(graymap-highpass! graymap theshold)
This procedure sets each gray pixel of a graymap to black (0) if its prior value is below the provided threshold. Any pixel values above or equal to the threshold are left untouched.
The graymap is modified.
The resulting graymap is returned.
> (define x (mandelbrot -2.0 2.0 -2.0 2.0 200 640))
x
> (graymap-minimum x)
2
> (graymap-highpass! x 20)
#<graymap(16) orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:640>
> (graymap-minimum x)
0
>