hmm, both the normals and the tangents are in model space. Normals will always be the same for Spine animations but tangents are unknown & have to be calcluated. They are only used for bump mapping though.
So for bump mapping the shader ultimately needs to work out a normal direction in worldspace to bounce light off correctly. This is done by reading a direction from the normal map texture, and then converting it into world space.
To do this for any vert you need the model space normal (which yep will be (0,0,-1) for spine sprites), plus the modelspace tangent which can be any perpendicular direction to that normal. The tangent will let the vert know what way 'up' its normal map textures is by pointing along the direction of its mapped texture.
If you had a spine animation where every textured mesh in it had its texture aligned with the screen (ie not rotated / just dragged and dropped from the image folder) then all the tangents would be (1,0,0).
If one of those meshes was rotated by 90cw then its verts tangents should be (0,1,0). If the tangent is still (1,0,0) then the world space normal caluclated by the shader will be incorrect (ie also 90 degrees out), so you get some meshes lit incorrectly like the dragon above.
So for correct bump mapped lighting in Spine we dont need to pass the normals to the shader in or even store them on the mesh, but we do need to calculate tangents using something like that tangent sovler. They might be a more optimised way of doing it though!
