Detecting drawn shapes in Canvas+Javascript?

I had an开发者_JAVA百科 idea for a WebOS, but it requires detecting drawn shapes. Ie: I want a user to be able to draw an image, then draw a big box around the whole image. Then the user can drag that box by the grabbing the border, and moving the whole image around. This requires that I be able to detect when such a box has been drawn, and turn it into a Rect type. Anyone have any pointers or tips on how to do this? I don't even know where to start.

There are Canvas functions for getting the pixels in a rectangle. But these functions do not work in IE (even with excanvas.js). Is that what you want to do?

You can attach mouse events to a Canvas, or to a div containing it.

If you could explain what you're trying to do exactly, I might be able to offer more help. If I understand you correctly, I think you could you what you're talking about if you don't care about Internet Explorer support.