iPhone: Get camera preview

问题:

I'd like to get the image that is being displayed on the UIImagePickerController when user uses the camera. And when I get I want to process the image and display instead of regular camera view.

But the problem is when I want to get the camera view, the image is just a black rectangle.

Here's my code:

UIView *cameraView = [[[[[[imagePicker.view subviews] objectAtIndex:0]
                         subviews] objectAtIndex: 0]
                       subviews] objectAtIndex: 0];

UIGraphicsBeginImageContext( CGSizeMake(320, 427) );
[cameraView.layer renderInContext: UIGraphicsGetCurrentContext()];
UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

imageToDisplay.image = [PixelProcessing processImage: viewImage];   //In this case the image is black
//imageToDisplay.image = viewImage; //In this case the image is black too
//imageToDisplay.image = [UIImage imageNamed: @"icon.png"];     //In this case image is being displayed properly

What am I doing wrong?

Thanks.

回答1:

This one is also working quite good. Use it when the camera preview is open:

UIImage *viewImage = [[(id)objc_getClass("PLCameraController") 
                      performSelector:@selector(sharedInstance)] 
                      performSelector:@selector(_createPreviewImage)];

But as far as I found out it brings the same results than the following solution which takes a 'screenshot' of the current screen:

extern CGImageRef UIGetScreenImage();

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

A problem I didn't still find a fix for is, how can one get the camera image very fast without any overlays?

回答2:

The unofficial call is:

UIGetScreenImage()

which you declare above the @implementation as:

extern CGImageRef UIGetScreenImage();

There may be a documented way to do this in 3.1, but I'm not sure. If not, please please file a Radar with Apple asking them to make some kind of screen grab access public!!!

That uses your same AppleID you log in to the iPhone development portal with.

Update: This call is not yet documented, but Apple explicitly has given the OK to use it with App Store apps.

回答3:

at least for now, there's no way to do this. (certainly no official documented way, and as far as I know nobody's figured out an unofficial way either.)

the camera preview data is being drawn by the OS in some way that bypasses the normal graphics methods.