF,
      ],
    },
    translation: [rand(-100, 100), rand(-100, 100), rand(-150, -50)],
    xRotationSpeed: rand(0.8, 1.2),
    yRotationSpeed: rand(0.8, 1.2),
  };

And where we compute the matrices for each object we no longer need to include the view projection matrix

-function computeMatrix(viewProjectionMatrix, translation, xRotation, yRotation) {
-  let matrix = m4.translate(viewProjectionMatrix,
+function computeMatrix(translation, xRotation, yRotation) {
+  let matrix = m4.translation(
      translation[0],
      translation[1],
      translation[2]);
  matrix = m4.xRotate(matrix, xRotation);
  return m4.yRotate(matrix, yRotation);
}
...

// Compute the matrices for each object.
objects.forEach(function(object) {
  object.uniforms.u_world = computeMatrix(
-      viewProjectionMatrix,
      object.translation,
      object.xRotationSpeed * time,
      object.yRotationSpeed * time);
});

We'll create just a 1x1 pixel texture and depth buffer

setFramebufferAttachmentSizes(1, 1);

...

// Draw the scene.
function drawScene(time) {
  time *= 0.0005;
  ++frameCount;

-  if (webglUtils.resizeCanvasToDisplaySize(gl.canvas)) {
-    // the canvas was resized, make the framebuffer attachments match
-    setFramebufferAttachmentSizes(gl.canvas.width, gl.canvas.height);
-  }
+  webglUtils.resizeCanvasToDisplaySize(gl.canvas);

Then before rendering the off screen ids we'll set the view projection using our 1 pixel projection matrix and then when drawing to the canvas we'll use the original projection matrix

-// Compute the projection matrix
-const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
-const projectionMatrix =
-    m4.perspective(fieldOfViewRadians, aspect, 1, 2000);

// Compute the camera's matrix using look at.
const cameraPosition = [0, 0, 100];
const target = [0, 0, 0];
const up = [0, 1, 0];
const cameraMatrix = m4.lookAt(cameraPosition, target, up);

// Make a view matrix from the camera matrix.
const viewMatrix = m4.inverse(cameraMatrix);

-const viewProjectionMatrix = m4.multiply(projectionMatrix, viewMatrix);

// Compute the matrices for each object.
objects.forEach(function(object) {
  object.uniforms.u_world = computeMatrix(
      object.translation,
      object.xRotationSpeed * time,
      object.yRotationSpeed * time);
});

// ------ Draw the objects to the texture --------

// Figure out what pixel is under the mouse and setup
// a frustum to render just that pixel

{
  // compute the rectangle the near plane of our frustum covers
  const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
  const top = Math.tan(fieldOfViewRadians * 0.5) * near;
  const bottom = -top;
  const left = aspect * bottom;
  const right = aspect * top;
  const width = Math.abs(right - left);
  const height = Math.abs(top - bottom);

  // compute the portion of the near plane covers the 1 pixel
  // under the mouse.
  const pixelX = mouseX * gl.canvas.width / gl.canvas.clientWidth;
  const pixelY = gl.canvas.height - mouseY * gl.canvas.height / gl.canvas.clientHeight - 1;

  const subLeft = left + pixelX * width / gl.canvas.width;
  const subBottom = bottom + pixelY * height / gl.canvas.height;
  const subWidth = 1 / gl.canvas.width;
  const subHeight = 1 / gl.canvas.height;

  // make a frustum for that 1 pixel
  const projectionMatrix = m4.frustum(
      subLeft,
      subLeft + subWidth,
      subBottom,
      subBottom + subHeight,
      near,
      far);
+  m4.multiply(projectionMatrix, viewMatrix, viewProjectionMatrix);
}

gl.bindFramebuffer(gl.FRAMEBUFFER, fb);
gl.viewport(0, 0, 1, 1);

gl.enable(gl.CULL_FACE);
gl.enable(gl.DEPTH_TEST);

// Clear the canvas AND the depth buffer.
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

drawObjects(objectsToDraw, pickingProgramInfo);

// read the 1 pixel
-const pixelX = mouseX * gl.canvas.width / gl.canvas.clientWidth;
-const pixelY = gl.canvas.height - mouseY * gl.canvas.height / gl.canvas.clientHeight - 1;
const data = new Uint8Array(4);
gl.readPixels(
-    pixelX,            // x
-    pixelY,            // y
+    0,                 // x
+    0,                 // y
    1,                 // width
    1,                 // height
    gl.RGBA,           // format
    gl.UNSIGNED_BYTE,  // type
    data);             // typed array to hold result
const id = data[0] + (data[1] << 8) + (data[2] << 16) + (data[3] << 24);

// restore the object's color
if (oldPickNdx >= 0) {
  const object = objects[oldPickNdx];
  object.uniforms.u_colorMult = oldPickColor;
  oldPickNdx = -1;
}

// highlight object under mouse
if (id > 0) {
  const pickNdx = id - 1;
  oldPickNdx = pickNdx;
  const object = objects[pickNdx];
  oldPickColor = object.uniforms.u_colorMult;
  object.uniforms.u_colorMult = (frameCount & 0x8) ? [1, 0, 0, 1] : [1, 1, 0, 1];
}

// ------ Draw the objects to the canvas

+{
+  // Compute the projection matrix
+  const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
+  const projectionMatrix =
+      m4.perspective(fieldOfViewRadians, aspect, near, far);
+
+  m4.multiply(projectionMatrix, viewMatrix, viewProjectionMatrix);
+}

gl.bindFramebuffer(gl.FRAMEBUFFER, null);
gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

drawObjects(objectsToDraw);

And you can see the math works, we're only drawing a single pixel and we're still figuring out what is under the mouse

Questions? Ask on stackoverflow.
Issue/Bug? Create an issue on github.
Use <pre><code>code goes here</code></pre> for code blocks
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