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Creating User Filters

Note 1: Creating user filters requires C/C++ programming skills.

Note 2: With your own C/C++ code you can easily crash the entire application. We are not able to protect against that.

Table of Contents

  1. Introduction
    1. Prerequisites
    2. User Filter Libraries Location
    3. Adding New Global User Filter Libraries
    4. Adding New Local User Filter Libraries
  2. Developing User Filters
    1. User Filter Project Configuration
    2. Basic User Filter Example
    3. Structure of User Filter Class
      1. Structure of Define Method
      2. Structure of Invoke Method
    4. Using Arrays
    5. Diagnostic Mode Execution and Diagnostic Outputs
    6. Filter Work Cancellation
    7. Using Dependent DLL
  3. Advanced Topics
    1. Using the Full Version of AVL
    2. Accessing Console from User Filter
    3. Generic User Filters
    4. Creating User Types in User Filters
  4. Troubleshooting and examples
    1. Upgrading User Filters to Newer Versions of Aurora Vision Studio
    2. Building x64 User Filters in Microsoft Visual Studio Express Edition
    3. Remarks
    4. Example: Image Acquisition from IDS Cameras
    5. Example: Using PCL library in Aurora Vision Studio


User filters are written in C/C++ and allow the advanced users to extend capabilities of Aurora Vision Studio with virtually no constraints. They can be used to support a new camera model, to communicate with external devices, to add application-specific image processing operations and more.


To create a user filter you will need:

  • an installed Microsoft Visual Studio 2015/2017/2019 for C++, Express Edition (free) or any higher edition,
  • the environment variable AVS_PROFESSIONAL_SDK1_0 in your system (depending on the edition; a proper value of the variable is set during the installation of Aurora Vision Studio),
  • C/C++ programming skills.

User filters are grouped in user filter libraries. Every user filter library is a single .dll file built using Microsoft Visual Studio. It can contain one or more filters that can be used in programs developed with Aurora Vision Studio.

User Filter Libraries Location

There are two types of user filter libraries:

  • Global – once created or imported to Aurora Vision Studio they can be used in all projects. The filters from such libraries are visible in the Libraries tab of the Toolbox.
  • Local – belong to specific projects of Aurora Vision Studio. The filters from such libraries are visible only in the project that the library has been added to.

A solution (.sln file) of a global user filter library can be located in any location on your hard disk, but the default and recommended location is Documents\Aurora Vision Studio 1.0 Professional\Sources\UserFilters (the exact path can vary depending on the version of Aurora Vision Studio). The output .dll file built using Microsoft Visual Studio and containing global user filters has to be located in Documents\Aurora Vision Studio 1.0 Professional\Filters\x64 (this time the exact path depends on the version and the edition) and this path is set in the initial settings of the generated Microsoft Visual Studio project. For global user filter libraries, this path must not be changed because Aurora Vision Studio monitors this directory for changes of the .dll files. The Global User Filter .dll file for Aurora Vision Executor has to be located in Documents\Aurora Vision Studio 1.0 Runtime\Filters\x64 (again, the exact path depends on the version and edition). For 32 bit edition the last subdirectory should be changed from x64 to Win32. The Local User Filter .dll file for Aurora Vision Executor has to be located in path configured in the User Filter properties. You can modify this path by editing user filters library properties in Project Explorer.

A local user filter library is a part of the project developed with Aurora Vision Studio and both source and output .dll files can be located anywhere on the hard drive. Use the Project Explorer task panel to check or modify paths to the output .dll and Microsoft Visual Studio solution files of the user filter library. The changes of the output .dll file are monitored by Aurora Vision Studio irrespectively of the file location. It's a good practice to keep the local user filter library sources (and the output .dll) relatively to the location of the developed project, for example in a subdirectory of the project.

Adding New Global User Filter Libraries

To add a new user filter library, start with File » Add/Modify Global User Filters » New Global User Filter Library....

The other option is to use Create New Local User Filter Library button from Project Explorer panel.

A dialog box will appear where you should choose:

  • name for the new library,
  • type of the library: local (available in current project only) or global (available in all projects),
  • location of the solution directory,
  • version of Microsoft Visual Studio (2015, 2017 or 2019),
  • whether Microsoft Visual Studio should be opened automatically,
  • whether the code of example user filters should be added to the solution - good idea for users with less experience with user filters programming.

If you choose Microsoft Visual Studio to be opened, you can build this solution instantly. A new library of filters will be created and after a few seconds loaded to Aurora Vision Studio. Switch back to Aurora Vision Studio to see the new filters in:

  • Appropriate categories of Libraries tab (global user filters, category in Libraries tab is based on the category set in filter code)
  • Project Explorer (local user filters)

You can work simultaneously in both Microsoft Visual Studio and Aurora Vision Studio. Any time the C++ code is built, the filters will get reloaded. Just re-run your program in Aurora Vision Studio to see what has changed.

If you do not see your filters in the above-mentioned locations, make sure that they have been compiled correctly in an architecture compatible with your Aurora Vision Studio architecture: x86 (Win32) or x64.

Adding New Local User Filter Libraries

To add a new local user filter use the "Create New Local User Filter Library.." button in the Project Explorer panel as on the image below:

Developing User Filters

User Filter Project Configuration

User filter project files (.sln and .vcproj) are generated by Aurora Vision Studio during adding new user filter library.

The settings of user filter project are gathered in .props file available in props subdirectory of the Aurora Vision Studio SDK (environment variable AVS_PROFESSIONAL_SDK1_0), typically C:\Program Files\Aurora Vision\Aurora Vision Studio 1.0 Professional\SDK\props.

If you want to configure the existing project to be a valid user filter library, please use the proper .props file (file with v140 suffix is dedicated for Microsoft Visual Studio 2015, v141 for 2017 and v142 for 2019).

Basic User Filter Example

Example below shows whole source code for basic user filter. In this example filter is making a simple threshold operation on an 8-bit image.

#include "UserFilter.h"
#include "AVL_Lite.h"

#include "UserFilterLibrary.hxx"

namespace avs
	// Example image processing filter
	class CustomThreshold : public UserFilter
		// Non-trivial outputs must be defined as a filed to retain data after filter execution. 
		avl::Image outImage;

		// Defines the inputs, the outputs and the filter metadata
		void Define() override
			SetName		(L"CustomThreshold");
			SetCategory	(L"Image::Image Thresholding");
			SetImage	(L"CustomThreshold_16.png");	
			SetImageBig	(L"CustomThreshold_48.png");
			SetTip		(L"Binarizes 8-bit images");

			//					 Name						Type				Default		Tool-tip
			AddInput			(L"inImage",				L"Image",			L"",		L"Input image"    );
			AddInput			(L"inThreshold",			L"Integer<0, 255>",	L"128",		L"Threshold value");
			AddOutput			(L"outImage",				L"Image",						L"Output image"   );

		// Computes output from input data
		int Invoke() override
			// Get data from the inputs
			avl::Image inImage;
			int inThreshold;

			ReadInput(L"inImage", inImage);
			ReadInput(L"inThreshold", inThreshold);

			if (inImage.Type() != avl::PlainType::UInt8)
				throw atl::DomainError("Only uint8 pixel type are supported.");

			// Get image properties
			int height = inImage.Height();

			// Prepare output image in this same format as input
			outImage.Reset(inImage, atl::NIL);

			// Enumerate each row
			for (int y = 0; y < height; ++y)
				// Get row pointers
				const atl::uint8* p = inImage.RowBegin<atl::uint8>(y);
				const atl::uint8* e = inImage.RowEnd<atl::uint8>(y);
				atl::uint8*       q = outImage.RowBegin<atl::uint8>(y);

				// Loop over the pixel components
				while (p < e)
					(*q++) = (*p++) < inThreshold ? 0 : 255;

			// Set output data
			WriteOutput(L"outImage", outImage);

			// Continue program
			return INVOKE_NORMAL;
	// Builds the filter factory
	class RegisterUserObjects
			// Remember to register every filter exported by the user filter library

	static RegisterUserObjects registerUserObjects;

Structure of User Filter Class

A user filter is a class derived from the UserFilter class defined in UserFilter.h header file. When creating a filter without state you have to override two methods:

  • Define – defines the interface of the filter, including its name, category, inputs and outputs.
  • Invoke – defines the routine that transforms inputs into outputs.

When creating a filter with state (storing information from previous invocations) the class is going to have some data fields and two additional methods have to be overridden:

  • Init – initializes the state variables, invoked at the beginning of a Task parenting the instance of the filter, may be invoked multiple times during filter instance lifetime. Always remember to invoke base type Invoke() method.
  • Stop – deinitializes the state variables, including releasing of external and I/O resources (like file handles or network connections), may not affect data on filter outputs, invoked at the end of a Task parenting the filter instance (to pair every Init call).
  • Release – releases output variables memory, invoked at the end of a Task macrofilters marked to release memory.

When a user filter class is created it has to be registered. This is done in the RegisterUserObjects function which is defined at the bottom of the sample user filters' code. You do not need to call it manually, it's called by Aurora Vision Studio while loading filters from the .dll file.

Structure of Define Method

Use the Define method to set the name, category, image (used as the icon of the filter) and tooltip for your filter. All of this can be set by using proper Set... methods.

The Define method should also contain a definition of the filter's external interface, which means: inputs, outputs and diagnostic outputs. The external interface should be defined using AddInput, AddOutput and AddDiagnosticOutput methods. These methods allow to define name, type and tooltip for every input/output of filter. For inputs a definition of the default value is also possible.

Aurora Vision Studio uses a set of additional attributes for ports. To apply attribute on a port use AddAttribute method. Example:

AddAttribute(L"ArraySync", L"inA inB");

List of attributes:

Attribute Name Description Example Comment
ArraySync Defines a set of synchronized ports.
L"inA inB"
Informs that arrays in inA and inB require the same number of elements.
UserLevel Defines user level access to the filter.
Only users with Advanced level will find this filter in Libraries tab.
UsageTips Defines additional documentation text.
L"Use this filter for creating a line."
This is instruction where this filter is needed.
AllowedSingleton Filter can accept singleton connections on input
User can connect a single value to inA which has Array type.
FilterGroup Defines element of filter group.
L"FilterName<VariantName> default ## Description for group"
Creates a FilterName with default element VariantName. More detailed description in "Defining Filters Groups"
Tags Defines alternative names for this filter.
L"DrawText DrawString PutText"
When user types "DrawText" this filter will be in result list.
CustomHelpUrl Defines alternative URL for this filter.
When user press F1 in the Program Editor alternative help page will be opened.

Defining Filters Groups

Several filters can be grouped into a single group, which can be very helpful for user to change variant of very similar operations.

To create filter group define attribute L"FilterGroup" for default filter with parameters. L"FilterName<VariantName> default ## Description for group". Notice "default" word. Text after "##" defines the tooltip for whole group.

If default filter is defined you can add another filter using L"FilterGroup" with parameter L"FilterName<NextVariant>"

Example usage:

// Default filter FindCircle -> Find: Circle
AddAttribute(L"FilterGroup", L"Find<Circle> default ## Finds an object on the image");

// Second variant FindRectangle -> Find: Rectangle
AddAttribute(L"FilterGroup", L"Find<Rectangle>");

// Third variant FindPolygon -> Find: Polygon
AddAttribute(L"FilterGroup", L"Find<Polygon>");

As result a filter group Find will be created with three variants: Circle, Rectangle, Polygon.

Using custom user filter icons

Using methods SetImage and SetImageBig user can assign a custom icon for user filter. Filter icon must be located in this same directory as output output user filter DLL file.

There are four types of icons:

  • Small Icon - icon with size 16x16 pixels used in Libraries tab, set by SetImage, name should end with "_16"
  • Medium Icon - icon of size 24x24 pixel, created automatically from Big Icon,
  • Big Icon - icon of size 48x48 pixel, set by SetImageBig, name should end with "_48",
  • Description Icon - icon of size 72x72 used in filter selection from group, name is created by replacing "_48" from SetImageBig by "_D". For given SetImageBig as "custom_48.png" a name "custom_D.png" will be generated.

Structure of Invoke Method

An Invoke method has to contain 3 elements:

  1. Reading data from inputs

    To read the value passed to the filter input, use the ReadInput method. This is a template method supporting all Aurora Vision Studio data types. ReadInput method returns the value (by reference) using its second parameter.

  2. Computing output data from input data

    It is the core part. Any computations can be done here

  3. Writing data to outputs

    Similarly to reading, there is a method WriteOutput that should be used to set values returned from filter on filter outputs.

    Data types that don't contain blobs (i.e. int, avl::Point2D, avl::Rectangle2D) can be simply returned by passing the local variable to the WriteOutput method. Output variables with blobs (i.e. avl::Image, avl::Region) should be declared at least in a class scope.

    	class MyOwnFilter : public UserFilter
    		int Invoke()
    			int length;
    			// ... computing the length value...
    			WriteOutput("outLength", length);
    		// ...

    All non-trivial data types like Image, Region or ByteBuffer should be defined as a filter class filed.

    This solution has two benefits:

    1. Reduces performance overhead for creating new objects in each filter execution,
    2. Assures that types which contains blobs are not released after the filter execution.

    For the sake of clarity it is good habit to define all filter variables as class members.

    	class MyOwnFilter : public UserFilter
                // Non-trivial type data
    			avl::Image image;
    		int Invoke()
    			// ... computing image ...
    			WriteOutput("outImage", image);
    		// ...

Invoke has to return one of the four possible values:

  • INVOKE_ERROR - when something went wrong and program cannot be continued.
  • INVOKE_NORMAL - when everything is OK and the filter can be invoked again.
  • INVOKE_LOOP - when everything is OK and the filter requests more iterations.
  • INVOKE_END - when everything is OK and the filter requests to stop the current loop.

For example the filter ReadVideo returns INVOKE_LOOP whenever a new frame is successfully read and INVOKE_END when there is the end. INVOKE_NORMAL is returned by filters that do not have any influence on the current loop continuation or exiting (for example ThresholdImage).

All filter outputs should be assigned by WriteOutput before filters return status. Missing assigned may result random data access in complex program structure. On INVOKE_END result filter should set up output values as last iterations of filter.

In case of error also exceptions can be thrown. User atl::DomainError for signaling problems connected with input data. All hardware problems should be signaled using atl::IoError. For more information please read Error Handling

Using Arrays

User filters can process not only single data objects, but also arrays of them. In Aurora Vision Studio, arrays are represented by data types with suffix Array (i.e. IntegerArray, ImageArray, RegionArrayArray). Multiple Array suffixes are used for multidimensional arrays. In C++ code of user filters, atl::Array<T> container is used for storing objects in arrays:

atl::Array< int > integers;
atl::Array< avl::Image > images;
atl::Array< atl::Array< avl::Region > > regions2Dim;

For more information about types from atl and avl namespaces, please refer the documentation of Aurora Vision Library.

Diagnostic Mode Execution and Diagnostic Outputs

User filters can have diagnostic outputs. Diagnostic outputs can be helpful during developing programs in Aurora Vision Studio. The main purpose of this feature is to allow the user to view diagnostic data on the Data Previews, but they can also participate in the data flow and can be connected to an input of any filter. This type of connection is called a diagnostic connection and makes the destination filter to be executed in the Diagnostic mode (filter will be invoked only in the Diagnostic mode of program execution).

When a program is executed in the Non-Diagnostic mode, values of the diagnostic outputs shouldn't be (for performance purposes) computed by any filter. In user filters, you should use the IsDiagnosticMode() method for conditional computation of the data generated by your filter for diagnostic outputs. If the method returns True, execution is in the Diagnostic mode and values of the diagnostic outputs should be computed, otherwise, the execution is in the Non-Diagnostic mode and your filter shouldn't compute such values.

Filter Work Cancellation

Aurora Vision Studio allows to stop execution of each filter during the time consuming computations. To use this option function IsWorkCancelled() can be used. If function returns value True the long computation should be finished because user pressed the "Stop" button.

Using Dependent DLL

User filter libraries are often created as wrappers of third party libraries, e.g. of APIs for some specific hardware. These libraries often come in the form of DLL files. For a user filter to work properly, the other DLL files must be located in an accessible disk location at runtime, or the user gets the error code 126, The specified module could not be found. MSDN documentation specifies possible options in the article Dynamic-Link Library Search Order. From the point of view of user filters in Aurora Vision Studio, the most typical option is the one related to changing the PATH environment variable – almost all camera manufacturers follow this way. For local user filters it is also allowed to add dependent dll in the same directory as the user filter dll directory.

Alternatively, it is possible to create a new directory for a global user filter library: Documents\Aurora Vision Studio 1.0 Runtime\Filters\Deps_x64, after which the user filter's dependent DLL files can be stored inside of it.

Advanced Topics

Using the Full Version of AVL

By default, user filters are based on Aurora Vision Library Lite library, which is a free edition of Aurora Vision Library Professional. It contains data types and basic functions from the 'full' edition of Aurora Vision Library. Please refer to the documentation of Aurora Vision Library Lite and Aurora Vision Library Professional to learn more about their features and capabilities.

If you have bought a license for the 'full' Aurora Vision Library, you can use it in user filters instead of the Lite edition. The following steps are required:

  • In compiler settings of the project, add additional include directory $(AVL_PATH1_0)\include (Configuration Properties | C/C++ | General | Additional Include Directories).
  • In linker settings of the project, add new additional library directory $(AVL_PATH1_0)\lib\$(Platform) (Configuration Properties | Linker | General | Additional Library Directories).
  • In linker settings of the project, replace AVL_Lite.lib additional dependency with AVL.lib (Configuration Properties | Linker | Input | Additional Dependencies).
  • In source code file, change including AVL_Lite.h to AVL.h.

Accessing Console from User Filter

It is possible to add messages to the console of Aurora Vision Studio from within the Invoke method. Logging messages can be used for problems visualization, but also for debugging. To add the message, use one of the following functions:

bool LogInfo   (const atl::String& message);
bool LogWarning(const atl::String& message);
bool LogError  (const atl::String& message);
bool LogFatal  (const atl::String& message);

Generic User Filters

Generic filters are filters that do not have a strictly defined type of the data they process. Generic filters have to be concretized with a data type before they can be used. There are many generic filters provided with Aurora Vision Studio (i.e. ArraySize) and user filters can be generic as well.

To create a generic user filter, you need to define one or more ports of the user filters as generic. In the call of AddInput method the second parameter (data type) has to contain < T >. Example usage:

AddInput ("inArray", "<T>Array", "", "Input array");
AddInput ("inObject", "<T>", "", "Object of any type");

In the Invoke method of a user filter, the GetTypeParam function can be used to resolve the data type that the filter has been concretized with. Once the data type is known, the data can be properly processed using the if-else statement. Please see the example below.

atl::String type = GetTypeParam(); // Getting type of generic instantiation as string.
int arrayByteSize = -1;

if (type == "Integer")
    atl::Array< int > ints = GetInputArray< int >("inArray");
    arrayByteSize = ints.Size() * sizeof(int);
else if (type == "Image")
    atl::Array< avs::Image > images = GetInputArray< avs::Image >("inArray");
    arrayByteSize = 0;
    for (int i = 0; i <  images.Size(); ++i)
        arrayByteSize += images[i].pitch * images[i].height;

Creating User Types in User Filters

When creating a User Filter add to the project an AVTYPE file with a user types description. The file should contain type descriptions in a format the same like the one used for creating User Types in a program. See Creating User Types. Sample user type description file:

enum PartType

struct Part
	String 	Name
	Real	Width
	Real	Height
	Real	Tolerance

In your C++ code declare structures/enums with the same field types, names and order. If you create an enum then you can start using this type in your project instantly. For structures you must provide ReadData and WriteData functions overrides for serialization and deserialization.

In these functions you should serialize/deserialize all fields of your structure in the same order you declared them in the type definition file.

To support structure Part from the previous example in your source code you should add:

Structure declaration:

struct Part
	atl::String 	Name;
	float			Width;
	float			Height;
	float			Tolerance;

Structure deserialization function:

void ReadData(atl::BinaryReader& reader, Part& outPart)
	ReadData(reader, outPart.Name);
	ReadData(reader, outPart.Width);
	ReadData(reader, outPart.Height);
	ReadData(reader, outPart.Tolerance);

Structure serialization function:

void WriteData(atl::BinaryWriter& writer, const Part& inValue)
	WriteData(writer, inValue.Name);
	WriteData(writer, inValue.Width);
	WriteData(writer, inValue.Height);
	WriteData(writer, inValue.Tolerance);

Enum declaration:

enum PartType

It is not required for custom serialization / deserialization of enum types.

The file with user type definitions has to be registered. This is done in the RegisterUserObjects class constructor which is defined at the bottom of the user filter code. You need to add there a registration of your file as RegisterTypeDefinitionFile("fileName.avtype"). The file name is a path to your type definitions file. The path should be absolute or relative to the User Filter dll file.

You can use types defined in a User Filter library in this User Filters library as well as in all other modules of the project. If you want to use the same type in multiple User Filters libraries then you should declare these types in each User Filters library.

The following Example Program: "User Filter With User Defined Types" demonstrates usage of User Types in User Filters.

Troubleshooting and Examples

Upgrading User Filters to Newer Versions of Aurora Vision Studio

When upgrading project with User Filters to more recent version of Aurora Vision you should manually edit the User Filter vcxproj file in your favorite text editor e.g. notepad. Make sure to close the solution file in Microsoft Visual Studio before performing the modifications. In this file you should change all occurrences of AVS_PROFESSIONAL_SDKxx (where is xx is your current version of Aurora Vision) to AVS_PROFESSIONAL_SDK1_0, save your changes and rebuild the project.

After successful build you can use your User Filter library in the new version of Aurora Vision.

During compilation you can receive some errors if you use in your code function which has changed its interface. In such case, please refer the documentation and release notes to find out how the function was changed in the current version.


  • If you get problems with PDB files being locked, kill the mspdbsrv.exe process using Windows Task Manager. It is a known issue in Microsoft Visual Studio. You can also switch to use the Release configuration instead.
  • User filters can be debugged. See Debugging User Filters.
  • A user filter library (in .dll file) that has been built using SDK from one version of Aurora Vision Studio is not always compatible with other versions. If you want to use the user filter library with a different version, it may be required to rebuild the library.
  • If you use Aurora Vision Library ('full' edition) in user filters, Aurora Vision Library and Aurora Vision Studio should be in the same version.
  • A solution of a user filter library can be generated with example filters. If you're a beginner in writing your own filters, it's probably a good idea to study these examples.
  • Only compiling your library in the release configuration lets you use it on other computer units. You cannot do so if you use a debug configuration.

Example: Image Acquisition from IDS Cameras

One of the most common uses of user filters is for communication with hardware, which does not (fully) support the standard GenICam industrial interface. Aurora Vision Studio comes with a ready example of such a user filter – for image acquisition from cameras manufactured by the IDS company. You can use this example as a reference when implementing support for your specific hardware.

The source code is located in the directory:

%PUBLIC%\Documents\Aurora Vision Studio 1.0 Professional\Sources\UserFilters\IDS

Here is a list of the most important classes in the code:

  • CameraManager – a singleton managing all connections with the IDS device drivers.
  • IDSCamera – a manager of a single image acquisition stream. It will be shared by multiple filters connected to the same device.
  • IDS_BaseClass – a common base class for all user filter classes.
  • IDS_GrabImage, IDS_GrabImage_WithTimeout, IDS_StartAcquisition – the classes of individual user filters.

The CameraManager constructor checks if an appropriate camera vendor's dll file is present in the system. The user filter project loads the library with the option of Delay-Loaded DLL turned on to correctly handle the case when the file is missing.

Requirement: To use the user filters for IDS cameras you need to install IDS Software Suite, which can be downloaded from IDS web page.

After the project is built in the appropriate Win32/x64 configuration, you will get the (global) user filters loaded to Aurora Vision Studio automatically. They will appear in the Libraries tab of the Toolbox, "User Filters" section.

Example: Using PCL library in Aurora Vision Studio

This example shows how to use PCL in an user filter.

The source code is located in the directory:

%PUBLIC%\Documents\Aurora Vision Studio 1.0 Professional\Sources\UserFilters\PCL

To run this example PCL Library must be installed and system PCL_ROOT must be defined.

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