Add section on obtaining vision measurements for pose estimation

[jasondaming]

Summary

Creates a comprehensive guide for using AprilTags for robot pose estimation, addressing the lack of documentation on obtaining vision measurements for pose estimators.

Changes

New file: apriltag-pose-estimation.rst in vision processing section

• Explains the complete process: detect tags → get tag pose → calculate robot pose → apply measurement
• Documents PhotonVision's PhotonPoseEstimator with code examples in Java, C++, and Python
• Documents Limelight's MegaTag with NetworkTables API usage
• Important sections covering:
  • Timestamp handling (using capture time, not processing time)
  • Standard deviation scaling based on distance, tag count, and ambiguity
  • Rejecting bad measurements (no tags, high ambiguity, unrealistic poses)
• Guidance for custom vision implementations

Modified: state-space-pose-estimators.rst

• Replaced detailed vision measurement section with concise seealso link
• Keeps pose estimator docs focused on the API, refers to vision docs for obtaining measurements

Modified: apriltag/index.rst

• Added new article to table of contents

Organization

Per review feedback, vision measurement content now lives in vision processing docs (where teams look for vision guidance) rather than pose estimator docs (which focus on the API).

Fixes #1413

[sciencewhiz]

Thinking about this more, I think most of this probably belongs in the vision processing docs, rather then here.

[Gold856]

Looks good to me, just some minor changes to match current photonlib.

[Gold856]

LL link is broken.

[Gold856]

Suggested change

	camera, // PhotonCamera
	robotToCam // Transform3d from robot to camera
	);
	// In your periodic method
	var result = photonPoseEstimator.update();
	if (result.isPresent()) {
	var estimatedPose = result.get();
	poseEstimator.addVisionMeasurement(
	estimatedPose.estimatedPose.toPose2d(),
	estimatedPose.timestampSeconds
	);
	}
	robotToCam // Transform3d from robot to camera
	);
	// In your periodic method
	for (var result : camera.getAllUnreadResults()) {
	var estimate = photonPoseEstimator.update(result);
	if (estimate.isPresent()) {
	var estimatedPose = estimate.get();
	poseEstimator.addVisionMeasurement(
	estimatedPose.estimatedPose.toPose2d(),
	estimatedPose.timestampSeconds
	);
	}
	}

[Gold856]

I'll PR an update to this when we've updated the API.

[Gold856]

Suggested change

	camera, // photon::PhotonCamera
	robotToCam // frc::Transform3d from robot to camera
	};
	// In your periodic method
	auto result = photonPoseEstimator.Update();
	if (result) {
	poseEstimator.AddVisionMeasurement(
	result->estimatedPose.ToPose2d(),
	result->timestamp
	);
	}
	robotToCam // frc::Transform3d from robot to camera
	};
	// In your periodic method
	for (const auto& result : camera.GetAllUnreadResults()) {
	auto estimate = photonPoseEstimator.Update(result);
	if (estimate) {
	poseEstimator.AddVisionMeasurement(
	estimate->estimatedPose.ToPose2d(),
	estimate->timestamp
	);
	}
	}

[Gold856]

Suggested change

	var result = photonPoseEstimator.update();
	if (result.isPresent()) {
	var estimatedPose = result.get();
	// Check if pose is reasonable (within field boundaries with tolerance)
	double margin = 0.5; // meters of tolerance for edge measurements
	if (estimatedPose.estimatedPose.getX() >= -margin &&
	estimatedPose.estimatedPose.getX() <= fieldLayout.getFieldLength() + margin &&
	estimatedPose.estimatedPose.getY() >= -margin &&
	estimatedPose.estimatedPose.getY() <= fieldLayout.getFieldWidth() + margin) {
	poseEstimator.addVisionMeasurement(
	estimatedPose.estimatedPose.toPose2d(),
	estimatedPose.timestampSeconds
	);
	}
	}
	for (var result : camera.getAllUnreadResults()) {
	var estimate = photonPoseEstimator.update(result);
	if (estimate.isPresent()) {
	var estimatedPose = estimate.get();
	// Check if pose is reasonable (within field boundaries with tolerance)
	double margin = 0.5; // meters of tolerance for edge measurements
	if (estimatedPose.estimatedPose.getX() >= -margin &&
	estimatedPose.estimatedPose.getX() <= fieldLayout.getFieldLength() + margin &&
	estimatedPose.estimatedPose.getY() >= -margin &&
	estimatedPose.estimatedPose.getY() <= fieldLayout.getFieldWidth() + margin) {
	poseEstimator.addVisionMeasurement(
	estimatedPose.estimatedPose.toPose2d(),
	estimatedPose.timestampSeconds
	);
	}
	}
	}

[Gold856]

Suggested change

	auto result = photonPoseEstimator.Update();
	if (result) {
	// Check if pose is reasonable (within field boundaries with tolerance)
	units::meter_t margin = 0.5_m; // tolerance for edge measurements
	if (result->estimatedPose.X() >= -margin &&
	result->estimatedPose.X() <= fieldLayout.GetFieldLength() + margin &&
	result->estimatedPose.Y() >= -margin &&
	result->estimatedPose.Y() <= fieldLayout.GetFieldWidth() + margin) {
	poseEstimator.AddVisionMeasurement(
	result->estimatedPose.ToPose2d(),
	result->timestamp
	);
	}
	}
	for (const auto& result : camera.GetAllUnreadResults()) {
	auto estimate = photonPoseEstimator.Update(result);
	if (estimate) {
	// Check if pose is reasonable (within field boundaries with tolerance)
	units::meter_t margin = 0.5_m; // tolerance for edge measurements
	if (estimate->estimatedPose.X() >= -margin &&
	estimate->estimatedPose.X() <= fieldLayout.GetFieldLength() + margin &&
	estimate->estimatedPose.Y() >= -margin &&
	estimate->estimatedPose.Y() <= fieldLayout.GetFieldWidth() + margin) {
	poseEstimator.AddVisionMeasurement(
	estimate->estimatedPose.ToPose2d(),
	estimate->timestamp
	);
	}
	}
	}

[Gold856]

Suggested change

	- PhotonVision: ``result.timestampSeconds``
	- PhotonVision: ``estimate.timestampSeconds``