docs/features/objects/transform.md

@@ -133,3 +133,39 @@ 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`/`rot`/`scale` and `dloc`/`drot`/`dscale` are combined into a whole transform as follows:
+
+1. `loc` and `dloc` are simply added together as vectors.
+2. `rot` and `drot` are combined by first creating a 3x3 matrix for each, and then multiplying them together.  The multiplication order is equivalent to first applying `rot`'s rotation to the object and then applying `drot`'s rotation.
+3. `scale` and `dscale` are simply component-wise multiplied together.
+4. Finally, the results of the previous steps are all combined together into a single transform matrix.
+
+> 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 this is in `blenkernel/intern/object.cc` in the function `BKE_object_apply_mat4_ex`.