HTML Simple Paint example

Introduction to Computer Graphics
Version 1.1, January 2016
Author:  David J. Eck  (

Outline and Notes:
  1. I have removed the "turn off text selection" lines in the header style so I can edit this page using SeaMonkey.
  2. I have changed the background color to lightyellow - which I find a lot more pleasing that some shade of gray.
  3. I have added an endnote style to make the end note work nicely.
  4. The overall structure of this page:
    1. <head> includes:
      1. <style> section to let the make page use <div> tags for a pleasing display
      2. <script> section to bring up the graphics and respond to events
        I'll elaborate on the functions and variables below
    2. <body>
      1. <body onload="init()">
        1. responding to a page load event
        2. call the init function in the <script> section
      2. <div> for canvas
      3. <div> for controls
        1. <select> tag gives the combo-box controls
  5. init () - sets up:
    1. drawing area and off-screen canvas (stuff not being dragged)
    2. mouse event handler code
    3. default values for the possible user selections
  6. Events:
    1. configured in setUpMouseHander function/class, using local variables and inner functions
    2. initializes mouse down event listener
    3. doMouseDown - sets mouse move and mouse up events, sets initial mouse drag coordinates
    4. doMouseMove - reacts to mouse movement during drags
    5. doMouseUp - removes move and up listeners, finalizes drag end locations
  7. Global variables:
    1. described carefully at the beginning of the script section
    2. canvas, graphics references to main display area
    3. OSC, OSG references to stable display (objects not currently being dragged), pixel map
    4. tool, color, lineWidth, dragShape, dragStartX, dragStartY, dragCurrentX, dragCurrentY - drawing controls
  8. Functions:
    1. repaint - first stable image, then image being drawn using drag
    2. draw (line, oval or rectangle) defined by drag coordinates
    3. grabSmudgeData, swapSmudgeData - mess with 9x9 area in stable canvas
    4. applyTool, applyToolAlongLine - erase or smudge to stable canvas
    5. setUpMouseHander (typo? should have been handler?)
      1. code adds and removes listeners according to current drag status
      2. doMouseDown, doMouseMove, doMouseUp - internal event handlers
      3. addEventListener doMouseDown - initial event handler
    6. init () - see above.

Source Code of Original Page:
<!DOCTYPE html>
   This page is a simple paint program that lets the user draw on a
   canvas.  It demonstrates using an "off-screen canvas."  The "smudge
   tool" demonstrates pixel manipulation in HTML canvas graphics.
   (Note: on my old desktop, drawing ovals crashes Firefox!)
   (I'm not sure that the off-screen canvas technique will work everywhere.)
<meta charset="UTF-8">
<title>A Simple Paint Program</title>
    body {
        background-color: #DDDDDD;
        -webkit-user-select: none; /* turn off text selection  / Webkit */
        -moz-user-select: none;    /* Firefox */
        -ms-user-select: none;     /* IE 10  */
        -o-user-select: none;      /* Opera */
        user-select: none;
    canvas {
        background-color: white;
        display: block;
        cursor: default;
    #canvasholder {
        border:2px solid black;
        float: left; /* This makes the border exactly fit the canvas. */
    #tools {
        margin-left: 15px;
        border: 2px solid black;
        padding: 8px
    #tools p {
        margin-top: 20px;
    #tools select {
        margin-left: 30px;
        margin-top: 5px;

    "use strict";

    var canvas;    // The canvas element on which the user will draw.
    var graphics;  // A 2D graphics context for drawing on the canvas.
    var OSC;  // A canvas element that is not a visible part of the document.
              // It is created programmatically and serves as an off-screen canvas.
              // The off-screen canvas holds the "official" copy of the picture
              // that the user has drawn.
    var OSG;  // A 2D graphics context for drawing on the off-screen canvas.
    var tool = "Sketch";  // The current drawing tool.
    var color = "black";  // The color that is currently being used for drawing
    var lineWidth = 5;    // The line width that is currently being used for strokes 
    var dragShape = null;   // When non-null, the user is dragging with
                            // the Oval, Rectangle, or Line tool.  The
                            // current shape is drawn in the repaint() function
                            // over the BufferedImage.  The shape is only added
                            // to the off-screen image when the drag action ends.
    var dragStartX, dragStartY;     // Start point of drag for use with dragShape.
    var dragCurrentX, dragCurrentY; // Current mouse position for use with dragShape.

    var smudgeColorArray = null, smudgeImageData; // Data used by "Smudge" tool.
     *  The main function for drawing the contents of the on-screen canvas.  it simply
     *  copies the content of the off-screen canvas onto the screen.  If the user is dragging
     *  to draw a line, oval, or rectangle, then that shape is drawn on top of the picture.
     *  (The shape is drawn to the off-screen canvas when the drag action ends.)
    function repaint() {
        graphics.drawImage(OSC,0,0);  // Copy the off-screen image onto the screen.
        if (dragShape) {
               // The user is performing a drag to draw a shape. Call putShape to
               // draw the shape over the content of the off-screen canvas.
     *  Draws a Line, Oval, or Rectangle to the graphics context g. The shape is determined
     *  by the points (x1,y1) and (x2,y2).  The second parameter is a string nameing the shape.
    function putShape(g, shape, x1, y1, x2, y2) { 
        var x = Math.min(x1,x2); // upper left corner, used for ovals and rectangles.
        var y = Math.min(y1,y2);
        var w = Math.abs(x1-x2); // size of rectangle
        var h = Math.abs(y1-y2);
        switch (shape) {
        case "Line":  // stroke a line from (x1,y1) to (x2,y2);
            g.strokeStyle = color;
            g.lineWidth = lineWidth;
        case "Rectangle": // fill a rectangle with corners at (x1,y1) and (x2,y2)
            g.fillStyle = color;
        case "Oval": // fill the oval contained in the rect with corners at (x1,y1) and (x2,y2)
            g.translate((x1+x2)/2, (y1+y2)/2);
            g.scale(Math.abs(x1-x2)/2, Math.abs(y1-y2)/2);
            g.fillStyle = color;

     *  Read the contents of a 9-by-9 square of pixels centered at (x,y), from the off-screen canvas.
     *  The data is obtained in an "ImageData" object.  It is copied into a Float32Array that can
     *  be used for floating-point computations.  If that array does not already exist, it is created.
     *  A new ImageData object is also created that will be used for putting new color data into the
     *  image, in the swapSmudgeData function.
    function grabSmudgeData(x, y) {
        var colors = OSG.getImageData(x-5,y-5,9,9);
        if (smudgeColorArray == null) {
            smudgeImageData = OSG.createImageData(9,9);
            smudgeColorArray = new Float32Array(;
        for (var i = 0; i <; i++) {
            smudgeColorArray[i] =[i];
     *  The data in the smudgeColorArray, which was set in the grabSmudgeData function,
     *  is blended with the colors in a square of pixels centered at (x,y).  Effectively,
     *  some of the color from the array is moved to the image, and some of the color from
     *  the image is moved to the array.  The effect is that the smudgeColorArray carries
     *  color from the point where the mouse was first pressed and drops some of that color
     *  at each point that is visited by the mouse as it moves (while picking up some
     *  new color from those points.)
    function swapSmudgeData(x, y) {
        var colors = OSG.getImageData(x-5,y-5,9,9);  // get color data form image
        for (var i = 0; i < smudgeColorArray.length; i += 4) {
                // The color arrays contain four numbers for each pixel, giving red, blue,
                // green, and alpha color components.  An alpha value of 0 means that the
                // pixel was outside the image; such pixels are ignored.  Otherwise,
                // the red, green, and blue components in the two color arrays,
                // smudgeColorArray and, are replaced by weighted averages
                // of the existing values.  The alpha component is simply set to 255 (fully
                // opaque), which is actually what it should already be.
            if (smudgeColorArray[i+3] &&[i+3]) {
                for (var j = i; j < i+3; j++) {
                    var newSmudge = smudgeColorArray[j]*0.8 +[j]*0.2;
                    var newImage  = smudgeColorArray[j]*0.2 +[j]*0.8;
          [j] = newImage;
                    smudgeColorArray[j] = newSmudge;
      [i+3] = 255;
            else {
                for (var j = i; j <= i+3; j++) {
          [j] = 0; // "transparent black"; will have no effect on the image
     *  Applies the "Erase" or "Smudge" tool at the point (x,y), as the mouse is being dragged.
    function applyTool(tool, x, y) {
        if (tool == "Erase") { // Clear a 10-by-10 square, centered at (x,y).
            OSG.fillStyle = "white";
            OSG.fillRect(x-5,y-5,10,10);  // Erase the sqaure in the BufferedImage.
        else { // For the "Smudge" tool, mix some of the "paint" on the tool with the image,
               // in a 7-by-7 square centered at x,y.
            swapSmudgeData(x, y);
     *  Applies the "Erase" or "Smudge" tool to each point along a line from (x1,y1)
     *  to (x2,y2).  This is used when the user drags the mouse, with (x1,y1) being the
     *  previous mouse location and (x2,y2) the current location.  This is necessary
     *  since the mouse can move by several pixels at each step, and it's necessary to
     *  apply the tools at every point along the path for the tool to work properly.
    function applyToolAlongLine(tool, x1, y1, x2, y2) {
        var x, y, slope;
        if (Math.abs(x1-x2) >= Math.abs(y1-y2)) {
               // Horizontal distance is greater than vertical distance.  Apply the
               // tool once for each x-value between x1 and x2, computing the
               // y-value for each x-value from the equation of a line. 
            slope = (y2-y1)/(x2-x1);
            if (x1 <= x2) { // Increment up from x1 to x2.
                for (x = x1; x <= x2; x++) {
                    y = Math.round(y1 + slope*(x-x1));
            else { // Decrement down from x1 to x2
                for (x = x1; x >= x2; x--) {
                    y = Math.round(y1 + slope*(x-x1));
        else {
               // Vertical distance is greater than horizontal distance.  Apply the
               // tool once for each y-value between y1 and y2, computing the
               // x-value for each y-value from the equation of a line. 
            slope = (x2-x1)/(y2-y1);
            if (y1 <= y2) {  // Increment up from y1 to y2.
                for (y = y1; y <= y2; y++) {
                    x = Math.round(x1 + slope*(y-y1));
            else {  // Decrement down from y1 to y2.
                for (y = y1; y >= y2; y--) {
                    x = Math.round(x1 + slope*(y-y1));
     * This is called in init() to set up the respons to mouse actions on the canvas.
    function setUpMouseHander() {
        var dragging = false;
        var startX, startY;
        var prevX, prevY;
        function doMouseDown(evt) {  // responds when the user presses a mouse button on the canvas.
            if (dragging || evt.button != 0) {
            var r = canvas.getBoundingClientRect();
            var x = Math.round(evt.clientX - r.left);  // Firefox, at least, can give non-integer values
            var y = Math.round(evt.clientY -;
            prevX = startX = x;
            prevY = startY = y;
            dragging = true;
            if (dragging) {  // install handler functions for mouse move and up, just during drag action
            if (tool == "Line" || tool == "Oval" || tool == "Rectangle") {
                dragShape = tool;  // Tells repaint() about the drag action.
                dragStartX = dragCurrentX = startX;
                dragStartY = dragCurrentY = startY;
            else if (tool == "Erase") {
            else if (tool == "Smudge") {
        function doMouseMove(evt) {  // Called when mouse moves during a drag operation.
            if (dragging) { // (actually, the test should not be necessary)
                var r = canvas.getBoundingClientRect();
                var x = Math.round(evt.clientX - r.left);
                var y = Math.round(evt.clientY -;
                if (tool == "Line" || tool == "Oval" || tool == "Rectangle") {
                    dragCurrentX = x;
                    dragCurrentY = y;
                    repaint(); // This will draw the shape in its new position on the on-screen canvas.
                else if (tool == "Sketch") {
                    putShape(OSG,"Line",prevX,prevY,x,y);  // Directly draw the line on the off-screen canvas.
                    repaint();  // Redraws the on-screen canvas, to make the change visible on screen.
                else if (tool == "Erase" || tool =="Smudge") {
                prevX = x;
                prevY = y;
        function doMouseUp(evt) {
            if (dragging) { // (actually, the test should not be necessary)
                dragging = false;
                if (dragShape) { // Draw the shape into the off-screen canvas.
                    dragShape = null;
                    repaint(); // (Just to make sure off-screen and on-screen pictures agree.)
     * The init() funciton is called after the page has been
     * loaded.  It initializes the canvas and graphics variables,
     * and sets up some event handlers with input elements on
     * the web page.
    function init() {
        try {
            canvas = document.getElementById("canvas");
            graphics = canvas.getContext("2d");
            OSC = document.createElement("canvas");
            OSC.width = canvas.width;
            OSC.height = canvas.height;
            OSG = OSC.getContext("2d");
            OSG.fillStyle = "white";
            graphics.lineCap = OSG.lineCap = "round";
            graphics.lineJoin = OSG.lineJoin = "round";
        } catch(e) {  // In case of error, replace the canvas with an error message.
            document.getElementById("canvasholder").innerHTML =
               "Canvas graphics is not supported?<br>" +
               "An error occurred while initializing graphics.";
        repaint();  // Just copies the blank white off-screen canvas to the screen.
        document.getElementById("tool").value = "Sketch";
        document.getElementById("tool").onchange = function() {
            tool = document.getElementById("tool").value;
        document.getElementById("color").value = "Black";
        document.getElementById("color").onchange = function() {
            color = document.getElementById("color").value;
        document.getElementById("linewidth").value = "5";
        document.getElementById("linewidth").onchange = function() {
            lineWidth = parseInt(document.getElementById("linewidth").value);
        document.getElementById("clear").onclick = function() {
            OSG.fillStyle = "white";
<body onload="init()"> <!-- the onload attribute here is what calls the init() function -->

<h2>A Simple Paint Program</h2>

    <!-- This message will be shown in the page if JavaScript is not available. -->
<p>JavaScript is required to use this page.</p>

<div id="canvasholder">
<canvas id="canvas" width="640" height="480">
    <!-- This message is shown on the page if the browser doesn't support the canvas element. -->
Canvas not supported.

<div id="tools">

<p><label><b>Drawing Tool:</b><br>
    <select id="tool">

<p><label><b>Drawing Color:</b><br>
    <select id="color">
<p><label><b>Line Width:</b><br>
    <select id="linewidth">
<p><button id="clear">Clear</button></p>


Running code:

A Simple Paint Program

Canvas not supported.

Nicholas Duchon - May 27, 2018