Diamond-Effect V1.0
Draws a colorful caro effect on the screen.

Compiled with CC65 2.11.9
Compatible with all systems supporting conio.h

by
Monte Carlos / Cascade on 24.3.2008
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This program draws a colorful diamond on the screen.
It can only be compiled, if the target system features
"conio.h".

The size of the diamond can be given by command line
parameters using the "run:rem par1, par2" syntax.
The spaces before the parameters are important!
Given too large x or y sizes or a wrong number of
parameters will be prompted with an error message.

The speciality of this routine is, that it always draws
a symmetric diamond independent on the odd- or evenness
of the x respectively y-size of the drawing area.
This is not trivial.
When the drawing area is of even size,
it is divided in a left and a right half and the
zero lies between the middle two chars.
When the drawing area is of odd size,
is is divided in three parts:
The left third, the middle char with the coordinate zero
and the right third.
How can we make a routine compatible to both cases?

The solution is to use virtual a double x and y-screensize.
All even virtual coordinates belong to the screenpixels
0,1,2,3,4....
All odd virtual coordinates belong to the mid of the screenpixels
0.5, 1.5, 2.5, ...
With this convention the start and end pixels have always the
virtual coordinates -(size-1) and (size-1).
Try to understand this with the help of the pdf-document, it
will get really clear.

In the loop, which draws the diamond the coordinates x and y
start with -(size-1) and run till (size-1) in steps
of 2.
The values given to gotoxy are therefore just the half
of x respectively y plus an offset.
Look into the pdf, which is included in the release.
There you can see, how the origin of the real screen
is moved by half an pixel and therefore the area is
placed always centro symmetric.