hou.Matrix4 class

A 4×4 matrix of floating point values.

See also: hou.Vector4, hou.Vector3, hou.hmath.identityMatrix, hou.hmath 3 more , hou.ObjNode, hou.Vector3, hou.hmath

4×4 matrices are typically used in Houdini to represent a 3D transformation (e.g. some combination of rotation, scaling, shearing, translation, etc.). A single matrix compactly represents a transformation, and is much easier to deal with than multiple translate, rotate, scale, shear, transformation order, and rotation order values.

Note that Houdini’s matrices are stored in row-major format, and vectors that are multiplied with matrices are treated as row vectors. So, if p is a hou.Vector4 representing a point and M is a Matrix4, you write p*M, not M*p. Similarly, p*M1*M2 will first transform p by M1, and then transform it by M2.

You can multiply Matrix4s by Vector3s or Vector4s. If you multiply by a Vector3, it is the same as multiplying by a Vector4 where the fourth component is 1.

To transform a vector (as opposed to a point), you need to multiply by the inverse transpose of the matrix. For example, if p is a hou.Vector3 object representing a point (a position in space) and v is a hou.Vector3 object representing a vector (a direction with a length but no fixed location in space, and m is a transform matrix, you would write:

p * m
v * m.inverted().transposed()

>>> m = hou.hmath.buildTranslate(1, 1, 2)
>>> m
<hou.Matrix4 [[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 1, 0], [1, 1, 2, 1]]>
>>> p = hou.Vector3(1, 2, 3)
>>> p * m
<hou.Vector3 [2, 3, 5]>

Both Vex and the UT_DMatrix4 class in the Houdini Development Kit (HDK) also store matrices in row-major format.