Documentation>MATLAB API>SIFT - vl_frame2oell

EFRAME = VL_FRAME2OELL(FRAME) converts the generic FRAME to an oriented ellipses EFRAME. FRAME and EFRAME can be matrices, with one frame per column.

A frame is either a point, a disc, an oriented disc, an ellipse, or an oriented ellipse. These are represented respectively by 2, 3, 4, 5 and 6 parameters each, as described in VL_PLOTFRAME(). An oriented ellipse is the most general geometric frame; hence, there is no loss of information in this conversion.

If FRAME is an oriented disc or ellipse, then the conversion is immediate. If, however, FRAME is not oriented (it is either a point or an unoriented disc or ellipse), then an orientation must be assigned. The orientation is chosen in such a way that the affine transformation that maps the standard oriented frame into the output EFRAME does not rotate the Y axis. If frames represent detected visual features, this convention corresponds to assume that features are upright.

If FRAME is a point, then the output is an ellipse with null area.

See: <a href="matlab:vl_help('tut.frame')">feature frames</a>, VL_PLOTFRAME(), VL_HELP().