# Coordinate System, Matrix Math, etc#

Coordinate System: Right handed, Y-Up

Matrix - Matrices are post multiplied (x * M * V * P) - Row addressable M[RowIndex][ColumnIndex] - Matrix stored as row major m-m = first row, m-m = second row

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. From http://www.sidefx.com/docs/houdini/hom/hou/Matrix4

IMPORTANT however, in the houdini network editor, some nodes like channel multiply can be premutliplied/postmultipleid. For left to right inputs A, B, C => - Post-multiply (eg Multiplynode) means v * C * B * A - Pre-multiply (eg Transform node) means v * A * B * C

are connected in a left to right order bc they are “post-multiply implying a premultiply order (eg: WorldSpace * ObjectSpace * v). Others like transform give you the option to premultiply or post multiply

Last update: July 3, 2020