(graymap-lowpass! graymap theshold)
This procedure sets each gray pixel of a graymap to black (0) if its prior value is above the provided threshold. Any pixel values below 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-maximum x)
253
> (graymap-lowpass! x 200)
#<graymap(16) orow:-1 ocol:-1 xres:-1 yres:-1 w:640 h:640>
> (graymap-maximum x)
200
>