Adds ordering parameter for raycaster grid pattern (#483)

# Description

Allow changing of the indexing mode for the grid pattern of the
RayCaster. Possible options are:

- “xy”: the points are ordered with their y-dimension iterated over
first.
- “yx”: the points are ordered with their x-dimension iterated over
first.

## Type of change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have run all the tests with `./orbit.sh --test` and they pass
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.15.7"
+version = "0.15.8"
 
 # Description
 title = "ORBIT framework for Robot Learning"

source/extensions/omni.isaac.orbit/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.15.8 (2024-04-02)
+~~~~~~~~~~~~~~~~~~~
+
+Added
+^^^^^
+* Adds option to define ordering of points for the mesh-grid generation in the
+  :func:`omni.isaac.orbit.sensors.ray_caster.patterns.grid_pattern`. This parameter defaults to 'xy'
+  for backward compatibility.
+
+
 0.15.7 (2024-03-28)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sensors/ray_caster/patterns/patterns.py

@@ -25,17 +25,35 @@ def grid_pattern(cfg: patterns_cfg.GridPatternCfg, device: str) -> tuple[torch.T
 
     Returns:
         The starting positions and directions of the rays.
+
+    Raises:
+        ValueError: If the ordering is not "xy" or "yx".
+        ValueError: If the resolution is less than or equal to 0.
     """
+    # check valid arguments
+    if cfg.ordering not in ["xy", "yx"]:
+        raise ValueError(f"Ordering must be 'xy' or 'yx'. Received: '{cfg.ordering}'.")
+    if cfg.resolution <= 0:
+        raise ValueError(f"Resolution must be greater than 0. Received: '{cfg.resolution}'.")
+
+    # resolve mesh grid indexing (note: torch meshgrid is different from numpy meshgrid)
+    # check: https://github.com/pytorch/pytorch/issues/15301
+    indexing = cfg.ordering if cfg.ordering == "xy" else "ij"
+    # define grid pattern
     x = torch.arange(start=-cfg.size[0] / 2, end=cfg.size[0] / 2 + 1.0e-9, step=cfg.resolution, device=device)
     y = torch.arange(start=-cfg.size[1] / 2, end=cfg.size[1] / 2 + 1.0e-9, step=cfg.resolution, device=device)
-    grid_x, grid_y = torch.meshgrid(x, y, indexing="xy")
+    grid_x, grid_y = torch.meshgrid(x, y, indexing=indexing)
+
+    # store into ray starts
     num_rays = grid_x.numel()
     ray_starts = torch.zeros(num_rays, 3, device=device)
     ray_starts[:, 0] = grid_x.flatten()
     ray_starts[:, 1] = grid_y.flatten()
 
+    # define ray-cast directions
     ray_directions = torch.zeros_like(ray_starts)
     ray_directions[..., :] = torch.tensor(list(cfg.direction), device=device)
+
     return ray_starts, ray_directions
 
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sensors/ray_caster/patterns/patterns_cfg.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 import torch
 from collections.abc import Callable, Sequence
 from dataclasses import MISSING
+from typing import Literal
 
 from omni.isaac.orbit.utils import configclass
 
@@ -33,17 +34,39 @@ class GridPatternCfg(PatternBaseCfg):
     """Configuration for the grid pattern for ray-casting.
 
     Defines a 2D grid of rays in the coordinates of the sensor.
+
+    .. attention::
+        The points are ordered based on the :attr:`ordering` attribute.
+
     """
 
     func: Callable = patterns.grid_pattern
 
     resolution: float = MISSING
     """Grid resolution (in meters)."""
+
     size: tuple[float, float] = MISSING
     """Grid size (length, width) (in meters)."""
+
     direction: tuple[float, float, float] = (0.0, 0.0, -1.0)
     """Ray direction. Defaults to (0.0, 0.0, -1.0)."""
 
+    ordering: Literal["xy", "yx"] = "xy"
+    """Specifies the ordering of points in the generated grid. Defaults to ``"xy"``.
+
+    Consider a grid pattern with points at :math:`(x, y)` where :math:`x` and :math:`y` are the grid indices.
+    The ordering of the points can be specified as "xy" or "yx". This determines the outer and inner loop order
+    when iterating over the grid points.
+
+    * If *"xy"* is selected, the points are ordered with outer loop over "x" and inner loop over "y".
+    * If *"yx"* is selected, the points are ordered with outer loop over "y" and inner loop over "x".
+
+    For example, the grid pattern points with :math:`X = (0, 1, 2)` and :math:`Y = (3, 4)`:
+
+    * *"xy"* ordering: :math:`[(0, 3), (0, 4), (1, 3), (1, 4), (2, 3), (2, 4)]`
+    * *"yx"* ordering: :math:`[(0, 3), (1, 3), (2, 3), (1, 4), (2, 4), (2, 4)]`
+    """
+
 
 @configclass
 class PinholeCameraPatternCfg(PatternBaseCfg):