Three primitives allow to colour a figure. The primitive fill, fillzone and fillpolygon. .
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:
- The pen must be lowered (pd).
- 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
pu rt 90 fd 20 lt 90 pd
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
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.