# [racket] Plot: #:samples parameter of function---is this what was meant?

On 04/28/2012 06:31 AM, Deren Dohoda wrote:
>* Thanks, Neil. I don't mind. Clearly rough sampling would make plots
*>* really icky. That's not at all my goal. It was more like localization
*>* of the parameters.--- which is still true under the current
*>* interpretation, just not in a way I expected.
*>*
*>* Backstory: I was plotting splines and it was extremely slow and at
*>* first all I found to speed things up was to decrease the number of
*>* samples... with predictable results. What I had done was interpolate
*>* 520 points, generating 519 polynomials evaluated over a short
*>* distance, which when plotted used (function ... ) 519 times. I was
*>* getting worried that it was trying to plot all 519 functions over the
*>* entire range but then discarding results not wanted to be seen or
*>* something...
*
Quick reminder: your worry was spot-on, and I was going to fix the problem.
I've just pushed a fix. This test program used to sample the function
`f' about 500,000 times:
#lang racket
(require plot plot/utils)
(define count 0)
(define (f x)
(set! count (+ count 1))
(sin x))
(define xs (linear-seq -4 4 501)) ; bounds for 500 function renderers
(time
(plot (for/list ([x1 (in-list xs)]
[x2 (in-list (rest xs))])
(function f x1 x2))))
count
It now samples `f' only 1000 times: twice for each renderer's endpoints.
You might need to be a bit careful rendering piecewise functions with
many function renderers. If the pieces are drawn partially transparent
and a bit thick, the endpoints will visibly overlap, giving the function
a "knotted" look. Here's an example:
#lang racket
(require plot plot/utils)
(define xs (linear-seq -4 4 50))
(plot (for/list ([x1 (in-list xs)]
[x2 (in-list (rest xs))])
(function sin x1 x2 #:width 5 #:alpha 1/2)))
Neil ⊥