Webgl Circle without using extrernal library - webgl

Question

I am new to webgl and at this point I am able to create a triangle and square.
I am finding it difficult to understand how to create a circle with out any external scripts or libraries.

Answer 1

var vertexShaderText =
[
'uniform vec2 u_resolution;',
'',
'attribute vec2 a_position;',
'',
'void main()',
'{',
'',
'vec2 clipspace = a_position / u_resolution * 1.0 ;',
'',
'gl_Position = vec4(clipspace * vec2(2.5, 2.0), 0, 1);',
'}'
].join("\n");
var fragmentShaderText =
[
'precision mediump float;',
'',
'void main(void)',
'{',
'',
'gl_FragColor = vec4(0, 0, 0, 1.0);',
'',
'}'
].join("\n");
var uni = function(){
var canvas = document.getElementById("game-surface");
var gl = canvas.getContext("webgl");
console.log("This is working");
gl.clearColor(0.412,0.412,0.412,1);
gl.clear(gl.COLOR_BUFFER_BIT);
var vertextShader = gl.createShader(gl.VERTEX_SHADER);
var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(vertextShader,vertexShaderText);
gl.shaderSource(fragmentShader,fragmentShaderText);
gl.compileShader(vertextShader);
gl.compileShader(fragmentShader);
if(!gl.getShaderParameter(vertextShader,gl.COMPILE_STATUS)){
console.error("Error with vertexshader",gl.getShaderInfoLog(vertextShader));
return;
}
if(!gl.getShaderParameter(fragmentShader,gl.COMPILE_STATUS)){
console.error("Error with fragmentShader",gl.getShaderInfoLog(fragmentShader));
return;
}
var program =gl.createProgram();
gl.attachShader(program,vertextShader);
gl.attachShader(program,fragmentShader);
gl.linkProgram(program);
gl.useProgram(program);
if(!gl.getProgramParameter(program,gl.LINK_STATUS)){
console.error("Error linking program",gl.getProgramInfoLog(program));
return;
}
gl.validateProgram(program);
if(!gl.getProgramParameter(program,gl.VALIDATE_STATUS)){
console.error("Error validating",gl.getProgramInfoLog(program));
}
var circle = {x: 0, y:0, r: 75};
var ATTRIBUTES = 2;
var numFans = 64;
var degreePerFan = (2* Math.PI) / numFans;
var vertexData = [circle.x, circle.y];
for(var i = 0; i <= numFans; i++) {
var index = ATTRIBUTES * i + 2; // there is already 2 items in array
var angle = degreePerFan * (i+0.1);
vertexData[index] = circle.x + Math.cos(angle) * circle.r;
vertexData[index + 1] = circle.y + Math.sin(angle) * circle.r;
}
var vertexDataTyped = new Float32Array(vertexData);
var buffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.bufferData(gl.ARRAY_BUFFER, vertexDataTyped, gl.STATIC_DRAW);
var resolutionLocation = gl.getUniformLocation(program, "u_resolution");
gl.uniform2f(resolutionLocation, canvas.width, canvas.height);
gl.enableVertexAttribArray(positionLocation);
var positionLocation = gl.getAttribLocation(program, "a_position");
gl.vertexAttribPointer(positionLocation, 2, gl.FLOAT, false, ATTRIBUTES * Float32Array.BYTES_PER_ELEMENT, 0);
gl.drawArrays(gl.TRIANGLE_FAN, 0, vertexData.length/ATTRIBUTES);
};