A.8 Advanced fill function:

Three primitives allow to colour a figure. The primitive fill, fillzone and fillpolygon. .

A.8.1 fill and fillzone

These primitives allow a shape to be coloured in. These primitives can be compared with the ’fill’ feature available in many image-retouching programs. This feature can extend to the margins of the design area. There are two rules that must be adhered to in order to use this primitive correctly:

  1. The pen must be lowered (pd).
  2. The turtle must not be located on a pixel of the colour with which the shape is to be filled. (If you want to colour things red, it can’t be sitting on red...)

Let’s take a look at an example to see the difference between fill and fillzone:


Figure A.1: At the beginning

The pixel under the turtle is white right now. The primitive fill will colour all the neighbouring white pixels with the current pen colour. If for example, you type: setpc 1 fill.


Figure A.2: With the primitive fill

Let’s now go back to the first case, if the pen colour of the turtle is black, the primitive fillzone colours all pixels until it encounters the current colour (here black).


Figure A.3: With the primitive fillzone, if you type: setpc 0 fillzone

This is a good example of the use of this primitive:

to halfcirc :c  
# draw a half-circle of diameter :c  
repeat 180 [fd  :c*tan 0.5 rt 1]  
fd :c*tan 0.5  
rt 90 fd :c  
to tan :angle  
# renders the tangent of the angle  
output (sin :angle)/cos :angle  
to rainbow :c  
if :c<100 [stop]  
halfcirc :c rt 180 fd 20 lt 90  
rainbow :c-40  
to dep  
pu rt 90 fd 20 lt 90 pd  
to arc  
ht rainbow 400 pe lt 90 fd 20 bk 120 ppt pu rt 90 fd 20 pd  
setpc 0 fill dep  
setpc 1 fill dep  
setpc 2 fill dep  
setpc 3 fill dep  
setpc 4 fill dep  
setpc 5 fill dep  
setpc 6 fill dep  


Figure A.4: Arc-in-LOGO

A.8.2 Primitive fillpolygon

fillpolygon list

It filled a shape using a series of triangles, so every time you drew a line the next triangle is filled. The list contains all instructions needed to draw the shape to fill.