How do I perform a fast pixellation filter on an image?

Question

I have a little problem with my pixellation image processing algorithm.

I load the image from the beginning into an array of type unsigned char* After t

Answer 1

How about the use the Core Image filter named CIPixellate? Here is a code snippet of how i implemented it. You can play with kCIInputScaleKey to get the intensity you want:

// initialize context and image
CIContext *context = [CIContext contextWithOptions:nil];
CIImage *logo = [CIImage imageWithData:UIImagePNGRepresentation([UIImage imageNamed:@"test"])];

// set filter and properties
CIFilter *filter = [CIFilter filterWithName:@"CIPixellate"];
[filter setValue:logo forKey:kCIInputImageKey];
[filter setValue:[[CIVector alloc] initWithX:150 Y:150] forKey:kCIInputCenterKey]; // default: 150, 150
[filter setValue:[NSNumber numberWithDouble:100.0] forKey:kCIInputScaleKey]; // default: 8.0

// render image
CIImage *result = (CIImage *) [filter valueForKey:kCIOutputImageKey];
CGRect extent = result.extent;
CGImageRef cgImage = [context createCGImage:result fromRect:extent];

// result
UIImage *image = [[UIImage alloc] initWithCGImage:cgImage];

Here is the official Apple Filter Tutorial and a List of available Filters.

Update #1

I just wrote a method to execute the rendering work in background:

- (void) pixelateImage:(UIImage *) image withIntensity:(NSNumber *) intensity completionHander:(void (^)(UIImage *pixelatedImage)) handler {

    // async task
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        // initialize context and image
        CIContext *context = [CIContext contextWithOptions:nil];
        CIImage *logo = [CIImage imageWithData:UIImagePNGRepresentation(image)];

        // set filter and properties
        CIFilter *filter = [CIFilter filterWithName:@"CIPixellate"];
        [filter setValue:logo forKey:kCIInputImageKey];
        [filter setValue:[[CIVector alloc] initWithX:150 Y:150] forKey:kCIInputCenterKey]; // default: 150, 150
        [filter setValue:intensity forKey:kCIInputScaleKey]; // default: 8.0

        // render image
        CIImage *result = (CIImage *) [filter valueForKey:kCIOutputImageKey];
        CGRect extent = result.extent;
        CGImageRef cgImage = [context createCGImage:result fromRect:extent];

        // result
        UIImage *image = [[UIImage alloc] initWithCGImage:cgImage];

        // dispatch to main thread
        dispatch_async(dispatch_get_main_queue(), ^{
            handler(image);
        });
    });
}

Call it like this:

[self pixelateImage:[UIImage imageNamed:@"test"] withIntensity:[NSNumber numberWithDouble:100.0] completionHander:^(UIImage *pixelatedImage) {
    self.logoImageView.image = pixelatedImage;
}];