how many fps can iPhone's UIGetScreenImage() actually do?

Question

Now that Apple is officially allowing UIGetScreenImage() to be used in iPhone apps, I've seen a number of blogs saying that this "opens the floodgates" for video capture on iPhones, including older models. But I've also seen blogs that say the fastest frame rate they can get with UIGetScreenImage() is like 6 FPS.

Can anyone share specific frame-rate results you've gotten with UIGetScreenImage() (or other approved APIs)? Does restricting the area of the screen captured improve frame rate significantly?

Also, for the wishful thinking segment of today's program, does anyone have pointers to code/library that uses UIGetScreenImage() to capture video? For instance, I'd like an API something like Capture( int fps, Rect bounds, int durationMs ) that would turn on the camera and for the given duration record a sequence of .png files at the given frame rate, copying from the given screen rect.

Answer 1

There is no specific frame rate. UIGetScreenImage() is not a movie recorder. It just try to return as soon as it could, unfortunately still very slow.

Restricting the area of the screen captured is useless. UIGetScreenImage doesn't take any input parameters. Cropping the output image could make the frame rate even worse due to the excess work.

Answer 2

UIGetScreenImage() returns an image of current screen display. It's said to be slow but whether it's fast enough depends on the use case. The video recording app iCamcorder is using this function.

According to their blog,

iCamcorder records at an remarkable average minimum of 10 frames per second and a maximum of 15 frames per second.

The UIGetScreenImage method Apple recently allowed developers to use captures the current screen contents. Unfortunately it is really slow, about 15% of the processing time of the App just goes into calling this method. http://www.drahtwerk.biz/EN/Blog.aspx/iCamcorder-v19-and-Giveaway/?newsID=27

So the raw performance of UIGetScreenImage() should be at least much higher than 15 fps.

To crop the returned image, you can try

extern CGImageRef UIGetScreenImage(void);

...
CGImageRef cgoriginal = UIGetScreenImage();
CGImageRef cgimg = CGImageCreateWithImageInRect(cgoriginal, rect);            
UIImage *viewImage = [UIImage imageWithCGImage:cgimg];
CGImageRelease(cgoriginal);                
CGImageRelease(cgimg);