I have been working on so many canvas libraries which enables us to build robust drawing app with unconditional customization to the library.
Found FabricJS as the best library till now, which supports multiple touch based devices and obviously most of the HTML 5 supported browsers.
You can find a sample application in below repository and can download also
This application would give an idea about building a simple and powerful drawing app using HTML 5 canvas. 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 oou
8) Reset zoom
Ref: Official sites for fabricjs
Send me a note if you need any info/help on this library.