I am following a book called professional WebGL, I am hand writing stuff making sure of my understanding, I have this code:
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>2-1 A first WebGL example</title>
<meta charset="utf-8">
<script id="shader-vs" type="x-shader/x-vertex">
attribute vec3 aVertexPosition;
void main() {
gl_Position = vec4(aVertexPosition, 1.0);
}
</script>
<script id="shader-fs" type="x-shader/x-fragment">
precision mediump float;
void main() {
gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
}
</script>
<script type="text/javascript">
var gl;
var canvas;
var shaderProgram;
var vertexBuffer;
/*Function to create the webgl context*/
function createGLContext(canvas) {
var names = ["webgl", "experimental-webgl"];
var context = null;
for(var i = 0; i < names.length; i++) {
try {
context = canvas.getContext(names[i]);
} catch(e) {
if(context) {
break;
}
}
}
if(context) {
context.viewportWidth = canvas.width;
context.viewportHeight = canvas.height;
} else {
alert("Failed to create webgl context");
}
return context;
}
function loadShaderFromDOM(id) {
var shaderScript = document.getElementById(id);
//if we don't find an element with the specified id
//we do an early exit
if(!shaderScript) {
return null;
}
//loop through the children for the found DOM element and
//build up the shader source code as a string.
var shaderSource = "";
var currentChild = shaderScript.firstChild;
while(currentChild) {
if(currentChild.nodeType == 3) {
//3 corresponds to TEXT_NODE
shaderSource += currentChild.textContent;
}
currentChild = currentChild.nextSibling;
}
var shader;
if(shaderScript.type == "x-shader/x-fragment") {
shader = gl.createShader(gl.FRAGMENT_SHADER);
} else if(shaderScript.type == "x-shader/x-vertex") {
shader = gl.createShader(gl.VERTEX_SHADER);
} else {
return null;
}
gl.shaderSource(shader, shaderSource);
gl.compileShader(shader);
if(!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
alert(gl.getShaderInfoLog(shader));
return null;
}
return shader;
}
/*Function to setup the shaders*/
function setupShaders() {
var vertexShader = loadShaderFromDOM("shader-vs");
var fragmentShader = loadShaderFromDOM("shader-fs");
shaderProgram = gl.createProgram();
gl.attachShader(shaderProgram, vertexShader);
gl.attachShader(shaderProgram, fragmentShader);
gl.linkProgram(shaderProgram);
if(!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
alert("Failed toe setup shaders");
}
gl.useProgram(shaderProgram);
shaderProgram.vertexPositionAttribute = gl.getAttribLocation(shaderProgram, "aVertexPosition");
}
/*Function to setup the buffers*/
function setupBuffers() {
vertexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
var triangleVertices = [
0.0, 0.5, 0.0,
-0.5, -0.5, 0.0,
0.5, -0.5, 0.0
];
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(triangleVertices), gl.STATIC_DRAW);
vertexBuffer.itemSize = 3;
vertexBuffer.numberOfItems = 3;
}
/*Function to draw the triangle*/
function draw() {
gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight);
gl.celar(gl.COLOR_BUFFER_BIT);
gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, vertexBuffer.itemSize, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(shaderProgram.vertexPositionAttribute);
gl.drawArrays(gl.TRIANGLES, 0, vertexBuffer.numberOfItems);
}
/*Function to kick everything off*/
function startup() {
canvas = document.getElementById("myGLCanvas");
gl = createGLContext(canvas);
setupShaders();
setupBuffers();
gl.clearColor(0.0, 0.0, 0.0, 0.1);
draw();
}
</script>
</head>
<body onload="startup()">
<canvas id="myGLCnvas" width="500" height="300"></canvas>
</body>
</html>
for some reason, it says that it can’t read property null for createShader here:
shader = gl.createShader(gl.VERTEX_SHADER);
But it doesn’t complain at all for the same declaration but with the fragment shader.
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Answer
Slightly changed version.
gl.celar was wrong and main loop for getting context.
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>2-1 A first WebGL example</title>
<meta charset="utf-8">
<script id="shader-vs" type="x-shader/x-vertex">
attribute vec3 aVertexPosition;
void main() {
gl_Position = vec4(aVertexPosition, 1.0);
}
</script>
<script id="shader-fs" type="x-shader/x-fragment">
precision mediump float;
void main() {
gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
}
</script>
<script type="text/javascript">
var gl;
var canvas;
var shaderProgram;
var vertexBuffer;
/*Function to create the webgl context*/
function createGLContext(canvas) {
var names = ["webgl", "experimental-webgl"];
gl = null;
for(var i = 0; i < names.length; i++) {
gl = canvas.getContext(names[i]);
if(gl)
break;
}
if(gl) {
gl.viewportWidth = canvas.width;
gl.viewportHeight = canvas.height;
} else {
console.log("Failed to create webgl context");
}
}
function loadShaderFromDOM(id) {
var shaderScript = document.getElementById(id);
//if we don't find an element with the specified id
//we do an early exit
if(!shaderScript) {
return null;
}
//loop through the children for the found DOM element and
//build up the shader source code as a string.
var shaderSource = "";
var currentChild = shaderScript.firstChild;
while(currentChild) {
if(currentChild.nodeType == 3) {
//3 corresponds to TEXT_NODE
shaderSource += currentChild.textContent;
}
currentChild = currentChild.nextSibling;
}
var shader;
if(shaderScript.type == "x-shader/x-fragment") {
shader = gl.createShader(gl.FRAGMENT_SHADER);
} else if(shaderScript.type == "x-shader/x-vertex") {
shader = gl.createShader(gl.VERTEX_SHADER);
} else {
return null;
}
gl.shaderSource(shader, shaderSource);
gl.compileShader(shader);
if(!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
alert(gl.getShaderInfoLog(shader));
return null;
}
return shader;
}
/*Function to setup the shaders*/
function setupShaders() {
var vertexShader = loadShaderFromDOM("shader-vs");
var fragmentShader = loadShaderFromDOM("shader-fs");
shaderProgram = gl.createProgram();
gl.attachShader(shaderProgram, vertexShader);
gl.attachShader(shaderProgram, fragmentShader);
gl.linkProgram(shaderProgram);
if(!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
alert("Failed toe setup shaders");
}
gl.useProgram(shaderProgram);
shaderProgram.vertexPositionAttribute = gl.getAttribLocation(shaderProgram, "aVertexPosition");
}
/*Function to setup the buffers*/
function setupBuffers() {
vertexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
var triangleVertices = [
0.0, 0.5, 0.0,
-0.5, -0.5, 0.0,
0.5, -0.5, 0.0
];
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(triangleVertices), gl.STATIC_DRAW);
vertexBuffer.itemSize = 3;
vertexBuffer.numberOfItems = 3;
}
/*Function to draw the triangle*/
function draw() {
gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, vertexBuffer.itemSize, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(shaderProgram.vertexPositionAttribute);
gl.drawArrays(gl.TRIANGLES, 0, vertexBuffer.numberOfItems);
}
/*Function to kick everything off*/
function startup() {
canvas = document.getElementById("myGLCnvas");
createGLContext(canvas);
setupShaders();
setupBuffers();
gl.clearColor(0.0, 0.0, 0.0, 0.1);
draw();
}
</script>
</head>
<body onload="startup()">
<canvas id="myGLCnvas" width="500" height="300"></canvas>
</body>
</html>