The correct bone data to export from blender to view in ogl?

Question

Edit (Original post below):

So I have come up with the following code. I can export the mesh, bone structure and animations. I can animate a simple skeleton. But for some reason if I animate more than one bone, something goes wrong and the arm will move in the wrong axis.

My cpp code is here: http://kyuu.co.uk/so/main.cpp

My python export code is here: http://kyuu.co.uk/so/test.py

Could someone please tell me what I am doing wrong? I think it might be something to do with the bone roll in blender. I have seen many posts about that.

Thanks.

(Original post:)

I have been working on this problem for a while now and still cannot figure out what I am missing, so I am hoping someone kind will help me :3

Right, I have something like the following code in my application:

class bone {
    bone * child;
    Eigen::Matrix4f local_bind_pose; // this i read from a file
    Eigen::Matrix4f final_skinning_matrix; // i then transform each vertex by this matrix

    // temporary variables
    Eigen::Matrix4f inv_bind_pose;
    Eigen::Matrix4f world_bind_pose;
}

I.e. a simple bone hierarchy.

I believe I can work out the inv_bind_pose with:

world_bind_pose = bone.parent.world_bind_pose * local_bind_pose
inv_bind_pose = world_bind_pose.inverse()

I know that the bind_pose must be relative to the parent bone.

I know that blender is z = up and I am using y = up.

But I cannot get this information exported from blender. I am using version 2.56.3.

Would the rotation part of the matrix be bone.matrix_local? Would the translation part be bone.tail() - bone.head()?

What about bone roll? It seems that does affect the result.

Some references:

http://www.gamedev.net/topic/571044-skeletal-animation-bonespace

http://blenderartists.org/forum/showthread.php?209221-calculate-bone-location-rotation-from-fcurve-animation-data

http://www.blender.org/development/release-logs/blender-240/how-armatures-work

http://code.google.com/p/gamekit/source/browse/trunk/Engine/Loaders/Blender/gkSkeletonLoader.cpp?spec=svn482&r=482

Thank-you so much!

In blender > 2.6, you can use bpy_extras.io_utils.axis_conversion (see: blender.org/documentation/blender_python_api_2_66a_release/…) to convert to openGL space. — Exilyth, May 13 '13 at 18:49

score 1 · Answer 1 · answered Sep 14 '22 at 18:22

We use blender bones extensively. I think you might find this snippet useful.

import gzip
import struct

import bpy

groups = [x.name for x in bpy.context.object.vertex_groups]
rig = bpy.context.object.parent

buf = bytearray()
buf.extend(struct.pack('ii', len(groups), 60))

for i in range(60):
    bpy.context.scene.frame_set(i)
    for name in groups:
        base = rig.pose.bones[name].bone.matrix_local.inverted()
        mat = rig.pose.bones[name].matrix @ base
        x, y, z = mat.to_translation()
        rw, rx, ry, rz = mat.to_quaternion()
        buf.extend(struct.pack('3f4x4f', x, y, z, rx, ry, rz, rw))

open('output.rig.gz', 'wb').write(gzip.compress(buf))

This exports blender bones directly for the GPU.

Here we use it

This script can be run in Object Mode. I hope it helps.

The correct bone data to export from blender to view in ogl?

1 Answers1