How to optimize the following CoreGraphics code - ios

Question

I want to make a screenshot of OpenGLES and UIKit at a time and after a big research I found a way exactly like this:
- (UIImage *)makeScreenshot {
GLint backingWidth, backingHeight;
// Bind the color renderbuffer used to render the OpenGL ES view
// If your application only creates a single color renderbuffer which is already bound at this point,
// this call is redundant, but it is needed if you're dealing with multiple renderbuffers.
// Note, replace "_colorRenderbuffer" with the actual name of the renderbuffer object defined in your class.
// glBindRenderbufferOES(GL_RENDERBUFFER_OES, _colorRenderbuffer);
// Get the size of the backing CAEAGLLayer
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);
// NSInteger x = 0, y = 0, width = backingWidth, height = backingHeight;
NSInteger x = _visibleFrame.origin.x, y = _visibleFrame.origin.y, width = _visibleFrame.size.width, height = _visibleFrame.size.height;
NSInteger dataLength = width * height * 4;
GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
// Read pixel data from the framebuffer
glPixelStorei(GL_PACK_ALIGNMENT, 4);
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
// Create a CGImage with the pixel data
// If your OpenGL ES content is opaque, use kCGImageAlphaNoneSkipLast to ignore the alpha channel
// otherwise, use kCGImageAlphaPremultipliedLast
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
// CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast, ref, NULL, true, kCGRenderingIntentDefault);
CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaNoneSkipLast, ref, NULL, true, kCGRenderingIntentDefault);
// OpenGL ES measures data in PIXELS
// Create a graphics context with the target size measured in POINTS
NSInteger widthInPoints, heightInPoints;
if (NULL != UIGraphicsBeginImageContextWithOptions) {
// On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
// Set the scale parameter to your OpenGL ES view's contentScaleFactor
// so that you get a high-resolution snapshot when its value is greater than 1.0
CGFloat scale = _baseView.contentScaleFactor;
widthInPoints = width / scale;
heightInPoints = height / scale;
UIGraphicsBeginImageContextWithOptions(CGSizeMake(widthInPoints, heightInPoints), NO, scale);
}
else {
// On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
widthInPoints = width;
heightInPoints = height;
UIGraphicsBeginImageContext(CGSizeMake(widthInPoints, heightInPoints));
}
CGContextRef cgcontext = UIGraphicsGetCurrentContext();
// UIKit coordinate system is upside down to GL/Quartz coordinate system
// Flip the CGImage by rendering it to the flipped bitmap context
// The size of the destination area is measured in POINTS
CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);
// Retrieve the UIImage from the current context
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Clean up
free(data);
CFRelease(ref);
CFRelease(colorspace);
CGImageRelease(iref);
// return image;
UIImageView *GLImage = [[UIImageView alloc] initWithImage:image];
UIGraphicsBeginImageContext(_visibleFrame.size);
//order of getting the context depends on what should be rendered first.
// this draws the UIKit on top of the gl image
[GLImage.layer renderInContext:UIGraphicsGetCurrentContext()];
CGContextTranslateCTM(UIGraphicsGetCurrentContext(), -_visibleFrame.origin.x, -_visibleFrame.origin.y);
[_baseView.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *finalImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Do something with resulting image
return finalImage;
}
but the interesting part would be the merging section. Where I am having two
UIGraphicsBeginImageContext();
.......
.......
UIGraphicsEndImageContext();
blocks. First generating the OpenGLES image and then merging with the UIKit image. Is there a better way to do that with single UIGraphicsBeginImageContext(); ... UIGraphicsEndImageContext(); block rather creating UIImageView and then perform the render?
something like:
CGContextRef cgcontext = UIGraphicsGetCurrentContext();
// UIKit coordinate system is upside down to GL/Quartz coordinate system
// Flip the CGImage by rendering it to the flipped bitmap context
// The size of the destination area is measured in POINTS
CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);
// the merging part starts
CGContextTranslateCTM(UIGraphicsGetCurrentContext(), -_visibleFrame.origin.x, -_visibleFrame.origin.y);
[_baseView.layer renderInContext:UIGraphicsGetCurrentContext()];
// the merging part ends
// Retrieve the UIImage from the current context
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
but unfortunately its not merging. Can anyone correct the mistake here and/or find the best way to do that?

Answer 1

UISnapshotting
With iOS 7 Apple introduced UISnapshotting and they claim it's really fast, much faster than renderInContext:.
UIView *snapshot = [view snapshotViewAfterScreenUpdates:YES];
This method captures the current visual contents of the screen from
the render server and uses them to build a new snapshot view. You can
use the returned snapshot view as a visual stand-in for the screen’s
contents in your app. (...) this method is faster than trying to
render the contents of the screen into a bitmap image yourself.
Moreover, have a look into links below. They should give you some insights and point to the right direction.
Implementing Engaging UI on iOS from WWDC 2013, slides 32-41
How to render view into image faster?