Get Pixel color of UIImage

OnTouch

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [[touches allObjects] objectAtIndex:0];
    CGPoint point1 = [touch locationInView:self.view];
    touch = [[event allTouches] anyObject]; 
    if ([touch view] == imgZoneWheel)
    {
        CGPoint location = [touch locationInView:imgZoneWheel];
        [self getPixelColorAtLocation:location];
        if(alpha==255)
        {
            NSLog(@"In Image Touch view alpha %d",alpha);
            [self translateCurrentTouchPoint:point1.x :point1.y];
            [imgZoneWheel setImage:[UIImage imageNamed:[NSString stringWithFormat:@"blue%d.png",GrndFild]]];
        }
    }
}



- (UIColor*) getPixelColorAtLocation:(CGPoint)point 
{

    UIColor* color = nil;

    CGImageRef inImage;

    inImage = imgZoneWheel.image.CGImage;


    // Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
    CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
    if (cgctx == NULL) { return nil; /* error */ }

    size_t w = CGImageGetWidth(inImage);
    size_t h = CGImageGetHeight(inImage);
    CGRect rect = {{0,0},{w,h}};


    // Draw the image to the bitmap context. Once we draw, the memory 
    // allocated for the context for rendering will then contain the 
    // raw image data in the specified color space.
    CGContextDrawImage(cgctx, rect, inImage); 

    // Now we can get a pointer to the image data associated with the bitmap
    // context.
    unsigned char* data = CGBitmapContextGetData (cgctx);
    if (data != NULL) {
        //offset locates the pixel in the data from x,y. 
        //4 for 4 bytes of data per pixel, w is width of one row of data.
        int offset = 4*((w*round(point.y))+round(point.x));
        alpha =  data[offset]; 
        int red = data[offset+1]; 
        int green = data[offset+2]; 
        int blue = data[offset+3]; 
        color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
    }

    // When finished, release the context
    //CGContextRelease(cgctx); 
    // Free image data memory for the context
    if (data) { free(data); }

    return color;
}

- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef)inImage 
{
    CGContextRef    context = NULL;
    CGColorSpaceRef colorSpace;
    void *          bitmapData;
    int             bitmapByteCount;
    int             bitmapBytesPerRow;

    // Get image width, height. We'll use the entire image.
    size_t pixelsWide = CGImageGetWidth(inImage);
    size_t pixelsHigh = CGImageGetHeight(inImage);

    // Declare the number of bytes per row. Each pixel in the bitmap in this
    // example is represented by 4 bytes; 8 bits each of red, green, blue, and
    // alpha.
    bitmapBytesPerRow   = (pixelsWide * 4);
    bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

    // Use the generic RGB color space.
    colorSpace = CGColorSpaceCreateDeviceRGB();

    if (colorSpace == NULL)
    {
        fprintf(stderr, "Error allocating color space\n");
        return NULL;
    }

    // Allocate memory for image data. This is the destination in memory
    // where any drawing to the bitmap context will be rendered.
    bitmapData = malloc( bitmapByteCount );
    if (bitmapData == NULL) 
    {
        fprintf (stderr, "Memory not allocated!");
        CGColorSpaceRelease( colorSpace );
        return NULL;
    }

    // Create the bitmap context. We want pre-multiplied ARGB, 8-bits 
    // per component. Regardless of what the source image format is 
    // (CMYK, Grayscale, and so on) it will be converted over to the format
    // specified here by CGBitmapContextCreate.
    context = CGBitmapContextCreate (bitmapData,
                                     pixelsWide,
                                     pixelsHigh,
                                     8,      // bits per component
                                     bitmapBytesPerRow,
                                     colorSpace,
                                     kCGImageAlphaPremultipliedFirst);
    if (context == NULL)
    {
        free (bitmapData);
        fprintf (stderr, "Context not created!");
    }

    // Make sure and release colorspace before returning
    CGColorSpaceRelease( colorSpace );

    return context;
}

Try this very simple code:

I used to detect a wall in my maze game (the only info that I need is the alpha channel, but I included the code to get the other colors for you):

- (BOOL)isWallPixel:(UIImage *)image xCoordinate:(int)x yCoordinate:(int)y {

    CFDataRef pixelData = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
    const UInt8* data = CFDataGetBytePtr(pixelData);

    int pixelInfo = ((image.size.width  * y) + x ) * 4; // The image is png

    //UInt8 red = data[pixelInfo];         // If you need this info, enable it
    //UInt8 green = data[(pixelInfo + 1)]; // If you need this info, enable it
    //UInt8 blue = data[pixelInfo + 2];    // If you need this info, enable it
    UInt8 alpha = data[pixelInfo + 3];     // I need only this info for my maze game
    CFRelease(pixelData);

    //UIColor* color = [UIColor colorWithRed:red/255.0f green:green/255.0f blue:blue/255.0f alpha:alpha/255.0f]; // The pixel color info

    if (alpha) return YES;
    else return NO;

}

Some Swift code based on Minas' answer. Originally I had some code to figure out the pixel stride, but I've updated the answer to use the ComponentLayout from Desmond's answer. I've also moved the extension to CGImage.

Swift 5:

public extension UIImage {
    func getPixelColor(_ point: CGPoint) -> UIColor {
        guard let cgImage = self.cgImage else {
            return UIColor.clear
        }
        return cgImage.getPixelColor(point)
    }
}
public extension CGBitmapInfo {
    // See https://stackoverflow.com/a/60247648/1765629
    // I've extended it to include .a
    enum ComponentLayout {

        case a
        case bgra
        case abgr
        case argb
        case rgba
        case bgr
        case rgb

        var count: Int {
            switch self {
            case .a: return 1
            case .bgr, .rgb: return 3
            default: return 4
            }
        }
    }

    var isAlphaPremultiplied: Bool {
        let alphaInfo = CGImageAlphaInfo(rawValue: rawValue & Self.alphaInfoMask.rawValue)
        return alphaInfo == .premultipliedFirst || alphaInfo == .premultipliedLast
    }

    // [...] skipping the rest
}

public extension CGImage {

    func getPixelColor(_ point: CGPoint) -> UIColor {
        guard let pixelData = self.dataProvider?.data, let layout = bitmapInfo.componentLayout, let data = CFDataGetBytePtr(pixelData) else {
            return .clear
        }
        let x = Int(point.x)
        let y = Int(point.y)
        let w = self.width
        let h = self.height
        let index = w * y + x
        let numBytes = CFDataGetLength(pixelData)
        let numComponents = layout.count
        if numBytes != w * h * numComponents {
            NSLog("Unexpected size: \(numBytes) != \(w)x\(h)x\(numComponents)")
            return .clear
        }
        let isAlphaPremultiplied = bitmapInfo.isAlphaPremultiplied
        switch numComponents {
        case 1:
            return UIColor(red: 0, green: 0, blue: 0, alpha: CGFloat(data[index])/255.0)
        case 3:
            let c0 = CGFloat((data[3*index])) / 255
            let c1 = CGFloat((data[3*index+1])) / 255
            let c2 = CGFloat((data[3*index+2])) / 255
            if layout == .bgr {
                return UIColor(red: c2, green: c1, blue: c0, alpha: 1.0)
            }
            return UIColor(red: c0, green: c1, blue: c2, alpha: 1.0)
        case 4:
            let c0 = CGFloat((data[4*index])) / 255
            let c1 = CGFloat((data[4*index+1])) / 255
            let c2 = CGFloat((data[4*index+2])) / 255
            let c3 = CGFloat((data[4*index+3])) / 255
            var r: CGFloat = 0
            var g: CGFloat = 0
            var b: CGFloat = 0
            var a: CGFloat = 0
            switch layout {
            case .abgr:
                a = c0; b = c1; g = c2; r = c3
            case .argb:
                a = c0; r = c1; g = c2; b = c3
            case .bgra:
                b = c0; g = c1; r = c2; a = c3
            case .rgba:
                r = c0; g = c1; b = c2; a = c3
            default:
                break
            }
            if isAlphaPremultiplied && a > 0 {
                r = r / a
                g = g / a
                b = b / a
            }
            return UIColor(red: r, green: g, blue: b, alpha: a)
        default:
            return .clear
        }
    }

I was trying to refactor it to use ranges, but this doesn't seem to work,

    let start = numComponents * index
    let end = numComponents * (index + 1)
    let c = data[start ..< end] // expects Int, not a Range...