Mitsubishi P91DW User Manual download pdf (Page 91)

Alpha Innotech User’s Manual, version 4

4-16

General Information on How Filters Work

To enhance an image, filters change the value assigned to each pixel. The new value

assigned to a pixel is determined based on the values of the other pixels in its local

vicinity (or “neighborhood”). The neighborhood is a two-dimensional matrix of pixel

values, where each dimension has an odd number of elements. The "pixel of interest" is

the one at the center of the neighborhood. This is the pixel whose old value is being

replaced with a new one as the result of the filtering algorithm.

The pixels in a neighborhood provide information about the brightness trend. This

information is important to the filtering process. The brightness trend is also referred to

as the "spatial frequency." Images with high spatial frequency content contain large,

closely spaced changes in pixel values. For example, on a black and white checkerboard,

the smaller the squares, the higher the frequency content.

Images with low spatial frequency content (for example, images of clouds) contain large

areas of slowly changing pixel values.

Most of the filter options available (with the exception of the Noise filters) use a

weighted summation process to determine the value assigned to the pixel of interest. Each

pixel in a 3x3 neighborhood is multiplied by a "convolution kernel" having the same

dimensions. The resulting sum is assigned to the pixel of interest.

(K1xP1)+

(K2xP2)+

(K3xP3)+

(K4xP4)+

P1 P2 P3 K1 K2 K3 (K5xP5)+

P4 P5 P6 X K4 K5 K6 (K6xP6)+

P7 P8 P9 K7 K8 K9 (K7xP7)+

(K8xP8)+

3x3 pixel neighborhoodconvolution kernel (K9xP9)

(P5 is being calculated) New Value for P5

Each element of the convolution kernel is a weighting factor, also called a "convolution

coefficient." The size and arrangement of these weighting factors determine the type of

transformation the image will undergo. Changing a weighting factor influences the

overall sum and, therefore, affects the value given to the pixel of interest.

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Mitsubishi P91DW User Manual Page 91