Mark Hutchinson's regular column for beginners
CONTINUING GRAPHICS
This issue I will continue with a look at the
Graphics modes but we will leave the text modes and have a quick
look around the modes available for proper graphics.
As I explained in the last issue, all the modes
are basically the same but they are utilised by the computer in
different ways. Have a look at figure 1. This will show you each
mode with its related colour, co-ordinates and memory size. This
should be self explanatory, the only point of interest is GR.8 which
has one plot colour, but has two luminances.
COLOR and SETCOLOR
These commands tend to cause a bit of confusion in
the graphic modes so let's have a closer look at them.
SETCOLOR does not change screen colours directly but changes the
colour stored in a certain COLOR register. There can be up to four
of these COLOR registers used in each graphic mode (see figure 2).
Colour can also be changed by POKEing the appropriate memory
location. For instance, SETCOLOR 0,hue,luminance or its equivalent
POKE 708,16*hue + luminance will do the same job. So, if hue = 4 and
luminance = 10, you would use POKE 708,74. Figure 2 shows the
SETCOLOR command and the equivalent memory location.
COLOR in a graphics mode will define the register
to be used for drawing points; in a text mode it will give the ASCII
character. For a four colour mode, such as GR.7, COLOR will be 0 for
background colour and 1 to 3 for points. If the value is 4 then the
real value for the register is set to 0 again, i.e. the value will
cycle from 0 to 3 then start off at 0 again. In a two colour mode,
such as GR.4, the values are 0 and 1 only. Above that the cycling
begins.
GR.8 is a one colour mode so COLOR is used to set
the luminance of the plotted point. The colour of this point will be
the same as the background, and luminance will either show or hide
the point.
To recap, COLOR 1, 2, 3 or 4 will display a point
in a certain colour and SETCOLOR will change this colour. COLOR 0
will set the point colour to that of the background, effectively
erasing the point. Figure 3 shows the modes and associated COLOR/SETCOLOR
commands.
A DEMONSTRATION
To demonstrate the differences between all of the
graphics modes, I wrote LISTING 2. This program will set up all of
the modes and flip between each. I had hoped to write about page
flipping during the summer but I was beaten to it by Allan Knopp in
issue 25! As can be seen from figure 1, graphics 8 uses a massive
amount of memory. This is because it uses very fine points (pixels
or picture elements) to draw lines. Each point will have its value
stored in RAM and this is termed a memory mapped screen. Because the
screen is so finely drawn, a strange effect occurs known as
artifacting. Listing 2 should should demonstrate this effect nicely.
|
GR. MODE |
HORIZ COLMS |
VERT ROWS |
No.of COLRS |
LINE TYPE |
MEM
SIZE |
|
0 |
40 |
24 |
2 |
TEXT |
993 |
|
1 |
20 |
24 |
5 |
TEXT |
543 |
|
2 |
20 |
12 |
5 |
TEXT |
261 |
|
3 |
40 |
24 |
4 |
PLOT |
273 |
|
4 |
80 |
48 |
2 |
PLOT |
537 |
|
5 |
80 |
48 |
4 |
PLOT |
1017 |
|
6 |
160 |
96 |
2 |
PLOT |
2025 |
|
7 |
160 |
96 |
4 |
PLOT |
3945 |
|
8 |
320 |
192 |
1/2 |
PLOT |
7900 |
Figure 1 - GRAPHICS MODES
If you find that the program will not run due to
memory constraints, GRAPHICS 8 (lines 639 to 660 and line 1050) can
be deleted and if necessary GRAPHICS 7 (lines 609 to 630 and line
1040). Remember to set counter G (line 710) to the appropriate
value.
THANKS FOR WRITING
I would like to thank Brian Trevett for his kind
letter. I am glad that I was able to be of such help. If any
beginners need some advice that would take too long to explain here,
I can be reached (with an SAE please) at the following new address.
1,
HOLLYMOUNT,
ERINVALE,
FINAGHY
BELFAST
BT10 OGL
Finally, a couple of tips from Mr. Robinson of
Cumbria. You may know that LIST will display the complete listing of
a program, but did you know that you can display a series of line
numbers? LIST 100,120 will display all line numbers from 100 to 120
inclusive. This will also apply to a single line, eg. LIST 100.
When you use the TAB key to move the cursor across
the screen, the default is 10. This is stored in location 201, so
POKE any suitable number in here to change the TAB spacing. Hands up
who knew that a comma causes tabbing. Try the following line to see
what I mean.
PRINT "A","B"„"C"
See you next issue when we might take a look at
animation.
___________
|
SETCOLOR NUMBER |
MEMORY
LOCATION |
| 0 |
708 |
| 1 |
709 |
| 2 |
710 |
| 3 |
711 |
| 4 |
712 |
Figure 2 - SETCOLOR EQUIVALENTS
|
MODE No. |
SETCOLOR NUMBER |
COLOR NO. |
USE |
| 0 |
0 |
0-255 |
- |
| |
1 |
GIVES |
CHARACTER LUM |
|
ALSO |
2 |
CHAR- |
BACKGROUND |
|
TEXT |
3 |
ACTER |
- |
|
WINDOWS |
4 |
|
BORDER |
| |
|
|
|
| 1,2 |
0 |
0-255 |
CHARACTER |
| |
1 |
GIVES |
CHARACTER |
|
TEXT |
2 |
CHAR- |
CHARACTER |
|
MODES |
3 |
ACTER |
CHARACTER |
| |
4 |
|
BACKGROUND,
BORDER |
| |
|
|
|
|
3,5,7 |
0 |
1 |
POINT |
| |
1 |
2 |
POINT |
|
FOUR |
2 |
3 |
POINT |
|
COLOR |
3 |
- |
- |
|
MODES |
4 |
0 |
BACKGROUND,
BORDER |
| |
|
|
|
| 4,6 |
0 |
1 |
POINT |
| |
1 |
- |
- |
| TWO |
2 |
- |
- |
|
COLOUR |
3 |
- |
- |
|
MODES |
4 |
0 |
BACKGROUND,
BORDER |
| |
|
|
|
| 8 |
0 |
- |
- |
| |
1 |
1 |
POINT (LUM) |
| 1
COLOR |
2 |
0 |
BACKGROUND |
| 2
LUM. |
3 |
- |
- |
| |
4 |
- |
BORDER |
| |
|
|
|
The COLOR number defines the
point to be used (with its associated SETCOLOR statement) in
a graphics mode. In a text mode, COLOR defines the character
to be plotted
Where background is stated in graphic modes, any point using
that colour number takes the background colour, effectively
disappearing.
Mode 8 has one colour, that of the background, but has two
luminances. One makes the point stand-out,-the other makes
it disappear. |
Figure 3
|
|
|
 |
Listing 1
|
|
|
|
Listing 2
