Visit the Ejecta website for more info on what it is and how to use it. I will talk a bit more about some implementation details for the Canvas API here.
Implementing a general purpose drawing API, such as the HTML5 Canvas API, on top of OpenGL is by no means an easy endeavor. Before I decided to roll my own solution (you know, I have this problem), I looked at a number of graphic libraries including Google's skia and OpenVG.
I discovered exactly what I feared beforehand: these libraries do way too much, are too large and too hard to implement. You can't just use them here and there to draw – instead they replace your whole drawing stack. Getting them to compile alone is a huge pain; getting them to compile on the iPhone and then get them do what you wanted to seemed close to impossible.
So I began working on my own solution. Implementing the path methods for
bezierCurveTo(), etc. was fairly straight forward: have an array of subpaths where each subpath is an array of points (x,y). Each call to the API methods pushes one or more points to the subpath or closes it.
However, I struggled a bit with getting bezier curves to behave in a manner that makes sense for the current scale; i.e. push more points for large bezier curves and at sharp corners, fewer points for smaller ones and straight lines. After a few days of reading and experimenting, I found this excellent article on adaptive bezier curves and adopted its solution.
The hard part was getting that array of points on the screen. For drawing lines (
.stroke()) I didn't want to go with the obvious solution of just using GL_LINES, because it has a number of drawbacks, especially on iOS: no anti aliasing, limited line width and no miters or line caps.
So instead of using GL_LINES to draw, I ended up creating 2 triangles for each line segment and calculate the miter values myself. This correctly honors the APIs
.miterLimit property, though the bevel it then draws is still a bit off. The code I ended up with is a bit on the ugly side, because it handles a lot of edge cases, but all in all this solution worked very well and is extremely fast.
.fill() proved to be yet another challenge. With OpenGL, before you can draw a primitive to the screen, you have to break it down into triangles first. This is quite easy to do for convex polygons, but not so much for concave ones that potentially have holes in them.
I spent a few days looking for triangulation library and soon realized that this is serious business. Triangle for instance, sports 16k loc – I'm quite allergic to libraries that need that much code to solve seemingly simple problems. Poly2Tri looked much more sane, but apparently has some stability problems.
After a bit of searching, I found libtess2, which is based on OpenGL's libtess and is supposed to be extremely robust and quite fast. The code base is excellent and I had no problem implementing it with Ejecta.
However, some tests showed that it's much slower than I hoped it would be. Realtime triangulation of complex polygons isn't very feasible on the iPhone.
In the end, I found a trick that lets you draw polygons in OpenGL without triangulating them first. It is so simple and elegant to implement, yet so ingenious: You can draw polygons with a simple triangle fan and mark those areas that you overdraw in the stencil buffer. See Drawing Filled, Concave Polygons Using the Stencil Buffer. It's a hacker's solution – thinking outside the box – and it fills me with joy.
There's still some parts missing in my Canvas implementation, namely gradients, shadows and most notably: text. I believe the best solution for drawing text in OpenGL, while honoring the Canvas spec, would be drawing to a texture using the iPhone's CG methods. This will make it quite slow, but should be good enough for a few paragraphs of text.
If you want to help out with anything grab the Ejecta source code on github – I'd be honored.