iOS grayscale/black & white - iphone

Question

I am having an issue with some code that changes a UIImage to grayscale. It works right on iPhone/iPod, but on iPad whatever is already drawn gets all stretched and skewed in the process.
It also sometimes crashes only on iPad on the line
imageRef = CGBitmapContextCreateImage (ctx);
Here is the code:
CGContextRef ctx;
CGImageRef imageRef = [self.drawImage.image CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = malloc(height * width * 4);
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);
int byteIndex = 0;
int grayScale;
for(int ii = 0 ; ii < width * height ; ++ii)
{
grayScale = (rawData[byteIndex] + rawData[byteIndex + 1] + rawData[byteIndex + 2]) / 3;
rawData[byteIndex] = (char)grayScale;
rawData[byteIndex+1] = (char)grayScale;
rawData[byteIndex+2] = (char)grayScale;
//rawData[byteIndex+3] = 255;
byteIndex += 4;
}
ctx = CGBitmapContextCreate(rawData,
CGImageGetWidth( imageRef ),
CGImageGetHeight( imageRef ),
8,
CGImageGetBytesPerRow( imageRef ),
CGImageGetColorSpace( imageRef ),
kCGImageAlphaPremultipliedLast );
imageRef = CGBitmapContextCreateImage (ctx);
UIImage* rawImage = [UIImage imageWithCGImage:imageRef];
CGContextRelease(ctx);
self.drawImage.image = rawImage;
free(rawData);

Answer 1

Consider leveraging the CIColorMonochrome filter available in iOS 6.
void ToMonochrome()
{
var mono = new CIColorMonochrome();
mono.Color = CIColor.FromRgb(1, 1, 1);
mono.Intensity = 1.0f;
var uiImage = new UIImage("Images/photo.jpg");
mono.Image = CIImage.FromCGImage(uiImage.CGImage);
DisplayFilterOutput(mono, ImageView);
}
static void DisplayFilterOutput(CIFilter filter, UIImageView imageView)
{
CIImage output = filter.OutputImage;
if (output == null)
return;
var context = CIContext.FromOptions(null);
var renderedImage = context.CreateCGImage(output, output.Extent);
var finalImage = new UIImage(renderedImage);
imageView.Image = finalImage;
}

Answer 2

Figured it out with some help elsewhere
Got rid of an extra context being used and changed the bitmap format
CGContextRef ctx;
CGImageRef imageRef = [self.drawImage.image CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = malloc(height * width * 4);
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
ctx = CGBitmapContextCreate(rawData,
width,
height,
bitsPerComponent,
bytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Host);
CGContextDrawImage(ctx, CGRectMake(0, 0, width, height), imageRef);
int byteIndex = 0;
int grayScale;
for(int ii = 0 ; ii < width * height ; ++ii)
{
grayScale = (rawData[byteIndex] + rawData[byteIndex + 1] + rawData[byteIndex + 2]) / 3;
rawData[byteIndex] = (char)grayScale;
rawData[byteIndex+1] = (char)grayScale;
rawData[byteIndex+2] = (char)grayScale;
//rawData[byteIndex+3] = 255;
byteIndex += 4;
}
imageRef = CGBitmapContextCreateImage(ctx);
UIImage* rawImage = [UIImage imageWithCGImage:imageRef];
CGColorSpaceRelease(colorSpace);
CGContextRelease(ctx);
self.drawImage.image = rawImage;
free(rawData);