How to get the image pixels in ios

Question

I have an color image. I want return pixel value on touch of image. So, I tried below code :

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

float x, y;

for (UITouch *touch in touches) {        
    x = [touch locationInView:imageView].x;
    y = [touch locationInView:imageView].y;
    if ((x >= 0) && (x <= imageView.frame.size.width) && (y >= 0) && (y <= imageView.frame.size.height)) {
        [self getRGBAFromImage:imageView.image atX:x andY:y];
    }
  }
}

- (void)getRGBAFromImage:(UIImage *)image atX:(int)xx andY:(int)yy {

CGImageRef imageRef = [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 = (bytesPerRow * yy) + xx * bytesPerPixel;


CGFloat red   = (rawData[byteIndex]     * 1.0) ;
CGFloat green = (rawData[byteIndex + 1] * 1.0) ;
CGFloat blue  = (rawData[byteIndex + 2] * 1.0) ;
CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;

self.textView.textColor=[UIColor colorWithRed:red/255.0 green:green/255.0 blue:blue/255.0 alpha:alpha];
free(rawData);
}

This will return the pixels but not the correct one !! For EXAMPLE : Touched color in ImageView is red , but my textView text is not red color.

Please help me confirm my code.

Thanks!

Answer 1

Here is the category for UIImage that I use when I need to get the colour of a specific pixel on a image:

@implementation UIImage (ColorImage)

/*
 Returns the color of the image pixel at point. Returns nil if point lies outside of the image bounds.
 If the point coordinates contains a decimal part, it will be truncated (trunc).

 This method retreives the pixel data of the image by drawing the specific pixel onto a bitmap.
 To query pixel colors of the same image many times, it is best to cache the drawn image for better performance.
 */
- (UIColor *)colorAtPixel:(CGPoint)point {
    // Cancel if the point is outside of the image coordinates
    if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, self.size.width, self.size.height), point)) {
        return nil;
    }

    // Create a 1x1 pixel byte array and bitmap context to draw the pixel onto.
    // Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage
    NSInteger pointX = trunc(point.x);
    NSInteger pointY = trunc(point.y);

    CGImageRef cgImage = self.CGImage;

    NSUInteger width = CGImageGetWidth(cgImage);
    NSUInteger height = CGImageGetHeight(cgImage);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    int bytesPerPixel = 4;
    int bytesPerRow = bytesPerPixel * 1;
    NSUInteger bitsPerComponent = 8;

    unsigned char pixelData[4] = { 0, 0, 0, 0 };

    CGContextRef context = CGBitmapContextCreate( pixelData, 1, 1, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

    CGColorSpaceRelease(colorSpace);

    CGContextSetBlendMode(context, kCGBlendModeCopy);

    // Draw the pixel we are interested in
    CGContextTranslateCTM(context, -pointX, -pointY);
    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
    CGContextRelease(context);

    // Convert the color values [0...255] to floats [0.0f...1.0f]
    CGFloat r = (CGFloat)pixelData[0] / 255.0f;
    CGFloat g = (CGFloat)pixelData[1] / 255.0f;
    CGFloat b = (CGFloat)pixelData[2] / 255.0f;
    CGFloat a = (CGFloat)pixelData[3] / 255.0f;

    return [UIColor colorWithRed:r green:g blue:b alpha:a];
}
@end

You can call -colorAtPixel directly on a UIImage and it will return the color as a UIColor.