High level image pixels enumeration:

Create UIImage on the fly:

UIImage* patternImage = [UIImage FX_imageWitSize:CGSizeMake(100, 100)
                                       generator:^RGBA(size_t x, size_t y){
                             if( (x%20 < 10) ^ (y%20 < 10) )
                                 return FX_RGBA_Make(200, 200, 200, 255);
                             else
                                 return FX_RGBA_Make(255, 255, 255, 255);
                         }];

Output image:

You can easy enumerate all pixels in the UIImage:

double brightnessSum = 0;
for (UIColor* color in uiImage){
    brightnessSum += color.FX_hsba.component.brightness;
}
double averageImageBrightness = brightnessSum / (uiImage.size.width*uiImage.size.height);

Or if you need X, Y coordinates of each pixel :

CGPoint somePoint = CGPointMake(20, 20);
[uiImage FX_enumerateAllPixelsRGBA:^(RGBA rgba, FXPoint point, BOOL *stop) {
    double manhattanDistance = ABS(somePoint.x - point.x) + ABS(somePoint.y - point.y);
    // do something with distance for point
}];

If you need change pixels values:

UIImage* redDecreasedImage = [uiImage FX_imageByMutatePixelsRGBA:^(RGBA rgba, FXPoint point, BOOL *update, BOOL *stop){
                                  if ((point.x % 10 < 5) ^ (point.y%10 < 5)){
                                      rgba.component.r -= 100;
                                  }
                                  return rgba;
                              }];

Output image:

Install

pod 'FXColorSpace'