#### One-a-Day: FRAG

The FRAG primitive creates a filled shape out of the current turtle position and her last two positions. For example:

FD 100 RT 90 FD 100 FRAG

will create a triangle.

While turtleSpaces has a variety of shape primitives, sometimes you need to create an arbitrary shape, and FRAG aids you in this.

Take this example, which draws a star:

TO star repeat 4 [ forward 100 right 170 forward 86 frag left 70 forward 86 right 170 forward 100 frag right 180 ]

We can change the number of sides the star has by changing the number of repeats and fiddling with the values a bit:

TO star2 repeat 5 [ fd 100 rt 170 fd 86 frag lt 88 fd 86 rt 170 fd 100 frag rt 180 ] END

First, let’s change our star procedures so they can take a :size parameter, like so:

TO star1 :size repeat 4 [ fd 100 * :size rt 170 fd 86 * :size frag lt 70 fd 86 * :size rt 170 fd 100 * :size frag rt 180 ] END

In this case, :size is a ratio, that is, 0.5 will make a star half the size, and 2 will make a star twice the size of the default.

You can change the color of the star using the SETFILLCOLOR primitive, or set a random fill color with RANDFC.

The following procedures create a sky full of stars:

TO star1 :size repeat 4 [ fd 100 * :size rt 170 fd 86 * :size frag lt 70 fd 86 * :size rt 170 fd 100 * :size frag rt 180 ] END TO star2 :size repeat 5 [fd 100 * :size rt 170 fd 86 * :size frag lt 88 fd 86 * :size rt 170 fd 100 * :size frag rt 180] END TO star3 :size repeat 6 [fd 100 * :size rt 170 fd 86 * :size frag lt 100 fd 86 * :size rt 170 fd 100 * :size frag rt 180] END TO star4 :size repeat 7 [fd 100 * :size rt 170 fd 86 * :size frag lt 108.625 fd 86 * :size rt 170 fd 100 * :size frag rt 180] END TO star5 :size repeat 8 [fd 100 * :size rt 170 fd 86 * :size frag lt 115 fd 86 * :size rt 170 fd 100 * :size frag rt 180] END TO stars reset cam:setposition [0 0 0] cam:fixate [0 0 0] cam:setviewpoint [0 0 0] cam:newworker [forever [up 0.1 lt 0.1 rr 0.1 wait 1]] repeat 200 [ pu home randori fd 400 + random 1000 up 90 lt random 60 pd randpc randfc randfs randps make "size (10 + random 90) / 100 run {pick [star1 star2 star3 star4 star5] :size} ] END

Type STARS and press Enter to see the stars!

FRAG’s sister, SHARD creates a three-dimensional FRAG with depth beneath it. This depth is supplied as a parameter, in turtle-units, eg. SHARD 5. Try replacing FRAG with SHARD 5 in one of your star procedures and see what happens! (You’ll need to drag the camera around to see the sides of the star)