UE Bone Structure and UE Mannequin
Thanks so much SAM-tak for they works -> https://github.com/xavier150/Blender-For-UnrealEngine-Addons/pull/175
This addon include a modified version of the Blender standard fbx I/O api, instead of using the original one.
Basically, the X-axis positive direction is the bone direction, but the right side of the body is inverted on all axes compared to the corresponding left side.
In addition, the pelvis bone and foot bone have the X-axis pointing straight up. (The right foot is further inverted, so the X-axis points straight down)
Also, for some reason, the thigh bone and calf bone have the opposite direction of the bone direction as the X-axis on the left side, and the right side is inverted, so the bone direction is the negative X-axis.
hus, depending on the part, the correspondence between the bone direction and the transform coordinate axis is different, and it is difficult to handle in Blender, where you cannot set the bone direction and transform coordinate axis independently.
First you need to set tht export procedure to UE Standard
As described above, in UE, the right side bones are inverted on all axes compared to the left side, so this is an option to invert the coordinate axes of the right side bones.
Even if compatibility with Mannequin is not important, this option is useful if you want to use the 'Mirror' function of the Physics Asset Editor, which assumes that the corresponding opposite side bones have all axes inverted.
Furthermore, it applies the hierarchy-specific alignment process mentioned earlier. That is, it makes the thigh and calf bones face the opposite direction, and the pelvis and foot bones face straight up.
This can only be enabled if 'Revert direction of symmetry right side bones' is enabled.
UE does not support bone animation with individual scale values for each XYZ axis ('non-uniform') by default.
You can enable it by checking the 'Preserve Local Transform' option in the import settings, but it is better not to use it as it may cause performance issues or other problems.
However, in some cases, such as animations generated by Visual Keying for animation retargeting, you may unintentionally end up with non-uniform scale animations. Stretch bone constraints can also cause these unintended non-uniform scale animations.
And it occurs taking a posture that is 180 degrees inverted from what you see in Blender, as shown below.
By turning on this feature, you can force a uniform scale value when exporting, instead of correcting non-uniform scale values
- BFU Docs: Skeleton & Root bone
- BFU Docs: Unit Scale
- For community support and discussions, join the discord BleuRaven Side Projects.