[Epic] Markerless image tracking

[kalwalt]

Goal

Develop markerless image tracking for WebARKit — tracking natural images / planar surfaces without a pre-registered marker — building on the marker-based planar tracker.

Context

The marker-based Teblid / AKAZE / ORB / FREAK planar tracker is being made correct and robust (pose, projection, tracking-loss, examples) under the Marker-based tracking foundation milestone. That pipeline (feature detection → matching → homography → solvePnP → pose) is the stepping stone toward markerless tracking. This epic seeds the Markerless tracking milestone and will be broken into sub-issues as the approach is decided.

Scope / open questions (to refine)

• Define the "markerless" target: on-the-fly reference acquisition (use the first frame / a captured keyframe as the reference), matching against a database of natural images, or instant tracking of an arbitrary planar surface.
• Reference/keyframe management without a pre-supplied marker image.
• Extend the existing OCVT pipeline (TrackingPointSelector, homography, optical flow) to a markerless workflow.
• Evaluate approaches (instant tracking, plane detection, SLAM-lite).
• New example(s) in webarkit-testing demonstrating markerless tracking.
• WASM performance budget (detection cost per frame — cf. Restore downsampled feature detection + single-marker detection guard for performance (reconcile with ArtoolkitX OCVT) #44).

Foundation it builds on

• Pose/projection correctness: Tracked pose renders behind the camera (OpenCV vs OpenGL convention); matrixGL_RH throws #34, Asymmetric -2*f_x/screenWidth in cameraProjectionMatrix — origin of X-mirror unclear #35
• Tracking-loss detection: Tracking never declared lost when the marker leaves the frame (_valid/_isTracking stuck) #46
• Detection performance: Restore downsampled feature detection + single-marker detection guard for performance (reconcile with ArtoolkitX OCVT) #44
• Examples (webarkit-testing): Feature improves transmatrix #30, Fix CV->GL pose conversion in updateTrackable() #36, Fix crash when a marker is detected on the first frame (#37) #38

Status

Planning / north-star epic. Sub-issues to follow.