Question

Based on the responses to a previous question, I ve created a category on UIImageView for extracting pixel data. This works fine in the simulator, but not when deployed to the device. I should say not always -- the odd thing is that it does fetch the correct pixel colour if point.x == point.y; otherwise, it gives me pixel data for a pixel on the other side of that line, as if mirrored. (So a tap on a pixel in the lower-right corner of the image gives me the pixel data for a corresponding pixel in the upper-left, but tapping on a pixel in the lower-left corner returns the correct pixel colour). The touch coordinates (CGPoint) are correct.

What am I doing wrong?

Here s my code:

@interface UIImageView (PixelColor)
- (UIColor*)getRGBPixelColorAtPoint:(CGPoint)point;
@end

@implementation UIImageView (PixelColor)

- (UIColor*)getRGBPixelColorAtPoint:(CGPoint)point
{
    UIColor* color = nil;

    CGImageRef cgImage = [self.image CGImage];
    size_t width = CGImageGetWidth(cgImage);
    size_t height = CGImageGetHeight(cgImage);
    NSUInteger x = (NSUInteger)floor(point.x);
    NSUInteger y = height - (NSUInteger)floor(point.y);

    if ((x < width) && (y < height))
    {
        CGDataProviderRef provider = CGImageGetDataProvider(cgImage);
        CFDataRef bitmapData = CGDataProviderCopyData(provider);
        const UInt8* data = CFDataGetBytePtr(bitmapData);
        size_t offset = ((width * y) + x) * 4;
        UInt8 red = data[offset];
        UInt8 blue = data[offset+1];
        UInt8 green = data[offset+2];
        UInt8 alpha = data[offset+3];
        CFRelease(bitmapData);
        color = [UIColor colorWithRed:red/255.0f green:green/255.0f blue:blue/255.0f alpha:alpha/255.0f];
    }

    return color;
}

Answer 1

I think R B G is wrong. You have:

UInt8 red =   data[offset];     
UInt8 blue =  data[offset+1];
UInt8 green = data[offset+2];

But don t you really mean R G B? :

UInt8 red =   data[offset];     
UInt8 green = data[offset+1];
UInt8 blue =  data[offset+2];

But even with that fixed there s still a problem as it turns out Apple byte swaps (great article) the R and B values when on the device, but not when on the simulator.

I had a similar simulator/device issue with a PNG s pixel buffer returned by CFDataGetBytePtr.

This resolved the issue for me:

#if TARGET_IPHONE_SIMULATOR
        UInt8 red =   data[offset];
        UInt8 green = data[offset + 1];
        UInt8 blue =  data[offset + 2];
#else
        //on device
        UInt8 blue =  data[offset];       //notice red and blue are swapped
        UInt8 green = data[offset + 1];
        UInt8 red =   data[offset + 2];
#endif

Not sure if this will fix your issue, but your misbehaving code looks close to what mine looked like before I fixed it.

One last thing: I believe the simulator will let you access your pixel buffer data[] even after CFRelease(bitmapData) is called. On the device this is not the case in my experience. Your code shouldn t be affected, but in case this helps someone else I thought I d mention it.

Answer 2

You could try the following alternative approach:

create a CGBitmapContext
draw the image into the context
call CGBitmapContextGetData on the context to get the underlying data
work out your offset into the raw data (based on how you created the bitmap context)
extract the value

This approach works for me on the simulator and device.

Answer 3

It looks like that in the code posted in the original questions instead of:

NSUInteger x = (NSUInteger)floor(point.x);
NSUInteger y = height - (NSUInteger)floor(point.y);

It should be:

NSUInteger x = (NSUInteger)floor(point.x);
NSUInteger y = (NSUInteger)floor(point.y);

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