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    <p>This is really part of brainjam's answer, but just in case that jsfiddle were to go away, I wanted to make sure the code was archived. Here is the primary bit:</p> <pre><code> function handleMouseUp(event) { var world1 = [0,0,0,0] ; var world2 = [0,0,0,0] ; var dir = [0,0,0] ; var w = event.srcElement.clientWidth ; var h = event.srcElement.clientHeight ; // calculate x,y clip space coordinates var x = (event.offsetX-w/2)/(w/2) ; var y = -(event.offsetY-h/2)/(h/2) ; mat4.inverse(pvMatrix, pvMatrixInverse) ; // convert clip space coordinates into world space mat4.multiplyVec4(pvMatrixInverse, [x,y,-1,1], world1) ; vec3.scale(world1,1/world1[3]) ; mat4.multiplyVec4(pvMatrixInverse, [x,y,0,1], world2) ; vec3.scale(world2,1/world2[3]) ; // calculate world space view vector vec3.subtract(world2,world1,dir) ; vec3.normalize(dir) ; vec3.scale(dir,0.3) ; // move eye in direction of world space view vector vec3.subtract(eye,dir) ; drawScene(); console.log(event) } </code></pre> <p>And the entirety of the JS...</p> <pre><code> var gl; function initGL(canvas) { try { gl = canvas.getContext("experimental-webgl"); gl.viewportWidth = canvas.width; gl.viewportHeight = canvas.height; } catch (e) { } if (!gl) { alert("Could not initialise WebGL, sorry :-("); } } function getShader(gl, id) { var shaderScript = document.getElementById(id); if (!shaderScript) { return null; } var str = ""; var k = shaderScript.firstChild; while (k) { if (k.nodeType == 3) { str += k.textContent; } k = k.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, str); gl.compileShader(shader); if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) { alert(gl.getShaderInfoLog(shader)); return null; } return shader; } var shaderProgram; function initShaders() { var fragmentShader = getShader(gl, "shader-fs"); var vertexShader = getShader(gl, "shader-vs"); shaderProgram = gl.createProgram(); gl.attachShader(shaderProgram, vertexShader); gl.attachShader(shaderProgram, fragmentShader); gl.linkProgram(shaderProgram); if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) { alert("Could not initialise shaders"); } gl.useProgram(shaderProgram); shaderProgram.vertexPositionAttribute = gl.getAttribLocation(shaderProgram, "aVertexPosition"); gl.enableVertexAttribArray(shaderProgram.vertexPositionAttribute); shaderProgram.pMatrixUniform = gl.getUniformLocation(shaderProgram, "uPMatrix"); shaderProgram.mvMatrixUniform = gl.getUniformLocation(shaderProgram, "uMVMatrix"); } var mvMatrix = mat4.create(); var pMatrix = mat4.create(); function setMatrixUniforms() { gl.uniformMatrix4fv(shaderProgram.pMatrixUniform, false, pMatrix); gl.uniformMatrix4fv(shaderProgram.mvMatrixUniform, false, mvMatrix); } var triangleVertexPositionBuffer; var squareVertexPositionBuffer; function initBuffers() { triangleVertexPositionBuffer = gl.createBuffer(); gl.bindBuffer(gl.ARRAY_BUFFER, triangleVertexPositionBuffer); var vertices = [ 0.0, 1.0, 0.0, -1.0, -1.0, 0.0, 1.0, -1.0, 0.0 ]; gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW); triangleVertexPositionBuffer.itemSize = 3; triangleVertexPositionBuffer.numItems = 3; squareVertexPositionBuffer = gl.createBuffer(); gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer); vertices = [ 1.0, 1.0, 0.0, -1.0, 1.0, 0.0, 1.0, -1.0, 0.0, -1.0, -1.0, 0.0 ]; gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW); squareVertexPositionBuffer.itemSize = 3; squareVertexPositionBuffer.numItems = 4; } var eye = vec3.create([0,0,0]) ; // negation of actual eye position var pvMatrix = mat4.create(); var pvMatrixInverse = mat4.create(); function drawScene() { gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight); gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT); mat4.perspective(45, gl.viewportWidth / gl.viewportHeight, 0.1, 100.0, pMatrix); mat4.identity(mvMatrix); // calculate the view transform mvMatrix, and the projection-view matrix pvMatrix mat4.translate(mvMatrix, eye); mat4.multiply(pMatrix,mvMatrix,pvMatrix) ; mat4.translate(mvMatrix, [-1.5, 0.0, -7.0]); gl.bindBuffer(gl.ARRAY_BUFFER, triangleVertexPositionBuffer); gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, triangleVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0); setMatrixUniforms(); gl.drawArrays(gl.TRIANGLES, 0, triangleVertexPositionBuffer.numItems); mat4.translate(mvMatrix, [3.0, 0.0, 0.0]); gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer); gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, squareVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0); setMatrixUniforms(); gl.drawArrays(gl.TRIANGLE_STRIP, 0, squareVertexPositionBuffer.numItems); } function handleMouseUp(event) { var world1 = [0,0,0,0] ; var world2 = [0,0,0,0] ; var dir = [0,0,0] ; var w = event.srcElement.clientWidth ; var h = event.srcElement.clientHeight ; // calculate x,y clip space coordinates var x = (event.offsetX-w/2)/(w/2) ; var y = -(event.offsetY-h/2)/(h/2) ; mat4.inverse(pvMatrix, pvMatrixInverse) ; // convert clip space coordinates into world space mat4.multiplyVec4(pvMatrixInverse, [x,y,-1,1], world1) ; vec3.scale(world1,1/world1[3]) ; mat4.multiplyVec4(pvMatrixInverse, [x,y,0,1], world2) ; vec3.scale(world2,1/world2[3]) ; // calculate world space view vector vec3.subtract(world2,world1,dir) ; vec3.normalize(dir) ; vec3.scale(dir,0.3) ; // move eye in direction of world space view vector vec3.subtract(eye,dir) ; drawScene(); console.log(event) } function webGLStart() { var canvas = document.getElementById("lesson01-canvas"); initGL(canvas); initShaders(); initBuffers(); gl.clearColor(0.0, 0.0, 0.0, 1.0); gl.enable(gl.DEPTH_TEST); canvas.onmouseup = handleMouseUp; drawScene(); } webGLStart(); </code></pre>
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