why is that gl.clear(gl.COLOR_BUFFER_BIT) and requestAnimationFrame will clear all the primitives I drew before

Question

Hi guys I been leanring WebGL and trying to make a Tetris game out of it.

I have a couple of questions I'd like to ask:

1. For this game I wanted to first draw the grid as the background. However I noticed that after I drew the line, if I use gl.clear(gl.COLOR_BUFFER_BIT ); after, it will clear all the lines I drew before. I understand that gl.clear(gl.COLOR_BUFFER_BIT ); is about clearing the color buffer (and you probably will ask why I would want to do that. Just bear with me. ). Then I tried use gl.uniform4f( uColor, 0, 0, 0, 1); to send the color again to the fragment shader but it doesn't help.

The snippet is like this

window.onload = function(){
    getGLContext();
    initShaders();
    drawLines( 0, 0, 400,400 );
    gl.clear(gl.COLOR_BUFFER_BIT );
    gl.uniform4f( uColor, 0, 0, 0, 1);
}

2. For the game I need the grid as background and I need requestAnimationFrame for the game loop and will render Tetrominos inside the loop. Therefore after drawing the line I used this draw() to draw other Tetrominos. However it removes the line I drew before. And when I comment out gl.clear(gl.COLOR_BUFFER_BIT ); inside draw(), it will remove the line along with background color.

   function draw() { gl.clear(gl.COLOR_BUFFER_BIT ); gl.drawArrays(gl.TRIANGLES, 0, index*6); requestAnimationFrame(draw); }

Here is the demo: https://codepen.io/zhenghaohe/pen/LqxpjB

Hope you could answer these two questions. Thanks!

Answer 1

This is generally the way WebGL works.

WebGL is just draws into a rectangle of pixels. There is no memory of primitives. There is no structure. There is just code and the resulting canvas which is an rectangle of pixels.

Most WebGL programs/pages clear the entire canvas every frame and redraw 100% of the things they want to show every time they draw. For tetris the general code might be something like

function render()  {
  clear the canvas
  draw the grid
  draw all the stable pieces
  draw the current piece
  draw the next piece
  draw the effects
  draw the score
}

Any knowledge of primitives or other structure is entirely up to your code.

If you want the grid lines to be static then either set a static background with CSS or use another canvas

Using a background:

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

function render(time) {
  time *= 0.001;

  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  drawBlocks(gl, time);
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

// --- below this line not important to the answer

function drawBlocks(gl, time) {
  gl.enable(gl.SCISSOR_TEST);
  
  const numBlocks = 5;
  for (let i = 0; i < numBlocks; ++i) {
    const u = i / numBlocks;
    gl.clearColor(i / 5, i / 2 % 1, i / 3 % 1, 1);
    const x = 150 + Math.sin(time + u * Math.PI * 2) * 130;
    const y = 75 + Math.cos(time + u * Math.PI * 2) * 55;
    gl.scissor(x, y, 20, 20);
    gl.clear(gl.COLOR_BUFFER_BIT);
  }
  
  gl.disable(gl.SCISSOR_TEST);
}

#c {
  background-image: url(https://i.imgur.com/ZCfccZh.png);
}

<canvas id="c"></canvas>

Using 2 canvases

// this is the context for the back canvas. It could also be webgl
// using a 2D context just to make the sample simpler
const ctx = document.querySelector('#back').getContext('2d');
drawGrid(ctx);

// this is the context for the front canvas
const gl = document.querySelector('#front').getContext('webgl');

function render(time) {
  time *= 0.001;

  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  drawBlocks(gl, time);
  
  requestAnimationFrame(render);
}
requestAnimationFrame(render);

// --- below this line not important to the answer

function drawBlocks(gl, time) {
  gl.enable(gl.SCISSOR_TEST);
  
  const numBlocks = 5;
  for (let i = 0; i < numBlocks; ++i) {
    const u = i / numBlocks;
    gl.clearColor(i / 5, i / 2 % 1, i / 3 % 1, 1);
    const x = 150 + Math.sin(time + u * Math.PI * 2) * 130;
    const y = 75 + Math.cos(time + u * Math.PI * 2) * 55;
    gl.scissor(x, y, 20, 20);
    gl.clear(gl.COLOR_BUFFER_BIT);
  }
  
  gl.disable(gl.SCISSOR_TEST);
}

function drawGrid(ctx) {
  // draw grid
  ctx.translate(-10.5, -5.5);
  ctx.beginPath();
  for (let i = 0; i < 330; i += 20) {
    ctx.moveTo(0, i);
    ctx.lineTo(330, i);
    ctx.moveTo(i, 0);
    ctx.lineTo(i, 300);
  }
  ctx.strokeStyle = "blue";
  ctx.stroke();
}

#container {
  position: relative; /* required so we can position child elements */
}
#front {
  position: absolute;
  left: 0;
  top: 0;
}

<div id="container">
  <canvas id="back"></canvas>
  <canvas id="front"></canvas>
</div>

As for why it clears even if you didn't call clear that's because that's whqt the spec says it's supposed to do

See: Why WebGL 'clear' draw to front buffer?