visionhdl.ImageFilter

2-D FIR filtering

Description

The visionhdl.ImageFilter System object™ performs two-dimensional finite-impulse-response (FIR) filtering on a pixel stream. It supports the use of programmable filter coefficients.

To filter a pixel stream:

1. Create the visionhdl.ImageFilter object and set its properties.

2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Creation

Syntax

F = visionhdl.ImageFilter(Name,Value)

F = visionhdl.ImageFilter(coeff,lineSize,Name,Value)

Description

F = visionhdl.ImageFilter(Name,Value) returns a 2-D FIR filter System object that filters a pixel stream. Set properties using one or more name-value pairs. Enclose each property name in single quotes.

F = visionhdl.ImageFilter(coeff,lineSize,Name,Value) returns a 2-D FIR filter System object, with the Coefficients property set to coeff, the LineBufferSize property set to lineSize, and additional options specified by one or more Name,Value pair arguments.

Properties

Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

CoefficientsSource — Source to provide filter coefficients

'Property' (default) | 'Input port'

Source of the filter coefficients, specified as one of these values.

• 'Property' — The object uses the filter coefficients specified in the Coefficients property.

• 'Input port' — The object uses the filter coefficients from the coeff argument.

Coefficients — Coefficients of filter

[1,0;0,-1] (default) | matrix

Coefficients of the filter, specified as a matrix. Each dimension of the matrix must have at least 2 elements and no more than 64 elements.

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

Dependencies

This property applies when you set CoefficientsSource to 'Property'.

PaddingMethod — Method for padding

'Constant' (default) | 'Replicate' | 'Symmetric' | 'Reflection'

Methods for padding the boundary of the input image, specified as one of these values.

• '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 object does not set the pixels outside the image frame to any particular value. This option reduces the hardware resources that are used by the object and reduces the blanking that is required between frames. However, this option 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. To avoid using pixels calculated from undefined padding values, mask off the n/2 pixels around the edge of the frame for downstream operations. n is the size of the operation kernel. For more details, see Increase Throughput by Omitting Padding.

For more information about these methods, see Edge Padding.

PaddingValue — Value used to pad boundary of input image

0 (default) | integer

Value used to pad the boundary of the input image, specified as an integer. The object casts this value to the same data type as the input pixel.

Dependencies

This parameter applies when you set PaddingMethod to 'Constant'.

LineBufferSize — 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.

The object allocates (coefficient rows – 1)-by-LineBufferSize memory locations to store the pixels.

CoefficientsDataType — Method to determine data type of filter coefficients

'Same as first input' (default) | 'Custom'

Method for determining the data type of the filter coefficients, specified as one of these values.

• 'Same as first input'' — Sets the data type of the coefficients to match the data type of the pixelin argument.

• 'Custom' — Sets the data type of the coefficients to match the data type defined in the CustomCoefficientsDataType property.

When converting the coefficients to the specified data type, the object rounds to the nearest representable value and saturates on overflow.

Dependencies

This property applies when you set CoefficientsSource to 'Property'.

CustomCoefficientsDataType — Data type for the filter coefficients

numerictype(true,16,15) (default) | numerictype(S,WL,FL)

Data type for the filter coefficients, specified as numerictype(S,WL,FL), where S is 1 (true) for signed and 0 (false) for unsigned, WL is the word length, and FL is the fraction length in bits.

Dependencies

This property applies when you set CoefficientsDataType to 'Custom'.

OutputDataType — Method to determine data type of output pixels

'Same as first input' (default) | 'Full precision' | 'Custom'

Method for determining the data type of the output pixels, specified as one of these values.

• 'Same as first input' — Sets the data type of the output pixels to match the data type of the pixelin argument.

• 'Full precision' — Computes internal and output data types using full precision rules. These rules provide accurate fixed-point numerics and prevent quantization within the object. The object adds bits as needed to prevent rounding and overflow.

• 'Custom' — Sets the data type of the output pixels to match the data type in the CustomOutputDataType property.

CustomOutputDataType — Data type for the filter coefficients

numerictype(true,8,0) (default) | numerictype(S,WL,FL)

Data type for the output pixels, specified as numerictype(S,WL,FL), where S is 1 (true) for signed and 0 (false) for unsigned, WL is the word length, and FL is the fraction length in bits.

Dependencies

This property applies only when you set OutputDataType to 'Custom'.

RoundingMethod — Rounding mode for fixed-point operations

'Floor' (default) | 'Ceiling' | 'Convergent' | 'Nearest' | 'Round' | 'Zero'

When the input is any integer or fixed-point data type, the algorithm uses fixed-point arithmetic for internal calculations. This property does not apply when the input data type is single or double.

OverflowAction — Overflow mode used for fixed-point operations

'Wrap' (default) | 'Saturate'

When the input is any integer or fixed-point data type, the algorithm uses fixed-point arithmetic for internal calculations. This property does not apply when the input data type is single or double.

Usage

Syntax

[pixelout,ctrlout] = F(pixelin,ctrlin)

[pixelout,ctrlout] = F(pixelin,ctrlin,coeff)

Description

[pixelout,ctrlout] = F(pixelin,ctrlin) returns the next pixel, pixelout, of the filtered image resulting from applying the coefficients in the Coefficients property to the image described by the input pixel stream, pixelin.

[pixelout,ctrlout] = F(pixelin,ctrlin,coeff) returns the next pixel, pixelout, of the filtered image resulting from applying the coefficients in the coeff argument to the image described by the input pixel stream, pixelin. The object samples the values from the coeff argument only at the start of a frame and ignores any changes within a frame. To enable this syntax, set the CoefficientsSource property to 'Input port'.

This object uses a streaming pixel interface with a structure for frame control signals. This interface enables the object to operate independently of image size and format and to connect with other Vision HDL Toolbox™ objects. The object accepts and returns a scalar pixel value and control signals as a structure containing five signals. The control signals indicate the validity of each pixel and its location in the frame. To convert a pixel matrix into a pixel stream and control signals, use the visionhdl.FrameToPixels object. For a description of the interface, see Streaming Pixel Interface.

Input Arguments

pixelin — Input pixel stream

scalar

Single pixel, specified as a scalar value.

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

You can simulate System objects with a multipixel streaming interface, but you cannot generate HDL code for System objects that use multipixel streams. To generate HDL code for multipixel algorithms, use the equivalent Simulink^® blocks.

Data Types: uint | int | fi | double | single

ctrlin — Control signals accompanying input pixel stream

pixelcontrol structure

Control signals accompanying the input pixel stream, specified as a pixelcontrol structure containing five logical data type signals. The signals describe the validity of the pixel and its location in the frame. For more details, see Pixel Control Structure.

Data Types: struct

coeff — Filter coefficients

matrix

Filter coefficients, specified as a matrix. Each dimension of the matrix must have at least 2 elements and no more than 16 elements. The object samples the values from the coeff argument only at the start of a frame and ignores any changes within a frame.

Dependencies

To enable this argument, set the CoefficientsSource property to 'Input port'.

Data Types: uint | int | fi | double | single

Output Arguments

pixelout — Output pixel

scalar

Single filtered pixel, returned as a scalar value.

Configure the data type of the output pixel by using the OutputDataType and CustomOutputDataType properties.

Data Types: uint | int | fi | double | single

ctrlout — Control signals accompanying output pixel stream

pixelcontrol structure

Control signals accompanying the output pixel stream, returned as a pixelcontrol structure containing five logical data type signals. The signals describe the validity of the pixel and its location in the frame. For more details, see Pixel Control Structure.

Data Types: struct

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

Filter Pixel Stream

This example implements a 2-D blur filter on a thumbnail image.

Load the source image from a file. Select a portion of the image matching the desired test size.

frmOrig = imread('rice.png');
frmActivePixels = 64;
frmActiveLines = 48;
frmInput = frmOrig(1:frmActiveLines,1:frmActivePixels);
figure
imshow(frmInput,'InitialMagnification',300)
title 'Input Image'

Create a serializer object and specify the size of the inactive pixel regions.

frm2pix = visionhdl.FrameToPixels( ...
      'NumComponents',1, ...
      'VideoFormat','custom', ...
      'ActivePixelsPerLine',frmActivePixels, ...
      'ActiveVideoLines',frmActiveLines, ...
      'TotalPixelsPerLine',frmActivePixels+10, ...
      'TotalVideoLines',frmActiveLines+10, ...
      'StartingActiveLine',6, ...     
      'FrontPorch',5);

Create a filter object.

 filt2d = visionhdl.ImageFilter( ...
          'Coefficients',ones(2,2)/4, ...
          'CoefficientsDataType','Custom', ...
          'CustomCoefficientsDataType',numerictype(0,1,2), ...
          'PaddingMethod','Symmetric');

Serialize the test image by calling the serializer object. pixIn is a vector of intensity values. ctrlIn is a vector of control signal structures.

[pixIn,ctrlIn] = frm2pix(frmInput);

Prepare to process pixels by preallocating output vectors.

[~,~,numPixelsPerFrame] = getparamfromfrm2pix(frm2pix);
pixOut = zeros(numPixelsPerFrame,1,'uint8');
ctrlOut  = repmat(pixelcontrolstruct,numPixelsPerFrame,1);

For each pixel in the padded frame, compute the filtered value. Monitor the control signals to determine the latency of the object. The latency of a filter configuration depends on:

• The number of active pixels in a line.

• The size of the filter kernel.

• Optimization of symmetric or duplicate coefficients.

foundValIn = false;
foundValOut = false;
for p = 1:numPixelsPerFrame  
    if (ctrlIn(p).valid && foundValIn==0)
        foundValIn = p;
    end
    [pixOut(p),ctrlOut(p)] = filt2d(pixIn(p),ctrlIn(p));
    if (ctrlOut(p).valid && foundValOut==0)
        foundValOut = p;
    end
end
sprintf('object latency is %d cycles',foundValOut-foundValIn)

ans = 
'object latency is 101 cycles'

Create a deserializer object with a format that matches the format of the serializer. Convert the pixel stream to an image frame by calling the deserializer object. Display the resulting image.

pix2frm = visionhdl.PixelsToFrame( ...
      'NumComponents',1, ...
      'VideoFormat','custom', ...
      'ActivePixelsPerLine',frmActivePixels, ...
      'ActiveVideoLines',frmActiveLines, ...
      'TotalPixelsPerLine',frmActivePixels+10);
[frmOutput,frmValid] = pix2frm(pixOut,ctrlOut);
if frmValid
    figure
    imshow(frmOutput, 'InitialMagnification',300)
    title 'Output Image'
end

Algorithms

This System object implements the algorithms described on the Image Filter block reference page.

Extended Capabilities

C/C++ Code Generation

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

This System object supports C/C++ code generation for accelerating MATLAB^® simulations, and for DPI component generation. For more information about acceleration, see Accelerate Pixel-Streaming Designs Using MATLAB Coder. For more information about DPI component generation, see Considerations for DPI Component Generation with MATLAB (HDL Verifier).

HDL Code Generation

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

To generate HDL code from Vision HDL Toolbox System objects, see Design Hardware-Targeted Image Filters in MATLAB.

Version History

Introduced in R2015a

R2022a: Two pixels-per-clock streaming

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

You can simulate System objects with a multipixel streaming interface, but you cannot generate HDL code for System objects that use multipixel streams. To generate HDL code for multipixel algorithms, use the equivalent Simulink blocks.

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 object to not add padding around the boundaries of the active frame. This option reduces the hardware resources used by the object and reduces the blanking interval required between frames but affects the accuracy of the output pixels at the edges of the frame. To use this option, set the PaddingMethod property to 'None'.

R2019b: Multipixel streaming

The object now supports multipixel streams. For multipixel streaming, the object supports input and output column vectors of 4 or 8 pixels. The ctrl argument remains scalar, and the control signals in the pixelcontrol structure apply to all pixels in the vector.

You can simulate System objects with a multipixel streaming interface, but you cannot generate HDL code for System objects that use multipixel streams. To generate HDL code for multipixel algorithms, use the equivalent Simulink blocks.

R2019a: Increased kernel size limits

The object now allows for a coefficient kernel with up to 64-by-64 elements.

R2018b: Programmable coefficients

The object now accepts coefficients from an input argument. Each dimension of the matrix must have at least 2 and no more than 16 elements. The object samples the values from the coeff argument at the start of a frame only and ignores any changes within a frame.

In previous releases, you could specify a row vector of coefficients, that is, a matrix of 1-by-N elements. Now, the coefficient matrix must have at least 2 elements in each dimension.

R2018b: Improved line buffer

The internal line buffer in this object 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 visionhdl.LineBuffer object and objects 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 designs. A best practice is to synchronize parallel paths in your designs by using the pixel stream control signals rather than by inserting a specific number of delays.