Perform 2D median filtering
Filtering and Analysis & Enhancement
visionanalysis
visionfilter
The Median Filter block replaces the central value of an MbyN 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 upperleft 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 saltandpepper 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.
The information in this section is applicable only when the dimensions of the neighborhood are even.
For fixedpoint inputs, you can specify accumulator and output data types as discussed in Dialog Box. Not all these fixedpoint parameters apply to all types of fixedpoint inputs. The following table shows the output and accumulator data type used for each fixedpoint input.
FixedPoint Input  Output Data Type  Accumulator Data Type 

Even  X  X 
Odd  X 

Odd  X  X 
Even  X  X 
When M
is even, fixedpoint 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 fixedpoint 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.
The Main pane of the Median Filter dialog box appears as shown in the following figure.
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.
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 port I
.
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
.
Specify how to pad the boundary of your input matrix.
Select Constant
to pad your matrix
with a constant value. The Pad value source parameter
appears in the dialog box
Select Replicate
to pad your input
matrix by repeating its border values.
Select Symmetric
to pad your input
matrix with its mirror image.
Select Circular
to pad your input
matrix using a circular repetition of its elements. This parameter
appears if, for the Output size parameter, you
select Same as input port I
.
For more information on padding, see the Image Pad block reference page.
Use this parameter to specify how to define your constant boundary value.
Select Specify via dialog
to enter
your value in the block parameters dialog box. The Pad value parameter
appears in the dialog box.
Select Input port
to specify your
constant value using the Val
port. This parameter
appears if, for the Padding options parameter,
you select Constant
.
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.
The Data Types pane of the Median Filter dialog box appears as follows. The parameters on this dialog box becomes visible only when the dimensions of the neighborhood are even.
Select the rounding mode for fixedpoint operations.
Select the overflow mode for fixedpoint operations.
Note Only certain cases require the use of the accumulator and output parameters. Refer to FixedPoint Data Types for more information. 
Use this parameter to specify the accumulator word and fraction lengths resulting from a complexcomplex 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 poweroftwo slope and a bias of 0.
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 poweroftwo slope and a bias of 0.
Select this parameter to prevent the fixedpoint tools from
overriding the data types you specify on the block mask. For more
information, see fxptdlg
,
a reference page on the FixedPoint Tool in the Simulink^{®} documentation.
[1] Gonzales, Rafael C. and Richard E. Woods. Digital Image Processing. 2nd ed. Englewood Cliffs, NJ: PrenticeHall, 2002.
Computer Vision System Toolbox™  
Computer Vision System Toolbox  
Image Processing Toolbox™ 