Saturday, August 1, 2009

Subcanvas with events

What is the picture of? It's a simple test for the Subcanvas graphics / events API. It's Squeak, but with Morphic temporarily disabled and a Subcanvas test running full-screen and reacting to keyboard and mouse events. The blue squares are from mouse move events (offset by a few mm). The big green rectangle is a subcanvas that draws little red squares rather than blue squares on mouse move events. All the other sized squares are from mouse click events. The text is from keyboard events; the little white lines above text are key press events and the white lines below are key release events.

Font rendering here is done very simply. It will be improved.

The performance of this implementation is currently usable with a single, small subcanvas, but the performance will suffer incredibly as subcanvases get bigger. This is because subcanvases are BitBltted in their entirety after each event; the implementation is currently naïve and there is a lot of room for optimisation.

I'm disappointed by Squeak's graphics performance. BitBlts on Forms are very slow. As a result, I'm not going to try to optimise this implementation of Subcanvas (based on Forms and BitBlt) and instead re-implement it in the future using either a raw X11 socket with XRender, XDamage etc, or use OpenGL in 2-D. Subcanvas at heart is an API, so any users of it should be unaffected. Currently I'm favouring reimplementing it to use X11 as most POSIX systems (Linux, Solaris, BSD, Apple with OS/X) have an accelerated X server, and I could bundle something like XMing with MS Windows. Forms, BitBLT and friends could then be completely purged from the VM leaving it headless.

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