Non-canonical matrix-vector product

[VladislavShirinkinITEM]

XbimEssentials/Xbim.Common/Geometry/XbimVector3D.cs

Line 188 in ac8b909

public static XbimVector3D Multiply(XbimVector3D vec, XbimMatrix3D m)

See for example https://mathinsight.org/matrix_vector_multiplication

[andyward]

Can you be clear about what the issue relates to? If there's an issue it would help to see a failing test case

[VladislavShirinkinITEM]

This relates to integration with other projects, which uses canonical matrix operations. OpenGL and DirectX shaders for example.
If we have many transforms we should know that multiplication order is different
The case:

1. M1 - element transform
2. M2 - model transform
3. M3 - world transform

multiplication order in GLSL to get screen coords from 3d-space:

$$u_1 = M_3 \times M_2 \times M_1 \times v$$

using xBim matrices we'll write

$$u_2 = v \times M_1 \times M_2 \times M_3$$

that because of

$$(M_1 \times M_2)^T = M_2^T \times M_1^T$$

Difference between u1 and u2 is transposition, but vector is just list of numbers we can't see difference.

[andyward]

Any thoughts @martin1cerny ? This is above my pay grade

I'm unsure of the implications. i.e. what is broken? what is this solution? what will break if we apply a solution? What test cases do we have around this.