Darknoon Blog » Core Animation debug mode

If you’re into optimizing scrolling or drawing performance on the iPhone, then you know that blended layers (ie, non-opaque) can really slow things down. Sometimes it’s hard to know what is opaque and what is being blended, however.

The good news is that there is a tool for this! You can easily get a visual readout of all of the layers in your app and their blending. Here is an example:

The red areas are being blended, whereas the green areas are totally opaque, giving the best performance.

Here’s how to set it up:

Debug Mode On-Device

Open up the Instruments tool, and select an iPhone or iPod touch as the applicable device, then add a new Core Animation instrument.

You’ll then need to show Detail (View > Detail, or the button in the bottom bar). Then there is a checkbox for “Color blended layers.” This is what you want.

Your iPhone or iPod Touch should now switch into red-green mode. This setting is persistent, so you can detach your phone and use it now. To disable, attach to Instruments again and uncheck the box or restart the phone.

Debug Mode in Simulator

Turns out, there is an easier way! You can do all of this fun in the simulator. Just launch it with the CA_COLOR_OPAQUE environment variable set:

CA_COLOR_OPAQUE=1 /Developer/Platforms/iPhoneSimulator.platform/Developer/Applications/iPhone\ Simulator.app/Contents/MacOS/iPhone\ Simulator

And, boom! You can now run your apps as usual using Xcode and they’ll show up in the simulator with color blended layers enabled. To disable it, just quit and relaunch Simulator.