Home // Projective Texturing with Canvas
Description
Canvas is still limited to 2D: its drawing operations can only do typical vector graphics with so-called affine transformations, i.e. scaling, rotating, skewing and translation. Though there have been some efforts to try and add a 3D context to Canvas, these efforts are still experimental and only available for a minority of browsers through plug-ins.
So when my colleague Ross asked me if we could build a Cover Flow-like widget with JavaScript, my initial reaction was no… but really, that’s just a cop out. All you need are textured rectangles drawn in a convincing perspective: a much simpler scenario than full blown 3D.
Perspective views are described by so-called projective transforms, which Canvas2D does not support. However, it does support arbitrary clipping masks as well as affine transforms of both entire and partial images. These can be used to do a fake projective transform: you cut up your textured surface into a bunch of smaller patches (which are almost-affine) and render each with a normal affine transform. Of course you need to place the patches just right, so as to cover any possible gaps. As long as the divisions are small enough, this looks convincingly 3D.
So some hacking later, I have a working projective transform renderer in JavaScript. The algorithm uses adaptive subdivision to maintain quality and can be tuned for detail or performance. At its core it’s really just a lot of linear algebra, though I did have to add a bunch of tweaks to make it look seamless due to annoying aliasing effects.
