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Conventions

Author
  Andrea Vedaldi

This page summarizes some of the conventions used by the library.

# Matrix and image storage conventions

If not otherwise specified, matrices in VLFeat are stored in memory in column major order. Given a matrix \([A_{ij}] \in \real^{m \times n}\), this amounts of enumerating the elements one column per time: \(A_{11}, A_{21}, \dots, A_{m1}, A_{12}, \dots, A_{mn}\). This convention is compatible with Fortran, MATLAB, and popular numerical libraries.

Matrices are often used in the library to pack a number data vectors \(\bx_1,\dots,\bx_n \in \real^m\) of equal dimension together. These are normally stored as the columns of the matrix:

\[ X = \begin{bmatrix} \bx_1, \dots, \bx_n \end{bmatrix}, \qquad X \in \real_{m\times n} \]

In this manner, consecutive elements of each data vector \(\bx_i\) is stored in consecutive memory locations, improving memory access locality in most algorithms.

Images \(I(x,y)\) are stored instead in row-major order, i.e. one row after the other. Note that an image can be naturally identified as a matrix \(I_{yx}\), where the vertical coordinate \(y\) indexes the rows and the horizontal coordinate \(x\) the columns. The image convention amounts to storing this matrix in row-major rather than column-major order, which is in conflict with the rule given above. The reason for this choice is that most image processing and graphical libraries assume this convention; it is, however, not the same as MATLAB's.