2011

Masking Tab
The Lens Blur tool has a masking input and a Masking tab—see Pixel Masking
on page 560.
Median Tool
The Median tool is an edge-preserving smoothing filter that works particularly
well for removing impulse noise.
For grayscale images, the median filter works by ranking the pixels under the
kernel according to their value and selecting the median to replace the pixel
at the center of the kernel. This approach effectively removes spikes in the
original image without the blurring typically introduced by common
smoothing kernels (e.g. Gaussian).
For color images, there is no single concept of ranking, so several different
criteria are supported. Component-wise ranking computes the median of each
color component independently, however, this can result in pixel colors that
did not belong in the original image. Luminance ranking computes the median
of the luminance of each pixel under the kernel. This approach is fast and
does not introduce new colors in the original image, however, luminance is
not the best criteria for similarity in a color image. RGB Vector does not really
rank the pixels under the kernel, but rather it chooses as the median the one
pixel with the smallest sum of square distances (in RGB space) to all the other
ones under the kernel. It selects the pixel that is closest to the center of the
point cloud obtained by looking at the pixels under the kernel as points in
3D space. This approach is computationally intensive, but can yield better
results than either of the other ranking criteria.
The median filter uses a square neighborhood and can round the corners of
axes-aligned rectangular objects in an image.
As any noise reduction filter, the median filter may also affect the sharpness
of small details in the input image. When this problem arises, the result of
the median filter can be blended with the original image to decrease its effect.
406 | Chapter 17 Image Processing Tools