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I'm working on an iPhone app that I'm going to be demo'ing to a live audience soon.

I'd really like to demo the app live over VGA to a projector, rather than show screenshots.

I bought a VGA adapter for iPhone, and have adapted Rob Terrell's TVOutManager to suit my needs. Unfortunately, the frame rate after testing on my television at home just isn't that good - even on an iPhone 4 (perhaps 4-5 frames per second, it varies).

I believe the reason for this slowness is that the main routine I'm using to capture the device's screen (which is then being displayed on an external display) is UIGetScreenImage(). This routine, which is no longer allowed to be part of shipping apps, is actually quite slow. Here's the code I'm using to capture the screen (FYI mirrorView is a UIImageView):

CGImageRef cgScreen = UIGetScreenImage();
self.mirrorView.image = [UIImage imageWithCGImage:cgScreen];
CGImageRelease(cgScreen);

Is there a faster method I can use to capture the iPhone's screen and achieve a better frame rate (shooting for 20+ fps)? It doesn't need to pass Apple's app review - this demo code won't be in the shipping app. If anyone knows of any faster private APIs, I'd really appreciate the help!

Also, the above code is being executed using a repeating NSTimer which fires every 1.0/desiredFrameRate seconds (currently every 0.1 seconds). I'm wondering if instead wrapping those calls in a block and using GCD or an NSOperationQueue might be more efficient than having the NSTimer invoke my updateTVOut obj-c method that currently contains those calls. Would appreciate some input on that too - some searching seems to indicate that obj-c message sending is somewhat slow compared to other operations.

Finally, as you can see above, the CGImageRef that UIGetScreenImage() returns is being turned into a UIImage and then that UIImage is being passed to a UIImageView, which is probably resizing the image on the fly. I'm wondering if the resizing might be slowing things down even more. Ideas of how to do this faster?

Jon G.
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4 Answers4

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Have you looked at Apple's recommended alternatives to UIGetScreenImage? From the "Notice regarding UIGetScreenImage()" thread:

Applications using UIGetScreenImage() to capture images from the camera should instead use AVCaptureSession and related classes in the AV Foundation Framework. For an example, see Technical Q&A QA1702, "How to capture video frames from the camera as images using AV Foundation". Note that use of AVCaptureSession is supported in iOS4 and above only.

Applications using UIGetScreenImage() to capture the contents of interface views and layers should instead use the -renderInContext: method of CALayer in the QuartzCore framework. For an example, see Technical Q&A QA1703, "Screen capture in UIKit applications".

Applications using UIGetScreenImage() to capture the contents of OpenGL ES based views and layers should instead use the glReadPixels() function to obtain pixel data. For an example, see Technical Q&A QA1704, "OpenGL ES View Snapshot".

Community
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Jonah
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    Hi Jonah, thanks for your suggestion. As it turns out, I did try using the renderInContext: method of CALayer. It's even slower. AVFoundation won't really help me either in this case since I'm not capturing from a camera or other AV source - I'm trying to capture the screen itself. Apple recommends alternatives to UIGetScreenImage because they don't allow it any more, but this code isn't for App Store submission, it's just for a demo. – Jon G. Jan 20 '11 at 19:43
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New solution: get an iPad 2 and mirror the output! :)

Jon G.
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I don't know how fast is this but it worth a try ;)

CGImageRef screenshot = [[UIApplication sharedApplication] _createDefaultImageSnapshot];
[myVGAView.layer setContents:(id)screenshot];

where _createDefaultImageSnapshot is a private API. (Since is for demo... its ok I suppose) and myVGAView is a normal UIView. If you get CGImageRefs then just pass them to the contents of a layer, its lighter and should be a little bit faster (but just a little bit ;) )

nacho4d
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I haven't got the solution you want (simulating video mirroring) but you can move your views to the external display. This is what I do and there is no appreciable impact on the frame rate. However, obviously since the view is no longer on the device's screen you can no longer directly interact with it or see it. If you have something like a game controlled with the accelerometer this shouldn't be a problem, however something touch based will require some work. What I do is have an alternative view on the device when the primary view is external. For me this is a 2D control view to "command" the normal 3D view. If you have a game you could perhaps create an alternative input view to control the game with (virtual buttons/joystick etc.) really depends on what you have as to how to work around it best.

Not having jailbroken myself I can't say for sure but I am under the impression that a jailbroken device can essentially enable video mirroring (like they use at the apple demos...). If true, that is likely your easiest route if all you want is a demo.

ima747
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