Performance when frequently drawing CGPaths

Question

I am working on an iOS App that visualizes data as a line-graph. The graph is drawn as a CGPath in a fullscreen custom UIView and contains at most 320 data-points. The data is frequently updated and the graph needs to be redrawn accordingly – a refresh rate of 10/sec would be nice.

So far so easy. It seems however, that my approach takes a lot of CPU time. Refreshing the graph with 320 segments at 10 times per second results in 45% CPU load for the process on an iPhone 4S.

Maybe I underestimate the graphics-work under the hood, but to me the CPU load seems a lot for that task.

Below is my drawRect() function that gets called each time a new set of data is ready. N holds the number of points and points is a CGPoint* vector with the coordinates to draw.

- (void)drawRect:(CGRect)rect {

    CGContextRef context = UIGraphicsGetCurrentContext();

    // set attributes
    CGContextSetStrokeColorWithColor(context, [UIColor lightGrayColor].CGColor);
    CGContextSetLineWidth(context, 1.f);

    // create path
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddLines(path, NULL, points, N+1);

    // stroke path
    CGContextAddPath(context, path);
    CGContextStrokePath(context);

    // clean up
    CGPathRelease(path); 
}

I tried rendering the path to an offline CGContext first before adding it to the current layer as suggested here, but without any positive result. I also fiddled with an approach drawing to the CALayer directly but that too made no difference.

Any suggestions how to improve performance for this task? Or is the rendering simply more work for the CPU that I realize? Would OpenGL make any sense/difference?

Thanks /Andi

Update: I also tried using UIBezierPath instead of CGPath. This post here gives a nice explanation why that didn't help. Tweaking CGContextSetMiterLimit et al. also didn't bring great relief.

Update #2: I eventually switched to OpenGL. It was a steep and frustrating learning curve, but the performance boost is just incredible. However, CoreGraphics' anti-aliasing algorithms do a nicer job than what can be achieved with 4x-multisampling in OpenGL.

Answer 1

This post here gives a nice explanation why that didn't help.

It also explains why your drawRect: method is slow.

You're creating a CGPath object every time you draw. You don't need to do that; you only need to create a new CGPath object every time you modify the set of points. Move the creation of the CGPath to a new method that you call only when the set of points changes, and keep the CGPath object around between calls to that method. Have drawRect: simply retrieve it.

You already found that rendering is the most expensive thing you're doing, which is good: You can't make rendering faster, can you? Indeed, drawRect: should ideally do nothing but rendering, so your goal should be to drive the time spent rendering as close as possible to 100%—which means moving everything else, as much as possible, out of drawing code.