Median Filter
Perform 2-D median filtering
Library
Filtering and Analysis & Enhancement
visionanalysis
visionfilter
Description
The Median Filter block replaces the central value of an M-by-N neighborhood with its median value. If the neighborhood has a center element, the block places the median value there, as illustrated in the following figure.
The block has a bias toward the upper-left corner when the neighborhood does not have an exact center. See the median value placement in the following figure.
The block pads the edge of the input image, which sometimes causes the pixels within [M/2 N/2] of the edges to appear distorted. The median value is less sensitive than the mean to extreme values. As a result, the Median Filter block can remove salt-and-pepper noise from an image without significantly reducing the sharpness of the image.
| Port | Input/Output | Supported Data Types | Complex Values Supported |
|---|---|---|---|
I | Matrix of intensity values |
| No |
Val | Scalar value that represents the constant pad value | Same as I port | No |
Output | Matrix of intensity values | Same as I port | No |
If the data type of the input signal is floating point, the output has the same data type. The data types of the signals input to the I and Val ports must be the same.
Fixed-Point Data Types
The information in this section is applicable only when the dimensions of the neighborhood are even.
For fixed-point inputs, you can specify accumulator and output data types as discussed in Parameters. Not all these fixed-point parameters apply to all types of fixed-point inputs. The following table shows the output and accumulator data type used for each fixed-point input.
Fixed-Point Input | Output Data Type | Accumulator Data Type |
|---|---|---|
Even | X | X |
Odd | X |
|
Odd | X | X |
Even | X | X |
When M is even, fixed-point signals use the
accumulator and output data types. The accumulator data type store
the result of the sum performed while calculating the average of the
two central rows of the input matrix. The output data type stores
the total result of the average.
Complex fixed-point inputs use the accumulator parameters. The calculation for the sum of the squares of the real and imaginary parts of the input occur, before sorting input elements. The accumulator data type stores the result of the sum of the squares.
Parameters
- Neighborhood size
Specify the size of the neighborhood over which the block computes the median.
Enter a scalar value that represents the number of rows and columns in a square matrix.
Enter a vector that represents the number of rows and columns in a rectangular matrix.
- Output size
This parameter controls the size of the output matrix.
If you choose
Same as input port I, the output has the same dimensions as the input to portI. The Padding options parameter appears in the dialog box. Use the Padding options parameter to specify how to pad the boundary of your input matrix.If you select
Valid, the block only computes the median where the neighborhood fits entirely within the input image, with no need for padding. The dimensions of the output image are,output rows = input rows - neighborhood rows + 1,
and
output columns = input columns - neighborhood columns + 1.
- Padding options
Specify how to pad the boundary of your input matrix.
Select
Constantto pad your matrix with a constant value. The Pad value source parameter appears in the dialog boxSelect
Replicateto pad your input matrix by repeating its border values.Select
Symmetricto pad your input matrix with its mirror image.Select
Circularto pad your input matrix using a circular repetition of its elements. This parameter appears if, for the Output size parameter, you selectSame as input port I.
For more information on padding, see the Image Pad block reference page.
- Pad value source
Use this parameter to specify how to define your constant boundary value.
Select
Specify via dialogto enter your value in the block parameters dialog box. The Pad value parameter appears in the dialog box.Select
Input portto specify your constant value using theValport. This parameter appears if, for the Padding options parameter, you selectConstant.
- Pad value
Enter the constant value with which to pad your matrix. This parameter appears if, for the Pad value source parameter, you select
Specify via dialog. Tunable.
- Rounding mode
Select the rounding mode for fixed-point operations.
- Overflow mode
Select the overflow mode for fixed-point operations.
Note
Only certain cases require the use of the accumulator and output parameters. Refer to Fixed-Point Data Types for more information.
- Accumulator
Use this parameter to specify the accumulator word and fraction lengths resulting from a complex-complex multiplication in the block:
When you select
Same as input, these characteristics match the related input to the block.When you select
Binary point scaling, you can enter the word length and the fraction length of the accumulator, in bits.When you select
Slope and bias scaling, you can enter the word length, in bits, and the slope of the accumulator. This block requires power-of-two slope and a bias of 0.
- Output
Choose how to specify the output word length and fraction length:
When you select
Same as input, these characteristics match the related input to the block.When you select
Binary point scaling, you can enter the word length and the fraction length of the output, in bits.When you select
Slope and bias scaling, you can enter the word length, in bits, and the slope of the output. This block requires power-of-two slope and a bias of 0.
- Lock data type settings against change by the fixed-point tools
Select this parameter to prevent the fixed-point tools from overriding the data types you specify on the block mask. For more information, see
fxptdlg(Fixed-Point Designer), a reference page on the Fixed-Point Tool in the Simulink® documentation.
References
[1] Gonzales, Rafael C. and Richard E. Woods. Digital Image Processing. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 2002.
See Also
Computer Vision Toolbox™ | |
Computer Vision Toolbox | |
Image Processing Toolbox™ |