How to Create a Simple Drawing App using HTML 5 Canvas [fabricJS]?

• Download the complete sample app and source code from Github

Introduction

This tip explains simple steps to build a simple drawing app using pure JavaScript and HTML5 Canvas. It is also useful for all who want to learn or integrate pure JavaScript based drawing app.

HTML5 Canvas?

The HTML5 <canvas> element is used to draw graphics, on the fly, via scripting (usually JavaScript). The <canvas> element is only a container for graphics. You must use a script to actually draw the graphics.

Canvas has several methods for drawing paths, boxes, circles, text, and adding images. So, in this tip, I've used FabricJS as external canvas library and written JS functions to handle different features.

Create Your Drawing App

Let's target the below features in your drawing app:

1. Drawing a rectangle
2. Drawing a circle
3. Drawing an image
4. Enable and disable drawing
5. Enable and disable pan
6. Zoom in
7. Zoom out
8. Reset zoom

And you'll need the below tools:

1. A HTML file
2. A JavaScript file
3. A canvas library - Fabric.js

STEP 1

Create the below files in your app directory:

1. index.html
2. main.js [or any file name]

STEP 2

Import FabricJS library

STEP 3

Import an image

App structure should look like below:

DrawingApp

• index.html
• main.js
• myimage.jpg
• fabric.js

Canvas and FabricJS

Go through the below links to understand more about HTML 5 canvas and FabricJS library:

• http://www.w3schools.com/html/html5_canvas.asp
• http://fabricjs.com/

Write Code

STEP 1

Load your image on canvas:

 var src = 'your_image_path';
 var canvas = new fabric.Canvas('canvas', {
                isDrawingMode: false
              });
              
fabric.Image.fromURL(src, function (oImg) {
                canvas.add(oImg);
            });

STEP 2

Add drawing functions to draw objects:

Draw Solid Circle

var onSolidCircle = function () {
            canvas.add(new fabric.Circle({ radius: 30, fill: '#f55', top: 150, left: 150 }));
        }

Draw Solid Rectangle

var rect = new fabric.Rect({
               top: 100,
               left: 100,
               width: 60,
               height: 70,
               fill: '',
               selection: false,
               fill: '#f55'
           });

           canvas.add(rect);

STEP 3

Start and Stop Drawing.

We need two functions which allow us to notify canvas when to start and when to stop drawing.

var onStartDrawing = function () {
           canvas.isDrawingMode = true;
       }

var onStopDrawing = function () {
           canvas.isDrawingMode = false;
      }

STEP 4

Add zoomin and zoomout functions.

Zoom-in and zoom-out are now a days a very basic feature in any drawing tool on any platform. Below functions will let the objects and canvas to scale to required scale factor.

Canvas needs two variables to control zooming:

var canvasScale = 1;
var SCALE_FACTOR = 1.2;

Zoom-in

var onZoomIn = function () {
            // TODO limit the max canvas zoom in

            canvasScale = canvasScale * SCALE_FACTOR;

            canvas.setHeight(canvas.getHeight() * SCALE_FACTOR);
            canvas.setWidth(canvas.getWidth() * SCALE_FACTOR);

            var objects = canvas.getObjects();
            for (var i in objects) {
                var scaleX = objects[i].scaleX;
                var scaleY = objects[i].scaleY;
                var left = objects[i].left;
                var top = objects[i].top;

                var tempScaleX = scaleX * SCALE_FACTOR;
                var tempScaleY = scaleY * SCALE_FACTOR;
                var tempLeft = left * SCALE_FACTOR;
                var tempTop = top * SCALE_FACTOR;

                objects[i].scaleX = tempScaleX;
                objects[i].scaleY = tempScaleY;
                objects[i].left = tempLeft;
                objects[i].top = tempTop;

                objects[i].setCoords();
            }

            canvas.renderAll();
        }

Zoom-out

var onZoomOut = function () {
           canvasScale = canvasScale / SCALE_FACTOR;

           canvas.setHeight(canvas.getHeight() * (1 / SCALE_FACTOR));
           canvas.setWidth(canvas.getWidth() * (1 / SCALE_FACTOR));

           var objects = canvas.getObjects();
           for (var i in objects) {
               var scaleX = objects[i].scaleX;
               var scaleY = objects[i].scaleY;
               var left = objects[i].left;
               var top = objects[i].top;

               var tempScaleX = scaleX * (1 / SCALE_FACTOR);
               var tempScaleY = scaleY * (1 / SCALE_FACTOR);
               var tempLeft = left * (1 / SCALE_FACTOR);
               var tempTop = top * (1 / SCALE_FACTOR);

               objects[i].scaleX = tempScaleX;
               objects[i].scaleY = tempScaleY;
               objects[i].left = tempLeft;
               objects[i].top = tempTop;

               objects[i].setCoords();
           }

           canvas.renderAll();
       }

Reset zoom

This function will bring all the objects to their respective original scale factors:

var onResetZoom = function () {

           canvas.setHeight(canvas.getHeight() * (1 / canvasScale));
           canvas.setWidth(canvas.getWidth() * (1 / canvasScale));

           var objects = canvas.getObjects();
           for (var i in objects) {
               var scaleX = objects[i].scaleX;
               var scaleY = objects[i].scaleY;
               var left = objects[i].left;
               var top = objects[i].top;

               var tempScaleX = scaleX * (1 / canvasScale);
               var tempScaleY = scaleY * (1 / canvasScale);
               var tempLeft = left * (1 / canvasScale);
               var tempTop = top * (1 / canvasScale);

               objects[i].scaleX = tempScaleX;
               objects[i].scaleY = tempScaleY;
               objects[i].left = tempLeft;
               objects[i].top = tempTop;

               objects[i].setCoords();
           }

           canvas.renderAll();

           canvasScale = 1;
       }

All the above codes are wrapped in the below github link:

• https://github.com/swagatblog/FabricJS_Drawing

Just download and play around.

This application would give an idea about building a simple and powerful drawing app using HTML 5 canvas. FabricJs is one of the most powerful canvas JS library which gives you an opportunity to build a browser also a touch based drawing application.

Few of the components have been used in this app:

1. Drawing a rectangle
2. Drawing a circle
3. Drawing an image
4. Enable and disable drawing
5. Enable and disable pan
6. Zoom in
7. Zoom out
8. Reset zoom

References

1. http://fabricjs.com/
2. http://fabricjs.com/events/
3. http://fabricjs.com/demos/