OpenGL Point Functionality in WebGL - opengl-es

Question

In OpenGL you can draw define points like this:
glBegin(GL_POINTS);
for(float theta=0, radius=60.0; radius>1.0; theta+=0.1, radius-=0.3){
glColor3f(radius/60.0,0.3,1-(radius/60.0));
glVertex2i(200+radius*cos(theta),200+radius*sin(theta));
}
glEnd();
How do you accomplish this same functionality in WebGL?

Answer 1

The code you wrote really doesn't do much except define some points. To do that in WebGL could do it like this
var colors = [];
var verts = [];
var theta=0
for(var radius=60.0; radius>1.0; radius-=0.3) {
colors.push(radius/60.0, 0.3, 1-(radius/60.0));
verts.push(200+radius*Math.cos(theta),200+radius*Math.sin(theta));
theta+=0.1;
}
var numPoints = colors.length / 3;
That would make 2 JavaScript arrays. You'd then need to put them to WebGLBuffers
var colorBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);
var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(verts), gl.STATIC_DRAW);
After that though you need to write a shader and set it up. Shaders are a huge topic. For your particular data I'm guessing these shader would do
A vertex shader
uniform mat4 u_matrix;
attribute vec4 a_vertex;
attribute vec4 a_color;
varying vec4 v_color;
void main() {
// Set the size of the point
gl_PointSize = 3.0;
// multiply each vertex by a matrix.
gl_Position = u_matrix * a_vertex;
// pass the color to the fragment shader
v_color = a_color;
}
A fragment shader
precision mediump float;
varying vec4 v_color;
void main() {
gl_FragColor = v_color;
}
Next you need to initialize the shaders and parameters. I'm going to assume I put the shaders in script tags with ids "vshader" and "fshader" and use this boilerplate code to load them.
var program = createProgramFromScriptTags(gl, "vshader", "fshader");
gl.useProgram(program);
// look up the locations for the inputs to our shaders.
var u_matLoc = gl.getUniformLocation(program, "u_matrix");
var colorLoc = gl.getAttribLocation(program, "a_color");
var vertLoc = gl.getAttribLocation(program, "a_vertex");
// Set the matrix to some that makes 1 unit 1 pixel.
gl.uniformMatrix4fv(u_matLoc, false, [
2 / width, 0, 0, 0,
0, 2 / height, 0, 0,
0, 0, 1, 0,
0, 0, 0, 1
]);
// Tell the shader how to get data out of the buffers.
gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
gl.vertexAttribPointer(colorLoc, 3, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(colorLoc);
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.vertexAttribPointer(vertLoc, 2, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(vertLoc);
and finally draw the points
gl.drawArrays(gl.POINTS, 0, numPoints);
Here's a snippet
var canvas = document.getElementById("c");
var gl = canvas.getContext("webgl");
if (!gl) {
alert("no WebGL");
//return;
}
var colors = [];
var verts = [];
var theta=0
for(var radius=160.0; radius>1.0; radius-=0.3) {
colors.push(radius/160.0, 0.3, 1-(radius/160.0));
verts.push(radius*Math.cos(theta),radius*Math.sin(theta));
theta+=0.1;
}
var numPoints = colors.length / 3;
var colorBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);
var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(verts), gl.STATIC_DRAW);
var program = twgl.createProgramFromScripts(gl, ["vshader", "fshader"]);
gl.useProgram(program);
// look up the locations for the inputs to our shaders.
var u_matLoc = gl.getUniformLocation(program, "u_matrix");
var colorLoc = gl.getAttribLocation(program, "a_color");
var vertLoc = gl.getAttribLocation(program, "a_vertex");
function draw() {
gl.clear(gl.COLOR_BUFFER_BIT);
gl.clearColor(1.0, 1.0, 1.0, 1.0);
// Set the matrix to some that makes 1 unit 1 pixel.
gl.uniformMatrix4fv(u_matLoc, false, [
2 / canvas.width, 0, 0, 0,
0, -2 / canvas.height, 0, 0,
0, 0, 1, 0,
0, 0, 0, 1
]);
gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
gl.vertexAttribPointer(colorLoc, 3, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(colorLoc);
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.vertexAttribPointer(vertLoc, 2, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(vertLoc);
gl.drawArrays(gl.POINTS, 0, numPoints);
requestAnimationFrame(draw, canvas);
}
draw();
canvas { border: 1px solid black; }
<script src="https://twgljs.org/dist/3.x/twgl.min.js"></script>
<script id="vshader" type="whatever">
uniform mat4 u_matrix;
attribute vec4 a_vertex;
attribute vec4 a_color;
varying vec4 v_color;
void main() {
// Set the size of the point
gl_PointSize = length(a_vertex) * 0.1;
// multiply each vertex by a matrix.
gl_Position = u_matrix * a_vertex;
// pass the color to the fragment shader
v_color = a_color;
}
</script>
<script id="fshader" type="whatever">
precision mediump float;
varying vec4 v_color;
void main() {
gl_FragColor = v_color;
}
</script>
<canvas id="c" width="400" height="400"></canvas>
you might find these WebGL tutorials helpful.

Answer 2

WebGL is based on OpenGL ES 2.0 (see here), which dropped immediate-mode support.
This specification describes an additional rendering context and support objects for the HTML 5 canvas element [CANVAS]. This context allows rendering using an API that conforms closely to the OpenGL ES 2.0 API.
You'll need to use vertex buffers to store vertex data. See here1 for a good explanation of how things work in retained mode. And there for a nice small example to get you started.
1: Kudos to whoever posted this here.