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Add docs for how object's stored transforms work #23

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Nathan Vegdahl wants to merge 4 commits from nathanvegdahl/blender-developer-docs:stored_object_transforms into main
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docs/features/objects/transform.md
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@ -137,7 +137,7 @@ be more or less complex. Steps in **bold** are rather mandatory.
## 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`.
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.
@ -157,23 +157,15 @@ 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.
`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
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`.
@ -194,5 +186,5 @@ 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
The relevant code for applying this transform is in *blenkernel/intern/object.cc* in the function
`BKE_object_apply_mat4_ex`.