(make-line <number x0> <number y0> <number x1> <number y1> . <number contrast>)Given <number x0>, <number y0>, <number x1>, <number y1>, i.e., the x and y locations of a line segment's initial and terminal endpoints, make-line returns a <sexpr> of type <line>. make-line takes an optional argument, <number contrast>, which can be used to specify the difference in brightness between the line's right and left sides, where right and left are defined relative to the vector, [x1 - x0 , y1 - y0]T. Following Boldt et al., the initial and terminal endpoints should be chosen so that contrast is positive, i.e., the darker region lies to the left. If <number contrast> is not specified, contrast is set to zero. For example,
In the method used by Boldt et al. (specified by setting <number> equal to one), the midpoints of the lines are the same as those computed using the above method. However, the lines are all of uniform length (one) and have orientation normal to the image gradient direction at the line's midpoint (estimated by bilinearly interpolating the convolutions of the image with horizontal and vertical Sobel masks). For example,
displays the following sketch:
In both methods, a zero crossing's contrast is defined to be the image gradient magnitude at the line's midpoint. Lines' contrasts are displayed using a hot color map.
(filter-on-contrast <sketch> <number cmin> . <number cmax>)Returns a new sketch consisting of lines from <sketch> with contrast greater than <number cmin>. If the optional argument is also specified, then the lines must also have contrast less than <number cmax>.
(filter-on-length <sketch> <number lmin> . <number lmax>)Returns a new sketch consisting of lines from <sketch> with length greater than <number lmin>. If the optional argument is also specified, then the lines must also have length less than <number lmax>.