Canvas graphic rotation support and canvas graphic redraw optimizations

[iacovlev-pavel]

#1

Added canvas graphic rotation support (style.rotation), at the moment works only with SVG (did not test VML).
#2

With the current implementation every time a graphic is drawn a new image is created, then the onload handler is attached and the render process is executed in this handler's success result.
This has some performance implication (flickering) - if many features have the same graphic (typical in maps), or when the graphic is redrawn often with a rotation, offset, position etc. I added a quick fix which caches the last image, it did get rid of flicking in all of my use cases.

[ahocevar]

This does not look like a real cache. It only stores a single graphic, and creating a new image is only avoided if the same graphic is used by subsequent features. Or am I missing something?

[iacovlev-pavel]

Exactly.

My first use case was rotating a feature and on each rotation the code waits for the load event to fire. Basically if I rotate by 1 degree a full circle I had to load 360 times which caused flickering on Android.
Another use case if all the features have the same graphic or "a lot of the same graphic" in a row.
In both cases the "hack" helped (no flickering).

If you have any proposals of how to make this properly without a hack, I will gladly implement it. I can also create a separate pull request for rotation and another one for "optimization" of graphic drawing.

[ahocevar]

The rotation work seems incomplete as well. Try the styles-rotation.html example with the Canvas renderer to see what I mean.

For the caching, you could keep a cache of let's say 100 images (configurable), with a LRU policy. Something like I do here for caching tile images.

Having said that, separate pull requests for caching and rotation would make sense. Thanks for your efforts so far!