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WebGL month. Day 5. Interleaved buffers

lesnitsky profile image Andrei Lesnitsky Updated on ・5 min read

Interleaved buffers

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

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Hey 👋 Welcome to a WebGL month. Yesterday we've learned how to use varyings. Today we're going to explore one more concept, but let's solve a homework from yesterday first

We need to define raingbow colors first

📄 src/webgl-hello-world.js


  gl.uniform2fv(resolutionUniformLocation, [canvas.width, canvas.height]);

+ const rainbowColors = [
+     [255, 0.0, 0.0, 255], // red
+     [255, 165, 0.0, 255], // orange
+     [255, 255, 0.0, 255], // yellow
+     [0.0, 255, 0.0, 255], // green
+     [0.0, 101, 255, 255], // skyblue
+     [0.0, 0.0, 255, 255], // blue,
+     [128, 0.0, 128, 255], // purple
+ ];
+ 
  const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 360);
  const colors = fillWithColors(360);


Render a 7-gon

📄 src/webgl-hello-world.js

      [128, 0.0, 128, 255], // purple
  ];

- const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 360);
- const colors = fillWithColors(360);
+ const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 7);
+ const colors = fillWithColors(7);

  function createHexagon(centerX, centerY, radius, segmentsCount) {
      const vertices = [];

Fill colors buffer with rainbow colors

📄 src/webgl-hello-world.js


      for (let i = 0; i < segmentsCount; i++) {
          for (let j = 0; j < 3; j++) {
-             if (j == 0) { // vertex in center of circle
-                 colors.push(0, 0, 0, 255);
-             } else {
-                 colors.push(i / 360 * 255, 0, 0, 255);
-             }
+             colors.push(...rainbowColors[i]);
          }
      }


Rainbow

Where's the red? Well, to render 7 polygons, we need 8-gon 🤦 My bad, sorry.

Now we have a colored 8-gon and we store vertices coordinates and colors in two separate buffers.
Having two separate buffers allows to update them separately (imagine we need to change colors, but not positions)

On the other hand if both positions and colors will be the same – we can store this data in a single buffer.

Let's refactor the code to acheive it

We need to structure our buffer data by attribute.

x1, y1, color.r, color.g, color.b, color.a
x2, y2, color.r, color.g, color.b, color.a
x3, y3, color.r, color.g, color.b, color.a
...

📄 src/webgl-hello-world.js

  ];

  const triangles = createHexagon(canvas.width / 2, canvas.height / 2, canvas.height / 2, 7);
- const colors = fillWithColors(7);

  function createHexagon(centerX, centerY, radius, segmentsCount) {
-     const vertices = [];
+     const vertexData = [];
      const segmentAngle =  Math.PI * 2 / (segmentsCount - 1);

      for (let i = 0; i < Math.PI * 2; i += segmentAngle) {
          const from = i;
          const to = i + segmentAngle;

-         vertices.push(centerX, centerY);
-         vertices.push(centerX + Math.cos(from) * radius, centerY + Math.sin(from) * radius);
-         vertices.push(centerX + Math.cos(to) * radius, centerY + Math.sin(to) * radius);
+         const color = rainbowColors[i / segmentAngle];
+ 
+         vertexData.push(centerX, centerY);
+         vertexData.push(...color);
+ 
+         vertexData.push(centerX + Math.cos(from) * radius, centerY + Math.sin(from) * radius);
+         vertexData.push(...color);
+ 
+         vertexData.push(centerX + Math.cos(to) * radius, centerY + Math.sin(to) * radius);
+         vertexData.push(...color);
      }

-     return vertices;
+     return vertexData;
  }

  function fillWithColors(segmentsCount) {

We don't need color buffer anymore

📄 src/webgl-hello-world.js

  }

  const positionData = new Float32Array(triangles);
- const colorData = new Float32Array(colors);
- 
  const positionBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
- const colorBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
- 
- gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
- gl.bufferData(gl.ARRAY_BUFFER, colorData, gl.STATIC_DRAW);

  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, positionData, gl.STATIC_DRAW);

and it also makes sense to rename positionData and positionBuffer to a vertexData and vertexBuffer

📄 src/webgl-hello-world.js

      return colors;
  }

- const positionData = new Float32Array(triangles);
- const positionBuffer = gl.createBuffer(gl.ARRAY_BUFFER);
+ const vertexData = new Float32Array(triangles);
+ const vertexBuffer = gl.createBuffer(gl.ARRAY_BUFFER);

- gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
- gl.bufferData(gl.ARRAY_BUFFER, positionData, gl.STATIC_DRAW);
+ gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
+ gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);
  gl.lineWidth(10);

  const attributeSize = 2;

But how do we specify how this data should be read from buffer and passed to a valid shader attributes

We can do this with vertexAttribPointer, stride and offset arguments

stride tells how much data should be read for each vertex in bytes

Each vertex contains:

  • position (x, y, 2 floats)
  • color (r, g, b, a, 4 floats)

So we have a total of 6 floats 4 bytes each
This means that stride is 6 * 4

Offset specifies how much data should be skipped in the beginning of the chunk

Color data goes right after position, position is 2 floats, so offset for color is 2 * 4

📄 src/webgl-hello-world.js

  const attributeSize = 2;
  const type = gl.FLOAT;
  const nomralized = false;
- const stride = 0;
+ const stride = 24;
  const offset = 0;

  gl.enableVertexAttribArray(positionLocation);
  gl.vertexAttribPointer(positionLocation, attributeSize, type, nomralized, stride, offset);

- gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
- 
  gl.enableVertexAttribArray(colorLocation);
- gl.vertexAttribPointer(colorLocation, 4, type, nomralized, stride, offset);
+ gl.vertexAttribPointer(colorLocation, 4, type, nomralized, stride, 8);

- gl.drawArrays(gl.TRIANGLES, 0, positionData.length / 2);
+ gl.drawArrays(gl.TRIANGLES, 0, vertexData.length / 6);

And voila, we have the same result, but with a single buffer 🎉

Conclusion

Let's summarize how vertexAttribPointer(location, size, type, normalized, stride offset) method works for a single buffer (this buffer is called interleavd)

  • location: specifies which attribute do we want to setup
  • size: how much data should be read for this exact attribute
  • type: type of data being read
  • normalized: whether the data should be "normalized" (clamped to [-1..1] for gl.BYTE and gl.SHORT, and to [0..1] for gl.UNSIGNED_BYTE and gl.UNSIGNED_SHORT)
  • stride: how much data is there for each vertex in total (in bytes)
  • offset: how much data should be skipped in a beginning of each chunk of data

So now you can use different combinations of buffers to fill your attributes with data

See you tomorrow 👋


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

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