Convert qLD to CSR format.

PiperOrigin-RevId: 728196818
Change-Id: I31bdf32e251293284426ed345019f27fd051a0e1

doc/APIreference/functions.rst

@@ -423,7 +423,8 @@ Get name of object with the specified mjtObj type and id, returns NULL if name n
 
 .. mujoco-include:: mj_fullM
 
-Convert sparse inertia matrix M into full (i.e. dense) matrix.
+Convert sparse inertia matrix ``M`` into full (i.e. dense) matrix.
+|br| ``dst`` must be of size ``nv x nv``, ``M`` must be of the same size as ``mjData.qM``.
 
 .. _mj_mulM:
 

doc/APIreference/functions_override.rst

@@ -233,6 +233,11 @@ found, the function will return ``distmax`` and ``fromto``, if given, will be se
    In order to determine whether a geom pair uses ``mjc_Convex``, inspect the table at the top of
    `engine_collision_driver.c <https://github.com/google-deepmind/mujoco/blob/main/src/engine/engine_collision_driver.c>`__.
 
+.. _mj_fullM:
+
+Convert sparse inertia matrix ``M`` into full (i.e. dense) matrix.
+|br| ``dst`` must be of size ``nv x nv``, ``M`` must be of the same size as ``mjData.qM``.
+
 .. _mj_mulM:
 
 This function multiplies the joint-space inertia matrix stored in mjData.qM by a vector. qM has a custom sparse format

doc/includes/references.h

@@ -272,7 +272,7 @@ struct mjData_ {
   mjtNum* qM;                // total inertia (sparse)                           (nM x 1)
 
   // computed by mj_fwdPosition/mj_factorM
-  mjtNum* qLD;               // L'*D*L factorization of M (sparse)               (nM x 1)
+  mjtNum* qLD;               // L'*D*L factorization of M (sparse)               (nC x 1)
   mjtNum* qLDiagInv;         // 1/diag(D)                                        (nv x 1)
 
   // computed by mj_collisionTree
@@ -305,7 +305,7 @@ struct mjData_ {
   mjtNum* subtree_angmom;    // angular momentum about subtree com               (nbody x 3)
 
   // computed by mj_Euler or mj_implicit
-  mjtNum* qH;                // L'*D*L factorization of modified M               (nM x 1)
+  mjtNum* qH;                // L'*D*L factorization of modified M               (nC x 1)
   mjtNum* qHDiagInv;         // 1/diag(D) of modified M                          (nv x 1)
 
   // computed by mj_resetData

doc/programming/simulation.rst

@@ -614,12 +614,14 @@ essential to keep it in mind at all times. All MuJoCo utility functions that ope
 :ref:`mju_mulMatMat`, :ref:`mju_mulMatVec` etc. assume this matrix layout. For vectors there is of course no
 difference between row-major and column-major formats.
 
+.. TODO(tassa): update this section when qM is migrated to CSR.
 When possible, MuJoCo exploits sparsity. This can make all the difference between O(N) and O(N^3) scaling. The inertia
-matrix ``mjData.qM`` and its LTDL factorization ``mjData.qLD`` are always represented as sparse, using a custom
-indexing format designed for matrices that correspond to tree topology. The functions :ref:`mj_factorM`,
-:ref:`mj_solveM`, :ref:`mj_solveM2` and :ref:`mj_mulM` are used for sparse factorization, substitution and
-matrix-vector multiplication. The user can also convert these matrices to dense format with the function
-:ref:`mj_fullM` although MuJoCo never does that internally.
+matrix ``mjData.qM`` and its LTDL factorization ``mjData.qLD`` are always represented as sparse. ``qM`` uses a custom
+indexing format designed for matrices that correspond to tree topology, while ``qLD`` uses the standard CSR format.
+``qM`` will be migrated to CSR in and upcoming change. The functions :ref:`mj_factorM`, :ref:`mj_solveM`,
+:ref:`mj_solveM2` and :ref:`mj_mulM` are used for sparse factorization, substitution and matrix-vector multiplication.
+The user can also convert these matrices to dense format with the function :ref:`mj_fullM` although MuJoCo never does
+that internally.
 
 The constraint Jacobian matrix ``mjData.efc_J`` is represented as sparse whenever the sparse Jacobian option is
 enabled. The function :ref:`mj_isSparse` can be used to determine if sparse format is currently in use. In that case