docs/features/objects/transform.md

@@ -133,3 +133,58 @@ be more or less complex. Steps in **bold** are rather mandatory.
 6.  **recalcData** : Flush updates to Blender data when needed
 7.  **headerPrint** : Send text to the header
 8.  **viewRedrawForce** : Send redraw events to the proper screen area
+
+## Stored Transforms
+
+Each object has a set of source-of-truth (i.e. not derived or cached) transforms
+stored directly in its DNA, in the `Object` struct in *DNA_object_types.h*.
+These specify the aspects of an object's transforms that are independent of
+other objects in the scene, and which do not result from constraints, etc.
+
+### Loc/Rot/Scale
+
+The simplest of these are the `loc`, `rot`, and `scale` properties, which form
+the base transform of an object.  These are the familiar properties that users
+use to set and animate an object's position in 3D space.
+
+`rot` also has counterparts for quaternion and axis-angle rotations: `quat` for
+quaternions and `rotAxis` + `rotAngle` for axis-angle.  These are used for the
+object's rotation when `rotmode` is set to the corresponding rotation mode.
+
+### Delta Transforms
+
+After the base transform, there are the delta transforms, stored in the `dloc`,
+`drot`, and `dscale` properties.  These can also be set by the user, and
+effectively define a "rest position" for the object.
+
+`loc`/`dloc`, `rot`/`drot`, and `scale`/`dscale` are first combined as pairs
+(e.g. `loc` with `dloc`), and then assembled together into a transform matrix
+afterward.
+
+> NOTE: Quaternion and axis-angle rotation modes also have corresponding `d*`
+> properties for delta transforms, for when the object is in those respective
+> rotation modes.  They are applied the same way as `drot`.
+
+The relevant code for this is in *blenkernel/intern/object.cc* in the functions
+`BKE_object_scale_to_mat3`, `BKE_object_rot_to_mat3`, `BKE_object_to_mat3`, and
+`BKE_object_to_mat4`.
+
+### Parent-Inverse Matrix
+
+Finally, there is one additional transform step that is internal to an object:
+the parent-inverse matrix.
+
+The parent-inverse matrix is a 4x4 matrix stored in the property `parentinv`.
+Importantly, it is *not* the transform (inverse or otherwise) of the object's
+parent.  Rather, it's an arbitrary 4x4 matrix that gets applied to an object's
+transforms if and only if it has a parent.
+
+Its intended purpose (and the source of its name) is to keep a child object
+where it is visually when a parent-child relationship is created, without
+changing the child's loc/rot/scale values.  To accomplish this, the matrix is
+automatically set to the inverse of the parent's transform *at the time the
+parent-child relationship is created*.  It is specifically *not* updated to
+continuously match the parent's inverse.
+
+The relevant code for applying this transform is in
+*blenkernel/intern/object.cc* in the function `BKE_object_apply_mat4_ex`.