Median Filter

2-D median filtering

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Libraries:
Vision HDL Toolbox / Analysis & Enhancement
Vision HDL Toolbox / Filtering

Description

The Median Filter block replaces each input pixel with the median value of a specified surrounding N-by-N neighborhood. The median is less sensitive to extreme values than the mean. You can use this block to remove salt-and-pepper noise from an image without significantly reducing the sharpness of the image. You can specify the neighborhood size and padding values for edges of the input image.

Examples

Noise Removal and Image Sharpening

Implement a front-end module of an image processing design. This front-end module removes noise and sharpens the image to provide a better initial condition for the subsequent processing.

• Open Model

Filter Multipixel Video Streams

Design filters that operate on a multipixel input video stream. Use multipixel streaming to process high-resolution or high-frame-rate video with the same synthesized clock frequency as a single-pixel streaming interface. Multipixel streaming also improves simulation speed and throughput because fewer iterations are required to process each frame, while maintaining the hardware benefits of a streaming interface.

• Open Model

Ports

This block uses a streaming pixel interface with a bus for frame control signals. This interface enables the block to operate independently of image size and format. The pixel ports on this block support single pixel streaming or multipixel streaming. Single pixel streaming accepts and returns a single pixel value each clock cycle. Multipixel streaming accepts and returns a vector of M pixels per clock cycle to support high-frame-rate or high-resolution formats. The M value corresponds to the Number of pixels parameter of the Frame To Pixels block. Along with the pixel, the block accepts and returns a pixelcontrol bus that contains five control signals. The control signals indicate the validity of each pixel and their location in the frame. For multipixel streaming, one set of control signals applies to all pixels in the vector. To convert a frame (pixel matrix) into a serial pixel stream and control signals, use the Frame To Pixels block. For a full description of the interface, see Streaming Pixel Interface.

Input

pixel — Input pixel stream

scalar | vector

This block supports single pixel streaming or multipixel streaming. For single pixel streaming, specify a single input pixel as a scalar intensity value. For multipixel streaming, specify a vector of two, four, or eight pixel intensity values. For details of how to set up your model for multipixel streaming, see Filter Multipixel Video Streams.

This block does not support multicomponent streaming. To process multicomponent streams, replicate the block for each component. The pixelcontrol bus for all components is identical, so you can connect a single bus to multiple replicated blocks.

The software supports double and single data types for simulation, but not for HDL code generation.

Data Types: uint | int | fixed point | Boolean | double | single

ctrl — Control signals associated with pixel stream

pixelcontrol bus

The pixelcontrol bus contains five signals. The signals describe the validity of the pixel and its location in the frame. For more information, see Pixel Control Bus.

For multipixel streaming, each vector of pixel values has one set of control signals. Because the vector has only one valid signal, the pixels in the vector must be either all valid or all invalid. The hStart and vStart signals apply to the pixel with the lowest index in the vector. The hEnd and vEnd signals apply to the pixel with the highest index in the vector.

Data Types: bus

Output

pixel — Output pixel stream

scalar | vector

This block supports single pixel streaming or multipixel streaming. When using single pixel streaming, the block returns a single pixel as a scalar intensity value. When using multipixel streaming, the block returns a vector of intensity values. This vector is the same size as the input pixel vector. For details of how to set up your model for multipixel streaming, see Filter Multipixel Video Streams.

The output pixel data type is the same as the data type of the input pixels.

The software supports double and single data types for simulation, but not for HDL code generation.

Data Types: uint | int | fixed point | Boolean | double | single

ctrl — Control signals associated with pixel stream

pixelcontrol bus

The pixelcontrol bus contains five signals. The signals describe the validity of the pixel and its location in the frame. For more information, see Pixel Control Bus.

For multipixel streaming, each vector of pixel values has one set of control signals. Because the vector has only one valid signal, the pixels in the vector must be either all valid or all invalid. The hStart and vStart signals apply to the pixel with the lowest index in the vector. The hEnd and vEnd signals apply to the pixel with the highest index in the vector.

Data Types: bus

Parameters

Neighborhood size — Size of image region used to compute median

3×3 (default) | 5×5 | 7×7

Select the neighborhood size, in pixels, as 3×3, 5×5, or 7×7.

You cannot use a neighborhood size of 7×7 when you use multipixel streaming and set Padding method to None.

Padding method — Method for padding boundary of input image

Symmetric (default) | Constant | Replicate | Reflection | None

Select one of these methods for padding the boundary of the input image. For more information about these methods, see Edge Padding.

• Constant — Interpret pixels outside the image frame as having a constant value.

• Replicate — Repeat the value of pixels at the edge of the image.

• Symmetric — Set the value of the padding pixels to mirror the edge of the image.

• Reflection — Set the value of the padding pixels to reflect around the pixel at the edge of the image.

• None — Exclude padding logic. The block does not set the pixels outside the image frame to any particular value. This option reduces the hardware resources used by the block and the blanking required between frames but affects the accuracy of the output pixels at the edges of the frame. To maintain pixel stream timing, the output frame is the same size as the input frame. However, to avoid using pixels calculated from undefined padding values, mask off the KernelSize/2 pixels around the edge of the frame for downstream operations. For details, see Increase Throughput by Omitting Padding.

You cannot use a neighborhood size of 7×7 when you set Padding method to None.

Padding value — Value used to pad boundary of input image

0 (default) | integer

Specify an integer to pad the boundary of the input image. The block casts this value to the same data type as the input pixel.

Dependencies

To enable this parameter, set the Padding method parameter to Constant.

Line buffer size — Size of line buffer

2048 (default) | integer

Size of the line memory buffer, specified as a positive integer. Choose a power of two that accommodates the number of active pixels in a horizontal line. If you specify a value that is not a power of two, the buffer uses the next largest power of two.

When you use multipixel input, this value must accommodate (Active pixels per line)/(Number of pixels) + 2.

The block allocates (N— 1) -by-Line buffer size memory locations to store the pixels used to compute the median value. N is the dimension of the square region specified by the Neighborhood size parameter.

Tips

• When you use a block with an internal line buffer inside an Enabled Subsystem (Simulink), the enable signal pattern must maintain the timing of the pixel stream, including the minimum blanking intervals. If the enable pattern corrupts the timing of the pixel stream, you might see partial output frames, corrupted pixel stream control signals, or mismatches between Simulink^® and HDL simulation results. You may need to extend the blanking intervals to accommodate for cycles when the enable is low. For more information, see Configure Blanking Intervals.

Algorithms

When you use multipixel streaming, the block uses a single line memory and implements one filter kernel for each of the M input pixels, in parallel. This increase in hardware resources is a trade off for increasing throughput compared to single-pixel streaming.

Latency

The latency of the block is the line buffer latency plus the latency of the kernel calculation. The line buffer latency includes edge padding by default. The latency of the padding operation depends on the size of the kernel. If edge padding is not necessary for your design, you can reduce the latency by setting the Padding method parameter to None. When you use this option, the block latency does not depend on your kernel size. To determine the exact latency for any configuration of the block, measure the number of time steps between the input and output control signals.

To find the median value of each neighborhood, the kernel logic compares the pixel values to each other. The latency of the filter kernel depends on the neighborhood size. The table shows the number of comparisons needed to find the median value of each size of neighborhood.

Neighborhood Size	Number of Comparisons Performed
3×3	11
5×5	75
7×7	230

Note

When you use edge padding, use a horizontal blanking interval of at least twice the kernel width. This interval lets the block finish processing one line before it starts processing the next one, including adding padding pixels before and after the active pixels in the line.

The horizontal blanking interval is equal to TotalPixelsPerLine – ActivePixelsPerLine or, equivalently, FrontPorch + BackPorch. Standard streaming video formats use a horizontal blanking interval of about 25% of the frame width. This interval is much larger than the filters applied to each frame.

The horizontal blanking interval must also meet these minimums:

• If the kernel size is less than 4, and you use edge padding, the total porch must be at least 8 pixels.

• When you disable edge padding, the horizontal blanking interval must be at least 12 cycles and is independent of the kernel size.

• The BackPorch must be at least 6 pixels. This parameter is the number of inactive pixels before the first valid pixel in a frame.

For more information, see Configure Blanking Intervals.

Performance

For the Median Filter block, the comparison logic that finds the median pixel value of each neighborhood is typically the critical path that determines the achievable clock frequency after synthesis. As the size of a neighborhood increases, the length of this path increases.

This table shows the resource use after synthesis of the block for the Xilinx^® Zynq^®-7000 SoC ZC706 Evaluation Kit with single-pixel uint8 input and the default parameter settings. The design achieves a clock frequency of 250 MHz.

Resource	Usage
Slice LUTs	533
Slice Registers	896
DSP48	0
Block RAM	1

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

This block supports C/C++ code generation for Simulink accelerator and rapid accelerator modes and for DPI component generation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

You cannot generate HDL code for this block if it is inside a Resettable Synchronous Subsystem (HDL Coder).

Version History

Introduced in R2015a

R2023a: Two pixels-per-clock streaming

The block now supports multipixel streams that have 2 pixels per clock cycle.

R2021b: Reflection padding

Pad the edge of a frame by reflecting around the edge-pixel value. This padding method helps reduce edge contrast effects and can improve results for machine learning while maintaining the original frame size.

R2020a: Option to omit padding

You can now configure the block to not add padding around the boundaries of the active frame. This option reduces the hardware resources used by the block and the blanking required between frames but affects the accuracy of the output pixels at the edges of the frame. To use this option, set the Padding method parameter to None. For an example, see Increase Throughput by Omitting Padding.

R2019b: Multipixel streaming

The Median Filter block now supports multipixel streams. The HDL implementation replicates the algorithm for each pixel in parallel.

For multipixel streaming, the block supports input and output column vectors of 4 or 8 pixels. The ctrl port remains scalar, and the control signals in the pixelcontrol bus apply to all pixels in the vector.

R2018b: Improved line buffer

The internal line buffer in this block now handles bursty data, that is, noncontiguous valid signals within a pixel line. This implementation uses fewer hardware resources due to improved padding logic and native support for kernel sizes with an even number of lines. This change affects the Line Buffer block and blocks that use an internal line buffer.

The latency of the line buffer is now reduced by a few cycles for some configurations. You might need to rebalance parallel path delays in your models. A best practice is to synchronize parallel paths in your models by using the pixel stream control signals rather than by inserting a specific number of delays.