A deterministic, constraint-aware grasp sampling library for parallel jaw grippers. Built on OpenCASCADE (OCCT) for geometrically exact CAD surface operations with FCL and Embree for collision checking. Unlike conventional samplers, sampling areas are defined and constrained before the sampling phase, allowing solutions with hard geometric and environmental requirements. Contact points are computed on actual CAD surfaces giving geometrically exact results. Currently supports CAD-based geometry input only.
The entry point for CAD geometry into the pipeline. Loads workpiece geometry from URDF or STEP files and extracts full topology — surfaces, edges, and corners with their connectivity relationships. The resulting topology and face map are used throughout the pipeline.
The nature of the sampling pipeline and CAD file geometry require surfaces to be divided in order to create more opposing surface candidates. Shape Refiner handles this with a three policy approach: U-periodic surface splitting (surfaces that wrap around a closed axis such as full cylinders or cones, split along the periodic direction before other processing), arc length based splitting, and area ratio based splitting.
In addition it removes enclaves — small surface features that would confuse the sampling pipeline without contributing meaningful grasp candidates in real applications. Shape healing is also applied at this stage before the refined geometry is passed downstream.
A configurable antipodal contact pair finder with filter and constraint awareness. It considers limitations coming from overlap areas of constraints and filter evaluations before sampling, ensuring only valid surface regions are sampled. Uses a grid based approach to achieve broad coverage without generating redundant clustered pairs.
Approach direction validity is determined here. Implemented in four phases.
Phase one builds a radial surface map around each contact point by casting classification rays in decreasing rings starting from finger length radius down to finger radius. Each ray classifies the surrounding surface as flat, elevated, or a graspable cliff. This analysis drives approach direction candidates.
Phase two clusters the candidates, scores them by grippable arc fraction, and applies randomization to diversify the result set.
Phase three runs full collision checks against the primary shape and exclusion zones via FCL and Embree. This is the second point in the pipeline where constraints directly influence results, the first being the Contact Sampler.
Phase four performs self-collision checks against secondary shapes as a separate step.
Candidates surviving all four phases are returned as grasp candidates for downstream processing.
Eliminates ungraspable surfaces or parts of surfaces before the sampling phase to avoid wasted computation on invalid grasp candidates. Surface filters operate on whole surfaces, region filters operate on areas within surfaces.
Currently base classes and interfaces are defined but pipeline integration is planned.
Limits or eliminates surfaces from sampling using 3D collision volumes. Unlike filters, constraints construct 3D shapes and determine forbidden areas from their collision geometry. The same shapes serve dual purpose — defining forbidden sampling regions and acting as collision objects during the Angle Finder phase.
Constraints can represent real objects like ground planes and fixtures or mission specific forbidden zones like weld seams and screw holes.
Grasp candidates example generated by hold_and_weld_gripper_sampler:
ros2 run hold_and_weld_gripper_sampler grasp_finder_node --config <path_to_yaml>See config/grasp_finder_example.yaml for a fully documented configuration example
covering all supported workpiece types and constraint configurations.
- CAD geometry input only. Mesh and point cloud support is planned.
- Parallel jaw grippers only. Other gripper types are not supported.
- Tested on box, prism, and small cylinder workpieces. Complex organic geometry is not yet validated.
- Ground plane rejection uses a halfspace model. Smart ground detection based on workpiece topology is planned.
- Filter pipeline is defined but not yet wired into the active pipeline.
- Asymmetric gripper opening not supported. Both fingers are assumed to travel equal distances from closed position.