Webgl doesn't pass my circle
I am trying to learn how to use Webgl and have already learned how to draw triangles, squares and lines. I am having problems creating my circle in webgl.
var InitDemo = function(){
var canvas = document.getElementById('circle-surface');
var gl = canvas.getContext('webgl');
//define geomtry
var vertices = [];
var origin = [0,0];
vertices.push(origin);
for(var i = 0; i <= 360; i+=1){
var j = i*Math.PI/180;
var vert = [Math.sin(j),Math.cos(j)];
vertices.push(vert);
}
console.log('the vertices are: '+vertices);
var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
//shaders
var vertSrc = 'attribute vec2 coordinates;'+
'void main(){' +
' gl_Position = vec4(coordinates,0.0,1.0);'+
'}';
var vertShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vertShader, vertSrc);
gl.compileShader(vertShader);
if (!gl.getShaderParameter(vertShader, gl.COMPILE_STATUS)) {
console.error('ERROR compiling vertex shader!', gl.getShaderInfoLog(vertShader));
return;
}
var fragSrc = 'void main(){'+
' gl_FragColor = vec4(1.0,0.0,1.0,1.0);'+
'}';
var fragShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragShader, fragSrc);
gl.compileShader(fragShader);
if (!gl.getShaderParameter(fragShader, gl.COMPILE_STATUS)) {
console.error('ERROR compiling fragment shader!', gl.getShaderInfoLog(fragShader));
return;
}
var shaderProg = gl.createProgram();
gl.attachShader(shaderProg, vertShader);
gl.attachShader(shaderProg, fragShader);
gl.linkProgram(shaderProg);
gl.useProgram(shaderProg);
if (!gl.getProgramParameter(shaderProg, gl.LINK_STATUS)) {
console.error('ERROR linking program!', gl.getProgramInfoLog(shaderProg));
return;
}
gl.validateProgram(shaderProg);
if (!gl.getProgramParameter(shaderProg, gl.VALIDATE_STATUS)) {
console.error('ERROR validating program!', gl.getProgramInfoLog(shaderProg));
return;
}
//associate shader and buffer
var coord = gl.getAttribLocation(shaderProg, 'coordinates');
gl.vertexAttribPointer(coord, 2, gl.FLOAT, gl.FALSE, 2*Float32Array.BYTES_PER_ELEMENT, 0);
gl.enableVertexAttribArray(coord);
//Draw
gl.clearColor(0.0, 0.0, 0.2, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.drawArrays(gl.TRIANGLE_FAN, 0, vertices.length/2);
}
<!DOCTYPE html>
<html>
<head>
<title>Circle Demo</title>
<link rel="stylesheet" type="text/css" href="tutorial.css">
</head>
<body onload="InitDemo();">
<canvas id = "circle-surface" width="800px" height="600px"></canvas>
<script src = "CircleApp.js"></script>
</body>
</html>
I don't get any errors on the console, but for some reason it doesn't display the circle that I want.
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1 answer
Your mistake is not a mistake in the OGL part, but a mistake in setting up vertex arrays. You created a 2D array for your vertex buffer instead of a linear 1D array.
You should use vertices.push(origin[0], origin[1]);
instead of vertices.push(origin);
(same for vert
).
Your code should look something like this:
var vertices = [];
var origin = [0,0];
vertices.push( origin[0], origin[1] ); <-------------------------
for(var i = 0; i <= 360; i+=1){
var j = i*Math.PI/180;
var vert = [Math.sin(j),Math.cos(j)];
vertices.push( vert[0], vert[1] ); <-------------------------
}
Fixed examples of WebGL:
var InitDemo = function(){
var canvas = document.getElementById('circle-surface');
var gl = canvas.getContext('webgl');
//define geomtry
var vertices = [];
var origin = [0,0];
vertices.push( origin[0], origin[1] );
for(var i = 0; i <= 360; i+=1){
var j = i*Math.PI/180;
var vert = [Math.sin(j),Math.cos(j)];
vertices.push( vert[0], vert[1] );
}
console.log('the vertices are: '+vertices);
var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
//shaders
var vertSrc = 'attribute vec2 coordinates;'+
'void main(){' +
' gl_Position = vec4(coordinates,0.0,1.0);'+
'}';
var vertShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vertShader, vertSrc);
gl.compileShader(vertShader);
if (!gl.getShaderParameter(vertShader, gl.COMPILE_STATUS)) {
console.error('ERROR compiling vertex shader!', gl.getShaderInfoLog(vertShader));
return;
}
var fragSrc = 'void main(){'+
' gl_FragColor = vec4(1.0,0.0,1.0,1.0);'+
'}';
var fragShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragShader, fragSrc);
gl.compileShader(fragShader);
if (!gl.getShaderParameter(fragShader, gl.COMPILE_STATUS)) {
console.error('ERROR compiling fragment shader!', gl.getShaderInfoLog(fragShader));
return;
}
var shaderProg = gl.createProgram();
gl.attachShader(shaderProg, vertShader);
gl.attachShader(shaderProg, fragShader);
gl.linkProgram(shaderProg);
gl.useProgram(shaderProg);
if (!gl.getProgramParameter(shaderProg, gl.LINK_STATUS)) {
console.error('ERROR linking program!', gl.getProgramInfoLog(shaderProg));
return;
}
gl.validateProgram(shaderProg);
if (!gl.getProgramParameter(shaderProg, gl.VALIDATE_STATUS)) {
console.error('ERROR validating program!', gl.getProgramInfoLog(shaderProg));
return;
}
//associate shader and buffer
var coord = gl.getAttribLocation(shaderProg, 'coordinates');
gl.vertexAttribPointer(coord, 2, gl.FLOAT, gl.FALSE, 2*Float32Array.BYTES_PER_ELEMENT, 0);
gl.enableVertexAttribArray(coord);
//Draw
gl.clearColor(0.0, 0.0, 0.2, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.drawArrays(gl.TRIANGLE_FAN, 0, vertices.length/2);
}
<body onload="InitDemo();">
<canvas id = "circle-surface" width="800px" height="600px"></canvas>
</body>
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