diff --git a/docs/features/objects/transform.md b/docs/features/objects/transform.md index 1a0911b7..f6f9182c 100644 --- a/docs/features/objects/transform.md +++ b/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`.