Animate vertex positions with webgl2 transform feedback using two programs

Question

I try to follow example from webgl2fundamentals regarding transform feedback.

My goal is to create animation of vertex positions in a way that there are two programs:

doing animation calculations each frame and writing output to a buffer
doing drawing on screen using buffer with calculated values from 1st program.

I made this code snippet on codesandbox.

It seems that varying vPosition from first program is not being feed to the second program. What am I doing wrong?

Is setup enough to handle this use case? I have one TFO and one VAO and one buffer. What would be best approach?

The output should look something like this:

here is the code:

const canvas = document.querySelector("canvas");
canvas.width = 500;
canvas.height = 500;

const gl = canvas.getContext("webgl2");

const genPointsVSGLSL = `#version 300 es
uniform float time;

in vec2 position;

out vec2 vPosition;

void main() {
  
    vPosition = vec2(position * (sin(time) + 1.0));
}
`;

const genPointsFSGLSL = `#version 300 es
void main() {
  discard;
}
`;

const drawVSGLSL = `#version 300 es
uniform float time;

in vec2 vPosition;

void main() {
  gl_PointSize = 20.0;
  gl_Position = vec4(vPosition, 0.0, 1.0);
}
`;

const drawFSGLSL = `#version 300 es
precision highp float;
out vec4 outColor;

void main() {
    outColor = vec4(0.0, 0.0, 0.0, 1.0);
}
`;

function generatePoints(num) {
  const arr = [];
  for (let i = 0; i < num; i++) {
    const u = i / num;
    const a = u * Math.PI * 2.0;
    arr.push(Math.cos(a) * 0.8, Math.sin(a) * 0.8);
  }

  return new Float32Array(arr);
}

const numPoints = 12;
let time = 0.0;

const createShader = function (gl, type, glsl) {
  const shader = gl.createShader(type);
  gl.shaderSource(shader, glsl);
  gl.compileShader(shader);
  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    throw new Error(gl.getShaderInfoLog(shader));
  }
  return shader;
};

const createProgram = function (gl, vsGLSL, fsGLSL, outVaryings) {
  const vs = createShader(gl, gl.VERTEX_SHADER, vsGLSL);
  const fs = createShader(gl, gl.FRAGMENT_SHADER, fsGLSL);
  const prg = gl.createProgram();
  gl.attachShader(prg, vs);
  gl.attachShader(prg, fs);
  if (outVaryings) {
    gl.transformFeedbackVaryings(prg, outVaryings, gl.SEPARATE_ATTRIBS);
  }
  gl.linkProgram(prg);
  if (!gl.getProgramParameter(prg, gl.LINK_STATUS)) {
    throw new Error(gl.getProgramInfoLog(prg));
  }

  return prg;
};

// gen prog
const genProg = createProgram(gl, genPointsVSGLSL, genPointsFSGLSL, [
  "vPosition"
]);

gl.bindAttribLocation(genProg, 0, "position");
const timeLocGen = gl.getUniformLocation(genProg, "time");

const drawProg = createProgram(gl, drawVSGLSL, drawFSGLSL);

gl.bindAttribLocation(drawProg, 0, "vPosition");
const timeLocDraw = gl.getUniformLocation(drawProg, "time");

const dotVertexArray = gl.createVertexArray();
gl.bindVertexArray(dotVertexArray);

const positionBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
// gl.bufferData(gl.ARRAY_BUFFER, numPoints * 2 * 4, gl.DYNAMIC_DRAW);
gl.bufferData(gl.ARRAY_BUFFER, generatePoints(12), gl.DYNAMIC_DRAW);

gl.enableVertexAttribArray(0);

gl.vertexAttribPointer(
  0, // location
  2, // size (components per iteration)
  gl.FLOAT, // type of to get from buffer
  false, // normalize
  0, // stride (bytes to advance each iteration)
  0 // offset (bytes from start of buffer)
);

gl.bindVertexArray(null);

const tf = gl.createTransformFeedback();

function loop() {
  // update positions
  gl.useProgram(genProg);

  gl.enable(gl.RASTERIZER_DISCARD);

  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, tf);

  gl.bindBufferBase(gl.TRANSFORM_FEEDBACK_BUFFER, 0, positionBuffer);

  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, null);

  gl.beginTransformFeedback(gl.POINTS);

  gl.uniform1f(timeLocGen, time);

  gl.drawArrays(gl.POINTS, 0, numPoints);

  gl.endTransformFeedback();

  gl.disable(gl.RASTERIZER_DISCARD);

  // draw vertices

  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  gl.bindVertexArray(dotVertexArray);

  gl.useProgram(drawProg);

  gl.uniform1f(timeLocDraw, time);

  gl.drawArrays(gl.POINTS, 0, numPoints);
}

function animate() {
  time += 0.01;

  loop();

  window.requestAnimationFrame(animate);
}

animate();

<!DOCTYPE html>
<html>
  <head>
    <title>Parcel Sandbox</title>
    <meta charset="UTF-8" />
  </head>

  <body>
    <canvas></canvas>

    <script src="src/index.js"></script>
  </body>
</html>

Rabbid76 · Accepted Answer · 2022-07-02T16:49:17.120

Actually you only have 1 buffer. You cannot read and write the same buffer. This is undefined behavior. Read the points from the 1st buffer and write the transformed points to the 2nd buffer. Draw the points from the 2nd buffer.

const canvas = document.querySelector("canvas");
canvas.width = 500;
canvas.height = 500;

const gl = canvas.getContext("webgl2");

const genPointsVSGLSL = `#version 300 es
uniform float time;

in vec2 position;

out vec2 vPosition;

void main() {
  
    vPosition = vec2(position * (sin(time) + 1.0)*0.5);
}
`;

const genPointsFSGLSL = `#version 300 es
void main() {
  discard;
}
`;

const drawVSGLSL = `#version 300 es
uniform float time;

in vec2 vPosition;

void main() {
  gl_PointSize = 20.0;
  gl_Position = vec4(vPosition, 0.0, 1.0);
}
`;

const drawFSGLSL = `#version 300 es
precision highp float;
out vec4 outColor;

void main() {
    outColor = vec4(0.0, 0.0, 0.0, 1.0);
}
`;

function generatePoints(num) {
  const arr = [];
  for (let i = 0; i < num; i++) {
    const u = i / num;
    const a = u * Math.PI * 2.0;
    arr.push(Math.cos(a) * 0.8, Math.sin(a) * 0.8);
  }

  return new Float32Array(arr);
}

const numPoints = 12;
let time = 0.0;

const createShader = function (gl, type, glsl) {
  const shader = gl.createShader(type);
  gl.shaderSource(shader, glsl);
  gl.compileShader(shader);
  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    throw new Error(gl.getShaderInfoLog(shader));
  }
  return shader;
};

const createProgram = function (gl, vsGLSL, fsGLSL, outVaryings) {
  const vs = createShader(gl, gl.VERTEX_SHADER, vsGLSL);
  const fs = createShader(gl, gl.FRAGMENT_SHADER, fsGLSL);
  const prg = gl.createProgram();
  gl.attachShader(prg, vs);
  gl.attachShader(prg, fs);
  if (outVaryings) {
    gl.transformFeedbackVaryings(prg, outVaryings, gl.SEPARATE_ATTRIBS);
  }
  gl.linkProgram(prg);
  if (!gl.getProgramParameter(prg, gl.LINK_STATUS)) {
    throw new Error(gl.getProgramInfoLog(prg));
  }

  return prg;
};

// gen prog
const genProg = createProgram(gl, genPointsVSGLSL, genPointsFSGLSL, [
  "vPosition"
]);

gl.bindAttribLocation(genProg, 0, "position");
const timeLocGen = gl.getUniformLocation(genProg, "time");

const drawProg = createProgram(gl, drawVSGLSL, drawFSGLSL);

gl.bindAttribLocation(drawProg, 0, "vPosition");
const timeLocDraw = gl.getUniformLocation(drawProg, "time");

// source buffer
const positionBuffer1 = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer1);
gl.bufferData(gl.ARRAY_BUFFER, generatePoints(12), gl.DYNAMIC_DRAW);

// trnsform feedback VAO
const dotVertexArray1 = gl.createVertexArray();
gl.bindVertexArray(dotVertexArray1);
gl.enableVertexAttribArray(0);
gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);

// traget buffer
const positionBuffer2 = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer2);
gl.bufferData(gl.ARRAY_BUFFER, 12*2*4, gl.DYNAMIC_DRAW);

// draw points VAO
const dotVertexArray2 = gl.createVertexArray();
gl.bindVertexArray(dotVertexArray2);
gl.enableVertexAttribArray(0);
gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);

gl.bindBuffer(gl.ARRAY_BUFFER, null);

const tf = gl.createTransformFeedback();

function loop() {
  // update positions
  gl.useProgram(genProg);

  gl.enable(gl.RASTERIZER_DISCARD);
  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, tf);
  gl.bindBufferBase(gl.TRANSFORM_FEEDBACK_BUFFER, 0, positionBuffer2);
  gl.beginTransformFeedback(gl.POINTS);
  gl.uniform1f(timeLocGen, time);
  gl.bindVertexArray(dotVertexArray1);
  gl.drawArrays(gl.POINTS, 0, numPoints);
  gl.endTransformFeedback();
  gl.disable(gl.RASTERIZER_DISCARD);
  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, null);

  // draw vertices

  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  gl.bindVertexArray(dotVertexArray2);
  gl.useProgram(drawProg);
  gl.uniform1f(timeLocDraw, time);
  gl.drawArrays(gl.POINTS, 0, numPoints);
}

function animate() {
  time += 0.01;

  loop();

  window.requestAnimationFrame(animate);
}

animate();

<canvas></canvas>

thanks for working example! i was checking other posts and [this](https://stackoverflow.com/a/42459922/2323086) answer suggests that it would be possible to read and write to the same buffer but not the "on top of the vertex data that you read from"... i dont fully get it. Anyway your answer seems to be step forward - so we need 2 VAOs and TFO and 2Buffers. it seems a lot of state :/ — mtx, Jul 02 '22 at 17:16
_"... this answer suggests that it would be possible ..."_ - not it is not possible. This answer is about OpenGL and not OpenGL ES or WebGL. On Google Chrome I get a warning that it is undefined behavior. You need 2 buffers that's it. You do not need 2 VAOs, but its easier to handle with 2 VAOs. You need just 1 TFO. You actually need positionBuffer1 and positionBuffer2. Read from positionBuffer1 and write to positionBuffer2 — Rabbid76, Jul 02 '22 at 18:01

mtx · Answer 2 · 2022-07-06T06:55:15.223

After reviewing above accepted answer from Rabbid76 I also created different solution, with only 1 VAO. This requires vertexAttribPointer changes.

const canvas = document.querySelector("canvas");
canvas.width = 500;
canvas.height = 500;

const gl = canvas.getContext("webgl2");

const genPointsVSGLSL = `#version 300 es
uniform float time;

in vec2 position;

out vec2 vPosition;

void main() {
  
    vPosition = vec2(position * (sin(time) + 1.1)*0.5);
}
`;

const genPointsFSGLSL = `#version 300 es
void main() {
  discard;
}
`;

const drawVSGLSL = `#version 300 es

in vec2 vPosition;

void main() {
  gl_PointSize = 20.0;
  gl_Position = vec4(vPosition, 0.0, 1.0);
}
`;

const drawFSGLSL = `#version 300 es
precision highp float;
out vec4 outColor;

void main() {
    outColor = vec4(0.0, 0.0, 0.0, 1.0);
}
`;

function generatePoints(num) {
  const arr = [];
  for (let i = 0; i < num; i++) {
    const u = i / num;
    const a = u * Math.PI * 2.0;
    arr.push(Math.cos(a) * 0.8, Math.sin(a) * 0.8);
  }

  return new Float32Array(arr);
}

const numPoints = 12;
let time = 0.0;

const createShader = function (gl, type, glsl) {
  const shader = gl.createShader(type);
  gl.shaderSource(shader, glsl);
  gl.compileShader(shader);
  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    throw new Error(gl.getShaderInfoLog(shader));
  }
  return shader;
};

const createProgram = function (gl, vsGLSL, fsGLSL, outVaryings) {
  const vs = createShader(gl, gl.VERTEX_SHADER, vsGLSL);
  const fs = createShader(gl, gl.FRAGMENT_SHADER, fsGLSL);
  const prg = gl.createProgram();
  gl.attachShader(prg, vs);
  gl.attachShader(prg, fs);
  if (outVaryings) {
    gl.transformFeedbackVaryings(prg, outVaryings, gl.SEPARATE_ATTRIBS);
  }
  gl.linkProgram(prg);
  if (!gl.getProgramParameter(prg, gl.LINK_STATUS)) {
    throw new Error(gl.getProgramInfoLog(prg));
  }

  return prg;
};

// calc prog
const calcProg = createProgram(gl, genPointsVSGLSL, genPointsFSGLSL, [
  "vPosition"
]);

gl.bindAttribLocation(calcProg, 0, "position");
const timeLocGen = gl.getUniformLocation(calcProg, "time");

// draw prog
const drawProg = createProgram(gl, drawVSGLSL, drawFSGLSL);

gl.bindAttribLocation(drawProg, 0, "vPosition");

// vao init
const vao = gl.createVertexArray();
gl.bindVertexArray(vao);
gl.enableVertexAttribArray(0);

const tf = gl.createTransformFeedback();

// source buffer
const srcBuff = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, srcBuff);
gl.bufferData(gl.ARRAY_BUFFER, generatePoints(numPoints), gl.DYNAMIC_DRAW);

// target buffer
const tgtBuff = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, tgtBuff);
gl.bufferData(gl.ARRAY_BUFFER, numPoints * 2 * 4, gl.DYNAMIC_DRAW);

function loop() {
  // update vertex positions
  gl.useProgram(calcProg);
  gl.uniform1f(timeLocGen, time);

  gl.enable(gl.RASTERIZER_DISCARD);

  // will be needed if more objects are present
  // gl.bindVertexArray(vao);

  // binding srcBuff as ARRAY_BUFFER so it can be used as
  //  attr0 in VAO so tgtBuff is unlocked for TF
  gl.bindBuffer(gl.ARRAY_BUFFER, srcBuff);
  // vertexAttribPointer implicitly binds currently bound ARRAY_BUFFER
  // to currently bound VAO using currently used program
  // and configures attr props.
  gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);

  // using tgtBuff as write buffer for TF
  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, tf);
  gl.bindBufferBase(gl.TRANSFORM_FEEDBACK_BUFFER, 0, tgtBuff);
  gl.beginTransformFeedback(gl.POINTS);

  // drawingArrays is not actually drawing anything but sends output varyings
  // from vertex shader to tfo bound buffer (tgtBuff)
  gl.drawArrays(gl.POINTS, 0, numPoints);
  gl.endTransformFeedback();
  gl.bindTransformFeedback(gl.TRANSFORM_FEEDBACK, null);

  gl.disable(gl.RASTERIZER_DISCARD);

  // draw vertices

  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  gl.useProgram(drawProg);

  // binding tgtBuff as ARRAY_BUFFER
  gl.bindBuffer(gl.ARRAY_BUFFER, tgtBuff);
  // pointer binds the ARRAY_BUFFER to VAO attribute
  gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);

  // drawing output of tgtBuff
  gl.drawArrays(gl.POINTS, 0, numPoints);
}

function animate() {
  time += 0.005;

  loop();

  window.requestAnimationFrame(animate);
}

animate();

<canvas></canvas>

Animate vertex positions with webgl2 transform feedback using two programs

2 Answers2