One of the simplest ways to achieve a professional polish to your
programs is to use colour effectively. Even the simplest of programs
in Graphics 1 or 2 can be made to look special by carefully choosing
the colours of the text. Remember, in these modes you can have five
colours including background on the screen without resorting to fancy
tricks-just use upper and lower case and inverse.
Various books and manuals have charts of SETCOLOR values and some
even have printed colour charts, but no printed page can convey the
full range of Atari's 256 colours, particularly as the colours can
look different depending on the background and will vary from
television to television. The usual way to achieve the right balance
is to experiment and change colours one by one until the desired
result is obtained. As this can be a long process, I decided to write
a short Utility which would take away the guesswork and show colours
in close relation to each other as well as providing all the important
SETCOLOR and POKE values. The result is Colour Selector.
The utility itself is very easy to use, just choose a Setcolor
statement by pressing keys 0-4 and a marker will indicate which
statement you will be changing. Press the right and left arrow keys to
cycle up and down through the colours available. To change to another
register, press SELECT and choose again. When you have achieved the
balance you require, note down the SETCOLOR or POKE values to use in
your own programs.
The program, although relatively simple, is interesting in that it
introduces both the concepts of custom display lists and display list
interrupts. Lines 1000-1050 set up a display list which comprises
three lines of Graphics 0 followed by two blank lines, four lines of
Graphics 1, two blank lines, four lines of Graphics 2, two blank
lines, one line of Graphics 0 followed by two more blank lines, five
lines of Graphics 0, two blank lines and finally two lines of Graphics
0. This may sound quite complicated but all the information to build
this display list is in line 1030 and the whole display list routine
only takes five program lines. The introduction of blank lines
in the display gives a very pleasing effect and also helps to
balance out the display list to the required 192 scan lines and
assists in overcoming a small problem in the display list interrupt
which does not always give a clean separation of colours when the
keyboard is in use.
Now, display lists are one thing but Display List Interrupts are an
entirely different proposition. Altering the display list itself is
easily accomplished from Basic but Interrupts require the use of
machine language and my understanding of machine language is about as
deep as the average man's knowledge of nuclear physics! What I
required was a way to keep the bottom half of the screen the same
colour irrespective of what was being done to the registers which
would normally control the background and text. Without this there
would be problems in reading the information when the colours were
changed. I remembered a routine in De Re Atari which does just this
and duly typed it in. There was just one small problem, the routine on
page 5-3 of De Re Atari does not work! Experimentation, they say, is
the secret of learning programming techniques and after a few hours I
had some small understanding of the routine, despite starting from
absolute scratch. I can now proudly present in lines 1110-1130, a
corrected version of a routine from De Re Atari!
To enable display list interrupts you must add 128 to the line
before the one you wish to change, which is why in line 1030 a 135
follows the three 7's. Try changing this back to 7 and adding 128 to
another number and see what happens. I won't explain the machine
language routine (I can't!) but if you want to change the colour of
the bottom half of the screen, just change the figure 18 in line 1120
to change the background and the figure 14 to change the text. The
numbers to use are the same as you would use to POKE a colour
register.
That's it then-Colour Selector-a simple utility which ended up
teaching me quite a lot about display lists. Don't be afraid to
experiment, you may surprise yourself. I would never have dreamed of
presenting a correction to De Re Atari. What next?
|
|
|
 |
top
