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POiOS CGPath Performance
 

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    <p><strong>UPDATE</strong></p> <p>I got around CG's limitations by drawing everything with OpenGL. Still some glitches, but so far it's working much, much faster.</p> <p>Some interesting points :</p> <ul> <li><strong><code>GLKView</code></strong> : That's an iOS-specific view, and it helps a lot in setting up the OpenGL context and rendering loop. If you're not on iOS, I'm afraid you're on your own.</li> <li><strong>Shader precision</strong> : The precision of shader variables in the current version of OpenGL ES (2.0) is <a href="https://stackoverflow.com/questions/4414041/what-is-the-precision-of-highp-floats-in-glsl-es-2-0-for-iphone-ipod-touch-ipad" title="16-bit">16-bit</a>. That was a little low for my purposes, so I emulated 32-bit arithmetics with pairs of 16-bit variables.</li> <li><strong><code>GL_LINES</code></strong> : OpenGL ES can natively draw simple lines. Not very well <em>(no joints, no caps, see the purple/grey line on the top of the screenshot below)</em>, but to improve that you'll have to write a custom shader, convert each line into a triangle strip and pray that it works! <em>(supposedly that's how browsers do that when they tell you that Canvas2D is GPU-accelerated)</em></li> </ul> <p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src="https://i.stack.imgur.com/y0lAI.png" alt="Example rendering"></p> <ul> <li>Draw <strong>as little as possible</strong>. I suppose that makes sense, but you can frequently avoid rendering things that are, for instance, outside of the viewport.</li> <li>OpenGL ES has <strong><em>no</em> support for filled polygons</strong>, so you have to tesselate them yourself. Consider using <a href="http://code.google.com/p/iphone-glu/" rel="nofollow noreferrer" title="iPhone-GLU">iPhone-GLU</a> : that's a port of the MESA code and it's pretty good, although it's a little hard to use (no standard Objective-C interface).</li> </ul> <p><strong>Original Question</strong></p> <p>I'm trying to draw lots of CGPaths (typically more than 1000) in the <code>drawRect</code> method of my scroll view, which is refreshed when the user pans with his finger. I have the same application in JavaScript for the browser, and I'm trying to port it to an iOS native app.</p> <p>The iOS test code is (with 100 line operations, <code>path</code> being a pre-made <code>CGMutablePathRef</code>) :</p> <pre><code>- (void) drawRect:(CGRect)rect { // Start the timer BSInitClass(@"Renderer"); BSStartTimedOp(@"Rendering"); // Get the context CGContextRef context = UIGraphicsGetCurrentContext(); CGContextSetLineWidth(context, 2.0); CGContextSetFillColorWithColor(context, [[UIColor redColor] CGColor]); CGContextSetStrokeColorWithColor(context, [[UIColor blueColor] CGColor]); CGContextTranslateCTM(context, 800, 800); // Draw the points CGContextAddPath(context, path); CGContextStrokePath(context); // Display the elapsed time BSEndTimedOp(@"Rendering"); } </code></pre> <p>In JavaScript, for reference, the code is (with 10000 line operations) :</p> <pre><code>window.onload = function() { canvas = document.getElementById("test"); ctx = canvas.getContext("2d"); // Prepare the points before drawing var data = []; for (var i = 0; i &lt; 100; i++) data.push ({x: Math.random()*canvas.width, y: Math.random()*canvas.height}); // Draw those points, and write the elapsed time var __start = new Date().getTime(); for (var i = 0; i &lt; 100; i++) { for (var j = 0; j &lt; data.length; j++) { var d = data[j]; if (j == 0) ctx.moveTo (d.x, d.y); else ctx.lineTo(d.x,d.y) } } ctx.stroke(); document.write ("Finished in " + (new Date().getTime() - __start) + "ms"); }; </code></pre> <p>Now, I'm much more proficient in optimizing JavaScript than I am at iOS, but, after some profiling, it seems that CGPath's overhead is absolutely, incredibly bad compared to JavaScript. Both snippets run at about <em><strong>the same speed</strong></em> on a real iOS device, and the JavaScript code has 100x the number of line operations of the Quartz2D code!</p> <p><em>EDIT: Here is the top of the time profiler in Instruments :</em></p> <pre><code>Running Time Self Symbol Name 6487.0ms 77.8% 6487.0 aa_render 449.0ms 5.3% 449.0 aa_intersection_event 112.0ms 1.3% 112.0 CGSColorMaskCopyARGB8888 73.0ms 0.8% 73.0 objc::DenseMap&lt;objc_object*, unsigned long, true, objc::DenseMapInfo&lt;objc_object*&gt;, objc::DenseMapInfo&lt;unsigned long&gt; &gt;::LookupBucketFor(objc_object* const&amp;, std::pair&lt;objc_object*, unsigned long&gt;*&amp;) const 69.0ms 0.8% 69.0 CGSFillDRAM8by1 66.0ms 0.7% 66.0 ml_set_interrupts_enabled 46.0ms 0.5% 46.0 objc_msgSend 42.0ms 0.5% 42.0 floor 29.0ms 0.3% 29.0 aa_ael_insert </code></pre> <p>It is my understanding that this should be much faster on iOS, simply because the code is native... So, do you know :</p> <ul> <li>...what I am doing wrong here?</li> <li>...and if there's another, better solution to draw that many lines in real-time?</li> </ul> <p>Thanks a lot!</p>
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    3. COTry drawing it in a non scrollview to see how slow it is. UIScrollView sometimes makes drawing very slow. If that's the case, you might be able to do your drawing in a plain UIView and then place that UIView into the scrollview. Also, make sure the view is opaque if possible.
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