Resizable Canvas (JQuery UI)

Question

ANY DOM element can be made resizable according to this page: http://jqueryui.com/demos/resizable/

However, it seems that this doesn\'t work for the CANVAS element.

Answer 1

Well why not apply a div before the canvas element, and resize that element, and when it stops apply width and height in canvas and then redraw canvas.

Answer 2

There's a bit of a funny quirk on the canvas co-ordinate system, with regards to using this :

#stretch canvas {
  width: 100%;
  height: 100%;
}

(from the example above)

You have to use these CSS rules because you can not specify % in the canvas' width/height attributes. But you'll find you get unexpected results when you try to draw objects on to it using just these rules- drawn objects appear squashed. Here's the above CSS, with a canvas border...

    /* The CSS */
    #stretch {
        width: 200px;
        height: 100px;          
    }
    #stretch canvas {
        border:1px solid red;
        width: 100%;
        height: 100%;
    }

The canvas is drawn to the right size and the border rule is implemented correctly. The blue rectangle, added in code elsewhere, is supposed to fill the canvas with a 20px padding all around, but this has not happened:

To fix this, make sure you also specify a width and height for the canvas element (either in the HTML or in javascript with setAttribute):

canvas.setAttribute('width', '200');
canvas.setAttribute('height', '100');

or

<canvas width=200 height=100></canvas>

Now when I refresh the page I get what I expected:

This 'double-check' will make sure the canvas is drawn using the correct co-ordinate system and still allow it to be resized, via the CSS rules. If this is a bug, I'm sure it will be fixed in due course.

This is just about providing a fix, but you can read more in to it here

Answer 3

Canvas has two types of resize behavior:

• Resizing where the contents are stretched to fit the canvas's new dimensions
• Resizing where the contents remain static while the canvas grows or shrinks

Here's a page that demonstrates the two types of "resizing": http://xavi.co/static/so-resizable-canvas.html

If you want the first type of resizing (stretch the content) then place the canvas into a container div and set the width and height of the canvas to 100% using CSS. Here's what that code might look like:

/* The CSS */
#stretch {
    height: 100px;
    width: 200px;
}

#stretch canvas {
    width: 100%;
    height: 100%;
}

<!-- The markup -->
<div id="stretch"><canvas></canvas></div>

// The JavaScript
$("#stretch").resizable();

The second type of resizing (static content) is a two step process. First you must adjust the width and height attributes of the canvas element. Unfortunately, doing this clears the canvas, so you must then re-draw all its contents. Here's bit of code that does this:

/* The CSS */
#resize {
    height: 100px;
    width: 200px;
}

<!-- The markup -->
<div id="resize"><canvas></canvas></div>

// The JavaScript
$("#resize").resizable({ stop: function(event, ui) {
    $("canvas", this).each(function() { 
        $(this).attr({ width: ui.size.width, height: ui.size.height });

        // Adjusting the width or height attribute clears the canvas of
        // its contents, so you are forced to redraw.
        reDraw(this);
    });
} });

Currently the code above re-draws the canvas's content when the user stops resizing the widget. It's possible to re-draw the canvas on resize, however resize events occur fairly often and re-draws are expensive operations -- approach with caution.