VLFeat - Documentation > MATLAB API > PLOTOP - vl_plotframe

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Documentation> MATLAB API> PLOTOP - vl_plotframe

VL_PLOTFRAME(FRAME) plots the feature frame FRAME. The frame can be either a 2D point, a circle, an oriented circle, an ellipse, or an oriented ellipse, as follows:

Point

  FRAME has 2 components. FRAME(1:2) are the x,y coordinates of the point.

Circle

  FRAME has 3 components. FRAME(1:2) are the x,y coordinates of the center and FRAME(3) is its radius.

Oriented circle

  FRAME has 4 components. FRAME(1:2) are the x,y coordiantes of the center of the circle, FRAME(3) is the radius, and FRAME(4) is the orientation, expressed as a rotation in radians of the standard oriented frame (see below). Positive rotations appear clockwise since the image coordinate system is left-handed.

Ellipse

  FRAME has 5 components. FRAME(1:2) are the x,y coordiantes of the center and FRAME(3:5) are the elements S11, S12, S22 of a 2x2 covariance matrix S (a positive semidefinite matrix) defining the ellipse shape. The ellipse is the set of points {x + T: x' inv(S) x = 1}, where T is the ellipse center.

Oriented ellipse

  FAME has 6 components. FRAME(1:2) are the coordiantes T=[x;y] of the center. FRAME(3:6) is the column-wise stacking of a 2x2 matrix A. The oriented ellipse is obtained by applying the affine transformation (A,T) to the standard oriented frame (see below).

A standard unoriented frame is a circle of unit radius centered at the origin; a standard oriented frame is the same, but marked with a radius pointing towards the positive Y axis (i.e. downwards according to the standard MATLAB image reference frame) to represent the frame orientation. All other frames can be obtained as affine transformations of these two. In the case of unoriented frames, this transformation is ambiguous up to a rotation.

VL_PLOTFRAME(FRAMES), where FRAMES is a D x N matrix, plots N frames, one per column. This is significantly more efficient than looping over frames explicitly.

H = VL_PLOTFRAME(...) returns the handle H of the graphical object representing the frames.

VL_PLOTFRAME(FRAMES,...) passes any extra argument to the underlying plotting function. The first optional argument, in particular, can be a line specification string such as the one used by PLOT().

See also: SIFT, covariant detectors, VL_FRAME2OELL(), VL_HELP().