How to iterate through all pixels of an UIImage? - cocoa

Question

Hey Guys i am currently trying to iterate through all pixels of an UIImage but the way i implemented it it takes sooo much time. So i thought it is the wrong way i implemented it.
This is my method how i get the RGBA Values of an Pixel :
+(NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy count:(int)count
{
// Initializing the result array
NSMutableArray *result = [NSMutableArray arrayWithCapacity:count];
// First get the image into your data buffer
CGImageRef imageRef = [image CGImage]; // creating an Instance of
NSUInteger width = CGImageGetWidth(imageRef); // Get width of our Image
NSUInteger height = CGImageGetHeight(imageRef); // Get height of our Image
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); // creating our colour Space
// Getting that raw Data out of an image
unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
NSUInteger bytesPerPixel = 4; // Bytes per pixel defined
NSUInteger bytesPerRow = bytesPerPixel * width; // Bytes per row
NSUInteger bitsPerComponent = 8; // Bytes per component
CGContextRef context = CGBitmapContextCreate(rawData, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace); // releasing the color space
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);
// Now your rawData contains the image data in the RGBA8888 pixel format.
int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
for (int ii = 0 ; ii < count ; ++ii)
{
CGFloat red = (rawData[byteIndex] * 1.0) / 255.0;
CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
CGFloat blue = (rawData[byteIndex + 2] * 1.0) / 255.0;
CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
byteIndex += 4;
UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
[result addObject:acolor];
}
free(rawData);
return result;
}
And this is the code how i parse through all the pixels :
for (NSUInteger y = 0 ; y < self.originalPictureWidth; y++) {
for (NSUInteger x = 0 ; x < self.originalPictureHeight; x++) {
NSArray * originalRGBA = [ComputerVisionHelperClass getRGBAsFromImage:self.originalPicture atX:(int)x andY:(int)y count:1];
NSArray * referenceRGBA = [ComputerVisionHelperClass getRGBAsFromImage:self.referencePicture atX:(int)referenceIndexX andY:(int)referenceIndexY count:1];
// Do something else ....
}
}
Is there a faster way of getting all RGBA values of an uiimage instance ?

Answer 1

For every pixel, you're generating a new copy of the image and then throwing it away. Yes, it would be much faster by just getting the data once and then processing on that byte array.
But it heavily depends on what is in "Do something else." There are many CoreImage and vImage functions that can do image processing very quickly, but you may need to approach the problem differently. It depends on what you're doing.