Android Canvas Off Screen Drawing Performance

Question

I'm developing an Android game using Canvas element. I have many graphic elements (sprites) drawn on a large game map. These elements are drawn by standard graphics functions like drawLine, drawPath, drawArc etc.

It's not hard to test if they are in screen or not. So, if they are out of the screen, i may skip their drawing routines completely. But even this has a CPU cost. I wonder if Android Graphics Library can do this faster than I can?

In short, should I try to draw everything even if they are completely out of the screen coordinates believing Android Graphics Library would take care of them and not spend much CPU trying to draw them or should I check their drawing area rectangle myself and if they are completely out of screen, skip the drawing routines? Which is the proper way? Which one is supposed to be faster?

p.s: I'm targeting Android v2.1 and above.

Answer 1

From a not-entirely-scientific test I did drawing Bitmaps tiled across a greater area than the screen, I found that checking beforehand if the Bitmap was onscreen doesn't seem to make a considerable different.

In one test I set a Rect to the screen size and set another Rect to the position of the Bitmap and checked Rect.intersects() before drawing. In the other test I just drew the Bitmap. After 300-ish draws there wasn't a visible trend - some went one way, others went another. I tried the 300-draw test every frame, and the variation from frame to frame was much greater than difference between checked and unchecked drawing.

From that I think it's safe to say Android checks bounds in its native code, or you'd expect a considerable difference. I'd share the code of my test, but I think it makes sense for you to do your own test in the context of your situation. It's possible points behave differently than Bitmaps, or some other feature of your paint or canvas changes things.

Hope that help you (or another to stumble across this thread as I did with the same question).