Median Filter for ios - ios

Question

I'm looking to apply Median Filter to a UIImage in my iOS application.
Due to my company restrictions, i cannot use openGL filters.
Any ideas or current implementations would be very welcomed.
Thanks.

Answer 1

Apple's Core Image framework may be your solution. To be precise, you need a subclass of CIFilter which implements a median filter. (Guess you would be interested in CIMedianFilter or have a look at the filter reference)
CIIImage *inputImage = //...
CIFilter *filter = [CIFilter filterWithName:#"CIMedianFilter"];
[filter setDefaults];
[filter setValue:inputImage forKey:#"inputImage"];
CIImage *outputImage = [filter outputImage];
To convert the CIImage to UIImage and vice versa:
CIImage *ciImage = [UIImage imageNamed:#"test.png"].CIImage;
UIImage *uiImage = [[UIImage alloc] initWithCIImage:ciImage];

Answer 2

Look at the github project:
https://github.com/BradLarson/GPUImage
These are a lot of CIFilter-type filters that are GPU accelerated.
Another alternate: Use a Convolution Filter and roll your own median filter using CI's kernel language.

Answer 3

For those of you who can use OpenGL ES in your iOS app, this is how you calculate the median in a pixel neighborhood radius of your choosing:
kernel vec4 medianUnsharpKernel(sampler u) {
vec4 pixel = unpremultiply(sample(u, samplerCoord(u)));
vec2 xy = destCoord();
int radius = 3;
int bounds = (radius - 1) / 2;
vec4 sum = vec4(0.0);
for (int i = (0 - bounds); i <= bounds; i++)
{
for (int j = (0 - bounds); j <= bounds; j++ )
{
sum += unpremultiply(sample(u, samplerTransform(u, vec2(xy + vec2(i, j)))));
}
}
vec4 mean = vec4(sum / vec4(pow(float(radius), 2.0)));
float mean_avg = float(mean);
float comp_avg = 0.0;
vec4 comp = vec4(0.0);
vec4 median = mean;
for (int i = (0 - bounds); i <= bounds; i++)
{
for (int j = (0 - bounds); j <= bounds; j++ )
{
comp = unpremultiply(sample(u, samplerTransform(u, vec2(xy + vec2(i, j)))));
comp_avg = float(comp);
median = (comp_avg < mean_avg) ? max(median, comp) : median;
}
}
return premultiply(vec4(vec3(abs(pixel.rgb - median.rgb)), 1.0));
}
A brief description of the steps
1. Calculate the mean of the values of the pixels surrounding the source pixel in a 3x3 neighborhood;
2. Find the maximum pixel value of all pixels in the same neighborhood that are less than the mean.
3. [OPTIONAL] Subtract the median pixel value from the source pixel value for edge detection.
If you're using the median value for edge detection, there are a couple of ways to modify the above code for better results, namely, hybrid median filtering and truncated media filtering (a substitute and a better 'mode' filtering). If you're interested, please ask.

Answer 4

If someone would like to do it in Xamarin (for iOS), here is the code:
public static UIImage MedianFilter(UIImage image)
{
CIImage ciImage = new CIImage(image);
var medianFilter = new CIMedianFilter() { Image = ciImage };
CIImage output = medianFilter.OutputImage;
var context = CIContext.FromOptions(null);
var cgimage = context.CreateCGImage(output, output.Extent);
var uiImage = UIImage.FromImage(cgimage);
return uiImage;
}