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HTML

              
                <script id="fragShader" type="shader-code">
		uniform sampler2D tex;
		uniform vec2 res;
uniform vec3 light;
		void main() {
        vec2 pixel = gl_FragCoord.xy / res.xy;
        vec4 color = texture2D(tex,pixel);
        //Distance of the current pixel from the light position
        float dist = distance(gl_FragCoord.xy,light.xy);
  
        if(light.z * res.x > dist){
          gl_FragColor = color;
        } else {
          gl_FragColor = vec4(0.0);
        }
		 }
	</script>
              
            
!

CSS

              
                body { margin: 0; }
              
            
!

JS

              
                //@author Omar Shehata. 2015.
//We are loading the Three.js library from the cdn here: https://cdnjs.com/libraries/three.js/
var scene;
var camera;
var renderer;

function scene_setup(){
  //This is all code needed to set up a basic ThreeJS scene

  //First we initialize the scene and our camera
  scene = new THREE.Scene();
  camera = new THREE.PerspectiveCamera( 75, window.innerWidth / window.innerHeight, 0.1, 1000 );

  //We create the WebGL renderer and add it to the document
  renderer = new THREE.WebGLRenderer();
  renderer.setSize( window.innerWidth, window.innerHeight );
  document.body.appendChild( renderer.domElement );
}

//Initialize the Threejs scene
scene_setup();

//Pull the shader code from the html
var shaderCode = document.getElementById("fragShader").innerHTML;

var textureURL = "https://raw.githubusercontent.com/tutsplus/Beginners-Guide-to-Shaders/master/Part3/images/blocks.JPG"
THREE.ImageUtils.crossOrigin = '';//Allows us to load an external image

//Load in the texture 
var texture = THREE.ImageUtils.loadTexture(textureURL);

//Set up the uniforms we'll send to our share
//More info on uniform types: https://threejs.org/docs/#Reference/Materials/ShaderMaterial
var uniforms = {
  //Add our light variable here
  light: {type:'v3', value:new THREE.Vector3()},
  tex : {type:'t',value:texture},//The texture
  res : {type: 'v2',value:new THREE.Vector2(window.innerWidth,window.innerHeight)}//Keeps the resolution
}
//We stick our shader onto a 2d plane big enough to fill the screen
var material = new THREE.ShaderMaterial({uniforms:uniforms,fragmentShader:shaderCode})
var geometry = new THREE.PlaneGeometry( 10, 10 );
var sprite = new THREE.Mesh( geometry,material );
//Add it to the scene
scene.add( sprite );
//Move the camera back so we can see it
camera.position.z = 2;

uniforms.light.value.z = 0.2;//Our radius

//Render everything!
function render() {
  requestAnimationFrame( render );
  renderer.render( scene, camera );
}
render();

document.onmousemove = function(event){
	//Update the light source to follow our mouse
	uniforms.light.value.x = event.clientX; 
	uniforms.light.value.y = event.clientY; 
}
              
            
!
999px

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