Programmatically create image from UILabel

Question

I want to use a UILabel to create an RGBA stream of bytes image representation programmatically (at runtime).

For example, I want to create a UILabel in a specific font and with specific text, turn this in to an NSData of RGBA unsigned bytes, and then from that point I can easily turn it in to an OpenGL texture and display it however I want.

It's important that I can know the dimensions of the resulting image, although if absolutely necessary I can create a very wide "blank" canvas to render this in to, and then at runtime detect the width and height myself by checking the bytes.

If I can achieve this through UIImage/CG it's a definite plus - and a solution with multiple render targets isn't reasonable/desired.

Edit: Placed a bounty; I had originally gotten this good-looking answer below from meronix, but now I got time to try it out and it does not work properly. I need this working "yesterday" and am quite frustrated. There's some nuance here, that someone who knows Cocoa drawing state better than me can surely catch.

Answer 1

You can "capture" (take a snapShot of) your UILabel, it's a subclass of UIView, so you can get its image.

use this class "CaptureView":

CaptureView.h

#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h>
#import <QuartzCore/CALayer.h>


@interface CaptureView : UIView {
@private
    UIImage *_imageCapture;
}

@property(nonatomic, retain) UIImage *imageCapture;

// Init
- (id)initWithView:(UIView *)view;

@end

CaptureView.m

#import "CaptureView.h"

// Private
@interface CaptureView (/* Private */)
- (void)settingImageFromView:(UIView *)view;
@end

// Public
@implementation CaptureView

@synthesize imageCapture = _imageCapture;

// Standard // UILabel
- (id)initWithFrame:(CGRect)frame {

    if ((self = [super initWithFrame:frame])) {
        // Initialization code.
    }
    return self;
}

// Init
- (id)initWithView:(UIView *)view {
    if ((self = [super initWithFrame:[view frame]])) {
//  if ((self = [super initWithFrame: CGRectMake(0, 0, 100, 100)])) {
        // Initialization code.
        [self settingImageFromView:view];
    }
    return self;  
}

/*!
 @method     imageFromView:view
 @abstract   Esegue una Grab di una intera view
 @discussion Imposta l'immagine catturata eseguendo una copia dell'intera view 
 */
- (void)settingImageFromView:(UIView *)view {
    CGRect rect = [view bounds];  
 //   CGRect rect = CGRectMake(20, 30, 100, 100);  
    UIGraphicsBeginImageContext(rect.size);  
    CGContextRef context = UIGraphicsGetCurrentContext();  
    [view.layer renderInContext:context];  
    UIImage *imageCaptureRect;

    imageCaptureRect = UIGraphicsGetImageFromCurrentImageContext();  
    _imageCapture = imageCaptureRect;
//    _imageCapture = UIGraphicsGetImageFromCurrentImageContext();  
//    [_imageCapture retain];
    UIGraphicsEndImageContext();   
}


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
    // Drawing code.
    CGPoint accPoint = CGPointMake(0,0);
    [_imageCapture drawAtPoint:accPoint];
}


- (void)dealloc {
    [_imageCapture release];
    [super dealloc];
}

@end

so just initiate it passing your UILabel to it, so you'll get a Class with the UIImage *imageCapture property, so just get it and use it...

// in your class which wanna manage the captured image:
// in .h : #import "CaptureView.h"
CaptureView *cloneView = [[CaptureView alloc] initWithView:aUILabel];
//  [yourUIView addSubview:cloneView]; // eventually, to see it...
UIImage *myCapt = cloneView.imageCapture;
// ... your code to use myCapt
[cloneView release];

EDIT:

download this project:

http://meronix.altervista.org/captureUILabel.zip

the centered black-backGround text is a capture (an instance of the class: CaptureView) of the Gray-BackGround text (a UILabel) on top

Answer 2

This should work:

UIGraphicsBeginImageContext(yourLabel.bounds.size);
[yourLabel.layer renderInContext:UIGraphicsGetCurrentContext()];
CGImageRef viewImage = [UIGraphicsGetImageFromCurrentImageContext() CGImage];
UIGraphicsEndImageContext();

UIGraphicsGetImageFromCurrentImageContext() returns an instance of UIImage.