MAKING IT MOVE
In issue 27 I said that GRAPHICS 0 and GRAPHICS 8
were similar. Perhaps you would like some further explanation? Each
mode has the same colour attributes, i.e. a border, a background and
a point on the background that is the same colour as the background
but which can have a different luminosity. Each can be addressed
down to a single pixel though GRAPHICS 0 is addressed in a block of
8 by 8 to make up a character.
The interesting part is that, if you can address
GRAPHICS 8 using a similar block, you can write to this mode as if
it were GRAPHICS 0. I have included LISTING 1 as an example of how
this can be done. Obviously, being in Basic, it is slower than the
operating system but it can come in useful.
The screen pointer (88,89) starts at the top left
hand side of the screen. If you POKE any number here it would show
as a character in a text mode, or as a coloured point in graphics
modes.
Location 57344 is the start of 1024 bytes that hold
the character set. Each character is 8 horizontal lines of one byte
each. So 1024/8 gives 128 characters. These can be toggled with the
inverse key (using an OR operation) to give another 128 characters.
For more information, I would recommend the purchase of a good
memory map.
The basis of the example listing is to to recognise the character
required, look up the appropriate location, get the character data
and then produce this data, one line at a time, as a block on the
screen. When you RUN the program, you will see the character written
quite slowly.
GETTING A MOVE ON
This issue I would like to have a look at movement (ANIMATION to
programmers). Movement, without using Player Missile Graphics, comes
in two forms. One form, published in issue 25 by Allan Knopp and in
the last FIRST STEPS column, is called page flipping. This is where
you draw several pages, with slight variations in each, and flick
through them very fast to simulate movement. The other method is
redrawing which is what we will look at now.
Moving a figure on a screen has to be done in steps. First the
figure must be drawn in one location, then drawn (perhaps slightly
different) in another position. Normally this is done by drawing the
figure in a certain colour on the screen, redrawing in the same
position using the background colour (effectively hiding the
figure), then moving to another position and redrawing using the
first colour. LISTING 2 is a good example of this technique. A
variation of this theme is to use the colours of the points to
create movement. You may wish to review the last column for details
of COLOR and SETCOLOR.
COLOUR CYCLING
Now let's look at how LISTING 3 works. A row of characters is set
up, each one using a different colour register. The colours are then
shifted from one register to another in a cycle thus making the
figure appear to change places. Your homework is to try this out in
GRAPHICS 7, making a straight line (remember the robot eyes in
Battlestar Galactica, the car in Knight Rider?).
Remember I mentioned memory locations 88 and 89 at the start of this
article? One of the reasons is that you can place points directly on
the screen using POKE instead of the slower PRINT or PLOT. A few
years ago I was given a demonstration of this technique, a program
called 'BUGS' written (in Basic) by none other than Les Ellingham! —
it was FAST.
USING PRINT
PRINT can, however, still be used to animate the screen with
reasonable speed. It is, after all, a form of redrawing. It can be
slow and jerky but can have uses in graphic modes. Text modes 1 and
2 use upper case characters only, but they use the same SETCOLOR
statements as the four COLOR statements. So if we were to print,
say, ABCDEFGH, the four registers would be used for ABCD and then
repeated for EFGH. Remember last time that I said GRAPHICS 7 uses
only four COLOR statements? Well we can in fact print characters to
a graphics screen, only thy will show as colour points only rather
than letters. One of the reasons for using PRINT in a graphics mode
is partly for speed. The character contains the colour register and
the point does not need to use PLOT every time. Another good point
is that the drawing can be stored as a string and ATARI is renowned
for its speed in string handling.
LISTING 4 will demonstrate the speed of using a string instead of
trying to PLOT points. I have also used the DRAWTO statement to show
that drawing lines is just as fast this way as using a string but I
wanted to point out the convenience of a using string — a method
which I am sure many of you were not aware of.
LISTING 5 is a complex diagram stored as a string. Try this out
using PLOT and DRAWTO and see if it just as fast. Experiment with
character strings and different COLOR and SETCOLOR values and you
should be able to build up some good graphics of your own.
I hope that this article will stimulate you enough to try out some
of your own exercises, you will only become proficient by practice!
Listing 1
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Listing
2
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Listing
3
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Listing
4
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Listing
5
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