initShaders.js

                                             //
//  initShaders.js: contains the code to read, compile, and link the shaders.
// Shaders are pro- grams that are compiled in the drivers. 
//We create them as null-terminated strings of characters, usually with a standard text editor, which are eventually passed into WebGL. 

function initShaders( gl, vertexShaderId, fragmentShaderId )
{
    var vertShdr;
    var fragShdr;

    var vertElem = document.getElementById( vertexShaderId );
    if ( !vertElem ) {
        alert( "Unable to load vertex shader " + vertexShaderId );
        return -1;
    }
    else {
        vertShdr = gl.createShader( gl.VERTEX_SHADER );
        gl.shaderSource( vertShdr, vertElem.textContent.replace(/^\s+|\s+$/g, '' ));
        gl.compileShader( vertShdr );
        if ( !gl.getShaderParameter(vertShdr, gl.COMPILE_STATUS) ) {
            var msg = "Vertex shader '"
                + vertexShaderId
                + "' failed to compile.  The error log is:\n\n"
        	    + gl.getShaderInfoLog( vertShdr );
            alert( msg );
            return -1;
        }
    }

    var fragElem = document.getElementById( fragmentShaderId );
    if ( !fragElem ) {
        alert( "Unable to load vertex shader " + fragmentShaderId );
        return -1;
    }
    else {
        fragShdr = gl.createShader( gl.FRAGMENT_SHADER );
        gl.shaderSource( fragShdr, fragElem.textContent.replace(/^\s+|\s+$/g, '' ) );
        gl.compileShader( fragShdr );
        if ( !gl.getShaderParameter(fragShdr, gl.COMPILE_STATUS) ) {
            var msg = "Fragment shader '"
                + fragmentShaderId
                + "' failed to compile.  The error log is:\n\n"
        	    + gl.getShaderInfoLog( fragShdr );
            alert( msg );
            return -1;
        }
    }

    var program = gl.createProgram();
    gl.attachShader( program, vertShdr );
    gl.attachShader( program, fragShdr );
    gl.linkProgram( program );

    if ( !gl.getProgramParameter(program, gl.LINK_STATUS) ) {
        var msg = "Shader program failed to link.  The error log is:\n\n"
            + gl.getProgramInfoLog( program );
        alert( msg );
        return -1;
    }

    return program;
}