Metapict questions on filling polygons from points

Questions & Answers
1

I am trying to draw something in metapict and I am having trouble simplifying code.

Lets say I have 2 rectangles side by side, roughly:

A            B            C
x------------x------------x
|            |            |
x------------x------------x
D            E            F

and I want to draw them. I can do:

(curve A -- B -- C -- F -- E -- D -- cycle)
(curve B -- E)

But then I run into trouble if I want to fill the left side and right rides with different colours.
So I need to do in other to fill both parts

(curve A -- B -- E -- D -- cycle)
(curve B -- C -- F -- E -- cycle)

But this is not great because it redraws B -- E. Is there a better way to do this?

Another issue is... Lets say I define a bunch of point in a list. Like

(define left  (list A B E D))
(define right (list B C F E))

How do I draw the above from these lists?
The obvious (apply curve left)... of course doesn't work.

2

If you want to avoid redrawing, you can draw the background (the two filled rectangles) first,
then draw the lines on top. See below.

But why is redrawing a problem in the first place - do you have an example of what it looks like?
[I get the best results, when I save as svg.]

The obvious (apply curve left) ... of course doesn't work.
I think (curve* left) is what you are looking for.

#lang racket
(require metapict)

;; A            B            C
;; x------------x------------x
;; |            |            |
;; x------------x------------x
;; D            E            F

(def A (pt 0 2))
(def B (pt 2 2))
(def C (pt 4 2))

(def D (pt 0 0))
(def E (pt 2 0))
(def F (pt 4 0))

(def ACFD  (curve A -- C -- F -- D -- cycle))
(def LEFT  (curve A -- B -- E -- D -- cycle))
(def RIGHT (curve B -- C -- F -- E -- cycle))

(def BE   (curve B -- E))

(define win (window -1 5 -1 5))

(set-curve-pict-size 401 401)
(with-window win
  (draw (color "red"  (fill LEFT))
        (color "blue" (fill RIGHT))
        ACFD
        BE))

image

3

I forgot to use rectangle. It takes two corners (or a corner and a vector) and
makes a rectangular path. The corners become a diagonal.

(def AF    (rectangle A F))  ; A -- C -- F -- D
(def LEFT  (rectangle A E))  ; A -- B -- E -- D
(def RIGHT (rectangle B F))  ; B -- C -- F -- E
4

Cool thanks.

Thanks for all the explanation. Somehow I missed curve*, and I noticed now that it's because it's undocumented.

The rectangle stuff is cool but unneeded since this was just a simple example. What I am currently doing needs no rectangles. Once I get this right, I will post here what I came up with. :slight_smile:

Paulo

5

About curve* and curve*: I forgot to say that you need "path joins" between the points.

For example: (curve* (add-between (list (pt 1 0) (pt 2 3) (pt 3 -1)) ..))

I did an experiment with redrawing the middle vertical line.
Both look fine when svg is used.

If you save the images as "png" you can see the difference.

(set-curve-pict-size 150 150)
(with-window win
  (draw (filldraw LEFT  "red")
        (filldraw RIGHT "blue")))

(set-curve-pict-size 151 151)
(with-window win
  (draw (filldraw LEFT  "red")
        (filldraw RIGHT "blue")))

image

The effect is explained in the documentation of racket/draw.
Basically the first version where the width is 150 draws between the pixels
and the second one draws perfectly in the middle of the pixels.

image
image1460×498 83 KB

The reason is due to the model used in Cairo:
http://stevehanov.ca/blog/?id=28