Message Filters

Agnocast provides message synchronization filters compatible with the ROS 2 message_filters API. These allow you to synchronize messages from multiple Agnocast topics based on their timestamps.

Component	Support
Synchronizer (2–9 inputs)	Supported
ExactTime policy	Supported
ApproximateTime policy	Supported
Subscriber filter	Supported
PassThrough filter	Supported
Cache	Not supported
Chain	Not supported

Migrating a Synchronizer

Before (rclcpp message_filters)

```cpp

include

include

include

using MsgA = sensor_msgs::msg::Image; using MsgB = sensor_msgs::msg::CameraInfo; using SyncPolicy = message_filters::sync_policies::ExactTime;

class MySyncNode : public rclcpp::Node { message_filters::Subscriber sub_a_; message_filters::Subscriber sub_b_; message_filters::Synchronizer sync_;

void callback( const MsgA::ConstSharedPtr & a, const MsgB::ConstSharedPtr & b) { // Process synchronized messages }

public: MySyncNode() : Node("sync_node"), sub_a_(this, "/image"), sub_b_(this, "/camera_info"), sync_(SyncPolicy(10), sub_a_, sub_b_) { sync_.registerCallback(&MySyncNode::callback, this); } }; ```

After (Agnocast Stage 1)

```cpp

include "agnocast/agnocast.hpp" // (1)

using MsgA = sensor_msgs::msg::Image; using MsgB = sensor_msgs::msg::CameraInfo; using SyncPolicy = agnocast::message_filters::sync_policies::ExactTime; // (2)

class MySyncNode : public rclcpp::Node { agnocast::message_filters::Subscriber sub_a_; // (3) agnocast::message_filters::Subscriber sub_b_; // (3) agnocast::message_filters::Synchronizer sync_; // (4)

void callback( const agnocast::ipc_shared_ptr & a, // (5) const agnocast::ipc_shared_ptr & b) { // Process synchronized messages }

public: MySyncNode() : Node("sync_node"), sub_a_(this, "/image"), sub_b_(this, "/camera_info"), sync_(SyncPolicy(10), sub_a_, sub_b_) { sync_.registerCallback(&MySyncNode::callback, this); } }; ```

Key changes:

1. Include changes to agnocast/agnocast.hpp
2. Sync policy namespace changes to agnocast::message_filters::sync_policies
3. message_filters::Subscriber → agnocast::message_filters::Subscriber
4. message_filters::Synchronizer → agnocast::message_filters::Synchronizer
5. Callback receives agnocast::ipc_shared_ptr<const T> instead of T::ConstSharedPtr

After (Agnocast Stage 2)

```cpp

include "agnocast/agnocast.hpp"

using MsgA = sensor_msgs::msg::Image; using MsgB = sensor_msgs::msg::CameraInfo; using SyncPolicy = agnocast::message_filters::sync_policies::ExactTime;

class MySyncNode : public agnocast::Node // (1) { agnocast::message_filters::Subscriber sub_a_; agnocast::message_filters::Subscriber sub_b_; agnocast::message_filters::Synchronizer sync_;

void callback( const agnocast::ipc_shared_ptr & a, const agnocast::ipc_shared_ptr & b) { // Process synchronized messages }

public: MySyncNode() : Node("sync_node"), sub_a_(this, "/image"), sub_b_(this, "/camera_info"), sync_(SyncPolicy(10), sub_a_, sub_b_) { sync_.registerCallback(&MySyncNode::callback, this); } }; ```

Additional change from Stage 1:

1. Base class changes to agnocast::Node

ApproximateTime Policy

The migration steps are the same as ExactTime — replace ExactTime with ApproximateTime in the policy type:

cpp using SyncPolicy = agnocast::message_filters::sync_policies::ApproximateTime<MsgA, MsgB>;

All parameter APIs from rclcpp's message_filters::sync_policies::ApproximateTime are supported with identical signatures:

```cpp SyncPolicy policy(10); // queue size policy.setMaxIntervalDuration(rclcpp::Duration(0, 100'000'000)); // 100ms max policy.setAgePenalty(0.5); policy.setInterMessageLowerBound(0, rclcpp::Duration(0, 30'000'000)); // 30ms for input 0

agnocast::message_filters::Synchronizer sync(policy, sub_a_, sub_b_); ```

Method	Description	Default
setMaxIntervalDuration(duration)	Maximum time difference between matched messages	No limit
setAgePenalty(double)	Weight given to message age in matching (must be >= 0)	0.1
setInterMessageLowerBound(i, duration)	Minimum expected interval between messages for input i	0 (auto-estimated)

Migrating a PassThrough Filter

PassThrough forwards messages without filtering — useful when you already have a subscription and want to feed it into a Synchronizer. The API is identical to rclcpp's message_filters::PassThrough.

Before (rclcpp message_filters)

```cpp message_filters::PassThrough pass_through;

// Feed messages manually pass_through.add(message); // message is MsgType::ConstSharedPtr ```

After (Agnocast)

```cpp agnocast::message_filters::PassThrough pass_through; // (1)

// Feed messages manually pass_through.add(message); // message is agnocast::ipc_shared_ptr // (2) ```

Key changes:

1. message_filters::PassThrough → agnocast::message_filters::PassThrough
2. add() takes agnocast::ipc_shared_ptr<const T> instead of T::ConstSharedPtr