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WebGL Month. Day 27. Click detection. Part I

lesnitsky profile image Andrei Lesnitsky ・6 min read

This is a series of blog posts related to WebGL. New post will be available every day

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Hey 👋

Yesterday we've learned how to render to a texture. This is a nice ability to make some nice effects after the scene was completely rendered, but we can get advantage of offscreen rendering for something else.

One important thing in interactive 3D is click detection. While it may be done with javascript, it involves some complex math. Instead we can:

  • assign a unique solid color to each object
  • render scene to a texture
  • read pixel color under cursor
  • match color with an object

Since we'll need another framebuffer, let's create a helper class

📄 src/RenderBuffer.js

export class RenderBuffer {
    constructor(gl) {
        this.framebuffer = gl.createFramebuffer();
        this.texture = gl.createTexture();
    }
}

Setup framebuffer and color texture

📄 src/RenderBuffer.js

      constructor(gl) {
          this.framebuffer = gl.createFramebuffer();
          this.texture = gl.createTexture();
+ 
+         gl.bindTexture(gl.TEXTURE_2D, this.texture);
+         gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.canvas.width, gl.canvas.height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
+ 
+         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);
+ 
+         gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer);
+         gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, this.texture, 0);
      }
  }

Setup depth buffer

📄 src/RenderBuffer.js


          gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer);
          gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, this.texture, 0);
+ 
+         this.depthBuffer = gl.createRenderbuffer();
+         gl.bindRenderbuffer(gl.RENDERBUFFER, this.depthBuffer);
+ 
+         gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, gl.canvas.width, gl.canvas.height);
+         gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, this.depthBuffer);
      }
  }

Implement bind method

📄 src/RenderBuffer.js

          gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, gl.canvas.width, gl.canvas.height);
          gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, this.depthBuffer);
      }
+ 
+     bind(gl) {
+         gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer);
+     }
  }

and clear

📄 src/RenderBuffer.js

      bind(gl) {
          gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer);
      }
+ 
+     clear(gl) {
+         this.bind(gl);
+         gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+     }
  }

Use new helper class

📄 src/minecraft.js

  import { setupShaderInput, compileShader } from './gl-helpers';
  import { GLBuffer } from './GLBuffer';
  import { createRect } from './shape-helpers';
+ import { RenderBuffer } from './RenderBuffer';

  const canvas = document.querySelector('canvas');
  const gl = canvas.getContext('webgl');

  mat4.fromTranslation(cameraFocusPointMatrix, cameraFocusPoint);

- const framebuffer = gl.createFramebuffer();
- 
- const texture = gl.createTexture();
- 
- gl.bindTexture(gl.TEXTURE_2D, texture);
- gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, canvas.width, canvas.height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
- 
- 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);
- 
- gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
- gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, texture, 0);
- 
- const depthBuffer = gl.createRenderbuffer();
- gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);
- 
- gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, canvas.width, canvas.height);
- gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);
+ const offscreenRenderBuffer = new RenderBuffer(gl);

  const vShader = gl.createShader(gl.VERTEX_SHADER);
  const fShader = gl.createShader(gl.FRAGMENT_SHADER);
  gl.uniform2f(programInfo.uniformLocations.resolution, canvas.width, canvas.height);

  function render() {
-     gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
- 
-     gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+     offscreenRenderBuffer.clear(gl);

      mat4.translate(cameraFocusPointMatrix, cameraFocusPointMatrix, [0, 0, -30]);
      mat4.rotateY(cameraFocusPointMatrix, cameraFocusPointMatrix, Math.PI / 360);
      gl.uniform2f(programInfo.uniformLocations.resolution, canvas.width, canvas.height);

      gl.bindFramebuffer(gl.FRAMEBUFFER, null);
-     gl.bindTexture(gl.TEXTURE_2D, texture);
+     gl.bindTexture(gl.TEXTURE_2D, offscreenRenderBuffer.texture);

      gl.drawElements(gl.TRIANGLES, indexBuffer.data.length, gl.UNSIGNED_BYTE, 0);


Instead of passing the whole unique color of the object, which is a vec3, we can pass only object index

📄 src/shaders/3d-textured.v.glsl

  attribute vec3 position;
  attribute vec2 texCoord;
  attribute mat4 modelMatrix;
+ attribute float index;

  uniform mat4 viewMatrix;
  uniform mat4 projectionMatrix;

and convert this float to a color right in the shader

📄 src/shaders/3d-textured.v.glsl


  varying vec2 vTexCoord;

+ vec3 encodeObject(float id) {
+     int b = int(mod(id, 255.0));
+     int r = int(id) / 255 / 255;
+     int g = (int(id) - b - r * 255 * 255) / 255;
+     return vec3(r, g, b) / 255.0;
+ }
+ 
  void main() {
      gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(position, 1.0);


Now we need to pass the color to a fragment shader via varying

📄 src/shaders/3d-textured.f.glsl

  uniform sampler2D texture;

  varying vec2 vTexCoord;
+ varying vec3 vColor;

  void main() {
      gl_FragColor = texture2D(texture, vTexCoord * vec2(1, -1) + vec2(0, 1));

📄 src/shaders/3d-textured.v.glsl

  uniform mat4 projectionMatrix;

  varying vec2 vTexCoord;
+ varying vec3 vColor;

  vec3 encodeObject(float id) {
      int b = int(mod(id, 255.0));
      gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(position, 1.0);

      vTexCoord = texCoord;
+     vColor = encodeObject(index);
  }

We also need to specify what do we want to render: textured object or colored, so let's use a uniform for it

📄 src/shaders/3d-textured.f.glsl

  varying vec2 vTexCoord;
  varying vec3 vColor;

+ uniform float renderIndices;
+ 
  void main() {
      gl_FragColor = texture2D(texture, vTexCoord * vec2(1, -1) + vec2(0, 1));
+ 
+     if (renderIndices == 1.0) {
+         gl_FragColor.rgb = vColor;
+     }
  }

Now let's create indices array

📄 src/minecraft-terrain.js

      State.modelMatrix = mat4.create();
      State.rotationMatrix = mat4.create();

+     const indices = new Float32Array(100 * 100);
+ 
      let cubeIndex = 0;

      for (let i = -50; i < 50; i++) {

Fill it with data and setup a GLBuffer

📄 src/minecraft-terrain.js

                  matrices[cubeIndex * 4 * 4 + index] = value;
              });

+             indices[cubeIndex] = cubeIndex;
+ 
              cubeIndex++;
          }
      }

      State.matricesBuffer = new GLBuffer(gl, gl.ARRAY_BUFFER, matrices, gl.STATIC_DRAW);
+     State.indexBuffer = new GLBuffer(gl, gl.ARRAY_BUFFER, indices, gl.STATIC_DRAW);

      State.offset = 4 * 4; // 4 floats 4 bytes each
      State.stride = State.offset * 4; // 4 rows of 4 floats

Since we have a new attribute, we need to update setupAttribute and resetDivisorAngles functions

📄 src/minecraft-terrain.js


          State.ext.vertexAttribDivisorANGLE(State.programInfo.attributeLocations.modelMatrix + i, 1);
      }
+ 
+     State.indexBuffer.bind(gl);
+     gl.vertexAttribPointer(State.programInfo.attributeLocations.index, 1, gl.FLOAT, false, 0, 0);
+     State.ext.vertexAttribDivisorANGLE(State.programInfo.attributeLocations.index, 1);
  }

  function resetDivisorAngles() {
      for (let i = 0; i < 4; i++) {
          State.ext.vertexAttribDivisorANGLE(State.programInfo.attributeLocations.modelMatrix + i, 0);
      }
+ 
+     State.ext.vertexAttribDivisorANGLE(State.programInfo.attributeLocations.index, 0);
  }

  export function render(gl, viewMatrix, projectionMatrix) {

And finally we need another argument of a render function to distinguish between "render modes" (either textured cubes or colored)

📄 src/minecraft-terrain.js

      State.ext.vertexAttribDivisorANGLE(State.programInfo.attributeLocations.index, 0);
  }

- export function render(gl, viewMatrix, projectionMatrix) {
+ export function render(gl, viewMatrix, projectionMatrix, renderIndices) {
      gl.useProgram(State.program);

      setupAttributes(gl);
      gl.uniformMatrix4fv(State.programInfo.uniformLocations.viewMatrix, false, viewMatrix);
      gl.uniformMatrix4fv(State.programInfo.uniformLocations.projectionMatrix, false, projectionMatrix);

+     if (renderIndices) {
+         gl.uniform1f(State.programInfo.uniformLocations.renderIndices, 1);
+     } else {
+         gl.uniform1f(State.programInfo.uniformLocations.renderIndices, 0);
+     }
+ 
      State.ext.drawArraysInstancedANGLE(gl.TRIANGLES, 0, State.vertexBuffer.data.length / 3, 100 * 100);

      resetDivisorAngles();

Now we need another render buffer to render colored cubes to

📄 src/minecraft.js

  mat4.fromTranslation(cameraFocusPointMatrix, cameraFocusPoint);

  const offscreenRenderBuffer = new RenderBuffer(gl);
+ const coloredCubesRenderBuffer = new RenderBuffer(gl);

  const vShader = gl.createShader(gl.VERTEX_SHADER);
  const fShader = gl.createShader(gl.FRAGMENT_SHADER);

Now let's add a click listeneer

📄 src/minecraft.js

      requestAnimationFrame(render);
  }

+ document.body.addEventListener('click', () => {
+     coloredCubesRenderBuffer.bind(gl);
+ });
+ 
  (async () => {
      await prepareSkybox(gl);
      await prepareTerrain(gl);

and render colored cubes to a texture each time user clicks on a canvas

📄 src/minecraft.js


  document.body.addEventListener('click', () => {
      coloredCubesRenderBuffer.bind(gl);
+ 
+     renderTerrain(gl, viewMatrix, projectionMatrix, true);
  });

  (async () => {

Now we need a storage to read pixel colors to

📄 src/minecraft.js

      coloredCubesRenderBuffer.bind(gl);

      renderTerrain(gl, viewMatrix, projectionMatrix, true);
+ 
+     const pixels = new Uint8Array(canvas.width * canvas.height * 4);
  });

  (async () => {

and actually read pixel colors

📄 src/minecraft.js

      renderTerrain(gl, viewMatrix, projectionMatrix, true);

      const pixels = new Uint8Array(canvas.width * canvas.height * 4);
+     gl.readPixels(0, 0, canvas.width, canvas.height, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
  });

  (async () => {

That's it, we now have the whole scene rendered to an offscreen texture, where each object has a unique color. We'll continue click detection tomorrow

Thanks for reading! 👋


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