CFDataRef and Pixel Data for iPhone Programming

Question

I am trying to access the raw pixels of an image, but I keep running into a problem that got me really stuck. I am pretty new to iPhone programming so it might be something really simple...

Here is the class I have, it uses a CGImageRef from an image and looks at the individual pixels. Here is my code

The header file:

#import <QuartzCore/QuartzCore.h>
#import <QuartzCore/QuartzCore.h>
#import <CoreImage/CoreImage.h>


@interface GraphingView : UIControl 
{
@private
    CAShapeLayer *shapeLayer;    
    CGImageRef pixelData;
}

@property(assign) CGImageRef pixelData;
@end

And my implementation file:

#import "GraphingView.h"
#import "iSpectrumViewController.h"

@implementation GraphingView

@synthesize pixelData;
- (void)awakeFromNib
{
    // set up a rounded border
    CALayer *layer = [self layer];

    // clear the view's background color so that our background
    // fits within the rounded border
    CGColorRef backgroundColor = [self.backgroundColor CGColor];
    self.backgroundColor = [UIColor clearColor];
    layer.backgroundColor = backgroundColor;


    shapeLayer = [CAShapeLayer layer];      

    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{

}

- (void)updateGraph
{
    // This is where my problem is:
     // Whenever the next line is executed the application stops working. It works fine in the setPixelData
     // down below.
    CFDataRef pixelRef = CGDataProviderCopyData(CGImageGetDataProvider(pixelData));
    CFIndex byteSize = CFDataGetLength(pixelRef);
    int intByteSize = (int) byteSize;
    NSLog(@"Number of Bytes contained %i",intByteSize);

     //Then get a pointer to the data so the information can be accessed
    const UInt8 *pixelPtr = CFDataGetBytePtr(pixelRef);


    // Do some drawing here using the data obtained from the image...

    [shapeLayer setPath:path];
    [shapeLayer setNeedsDisplay];
    CFRelease(path);    
}

-(void)setPixelData:(CGImageRef)pixelImage
{
     //This code takes the CGImage from my image and copies it to the pixelData CGImageRef defined in the header file


    //This gets the CFDataRef from the CGImage so the pixel information is available
    CFDataRef PixelCopy = CGDataProviderCopyData(CGImageGetDataProvider(pixelImage));
    CFIndex byteSize = CFDataGetLength(PixelCopy);
    int intByteSize = (int) byteSize;
    NSLog(@"Number of Bytes contained %i",intByteSize);

    pixelData = CGImageCreateCopy(pixelImage);
    [self updateGraph];

}
@end

The problem I have is:

The function setPixelData gets the CGImageRef image and copies it to pixelData. Then, up in the (void) updateGraph method, I use the CGImageRef pixelData and create a CFDataRef to get the raw pixel data. However, there is something wrong. Because when I use the CFDataGetLength method using the CFDataRef pixelRef as input I get the EXC_BAD_ACCESS error when the program is run. What confuses me is that when I then put the same two lines of code in the (void) setPixelData function (shown in the code above) and comment out the code in updateGraph then the program runs perfectly fine.

Any ideas of what could be causing this error, and how to eliminate it?

Thanks,

Sam

Answer 1

Ah... I found it,

The reason it didn't work was because I had two setter methods (one of them is not shown in the code). That one always ran before the setPixelData function, so PixelData CGImageRef was never set, therefore, CFDataRef did not exist, giving the memory error.