Part 16
Colours

Subjects covered...

	INK, PAPER, FLASH, BRIGHT, INVERSE, OVER
	BORDER

Run this program...

	 10 FOR m=0 TO 1: BRIGHT m
	 20 FOR n=1 TO 10
	 30 FOR c=0 TO 7
	 40 PAPER c: PRINT "    ";: REM 4 coloured spaces
	 50 NEXT c: NEXT n: NEXT m
	 60 FOR m=0 TO 1: BRIGHT m: PAPER 7
	 70 FOR c=0 TO 3
	 80 INK c: PRINT c;"  ":
	 90 NEXT c: PAPER 0
	100 FOR c=4 TO 7
	110 INK c: PRINT c;"  ";
	120 NEXT c: NEXT m
	130 PAPER 7: INK 0: BRIGHT 0

This shows the eight colours (including white and black) and the two
levels of brightness that the +3 can produce on a colour TV. (If your
TV is black-and-white, then you will see just various shades of grey.)
A quicker way to achieve a similar result is to RESET the +3 whilst
holding down BREAK - but that's a little drastic. Here is a list of
which numbers produce which colours (for your reference)...

	0 - black
	1 - blue
	2 - red
	3 - magenta
	4 - green
	5 - cyan
	6 - yellow
	7 - white

On a black-and-white TV, these numbers are in order of brightness. To
use these colours properly, you will need to understand a bit about
how the picture is arranged.

The picture is divided up into 768 (24 lines of 32) positions (cells)
where characters can be printed.


                    _______________________________
                   |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|
                   |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|
                   |   |   |###|###|###|   |   |   |
                   |___|___|###|###|###|___|___|___|
                   |   |   |   |   |   |###|   |   |
                   |___|___|___|___|___|###|___|___|
                   |   |   |###|###|###|###|   |   |
                   |___|___|###|###|###|###|___|___|
                   |   |###|   |   |   |###|   |   |
                   |___|###|___|___|___|###|___|___|
                   |   |   |###|###|###|###|   |   |
                   |___|___|###|###|###|###|___|___|
                   |   |   |   |   |   |   |   |   |
                   |___|___|___|___|___|___|___|___|

                       A typical character cell


Each character cell consists of an 8 x 8 grid (such as above). This
should remind you of the user-defined graphics in part 14, where we
had 0s for the white dots and 1s for the black dots.

The character has two colours associated with it: the ink, or
foreground colour, which is the colour for the black dots in our
square, and the paper, or background colour, which is used for the
white dots. To start off with, every cell has black ink and white
paper so writing appears as black on white.

The character also has a brightness (normal or extra bright), and
something to say whether it flashes or not. Flashing is done by
continuously swapping the ink and paper colours. All this information
can be coded into numbers, so a character then has the following...

(i)	An 8 x 8 grid of 0s and 1s to define the shape of the
	character, with 0 for paper and 1 for ink.

(ii)	Ink and paper colours, each coded into a number between 0 and
	7.

(iii)	A brightness - 0 for normal, 1 for extra bright.

(iv)	A flash number - 0 for steady, 1 for flashing.

Note that since the ink and paper colours cover a whole character
cell, you cannot possibly have more than two colours in a given block
of 64 dots. The same goes for the brightness and flash numbers - they
refer to the whole character cell, not individual dots within the
cell. The colour, brightness and flash number for a given character
cell are called attributes.

When you print something on the screen, you change the dot pattern for
that character cell. It is less obvious, but still true, that you also
change the cell's attributes. To start off with you do not notice this
because everything is printed with black ink on white paper (at normal
brightness and no flashing); however, you can vary this with the INK,
PAPER, BRIGHT and FLASH statements. Using the edit menu's 'Screen'
option, go to the bottom screen, and try...

	PAPER 5

...and then PRINT a few items on the screen - they will appear on cyan
paper, because as they are printed, the paper colour for the cells
they occupy are set to cyan (which has code 5).

The others work the same way, so you may use the settings...

	PAPER	(whole number between 0 and 7)
	INK	(whole number between 0 and 7)
	BRIGHT	(whole number between 0 and 1)
	FLASH	(whole number between 0 and 1)

...and any printing will set the corresponding attributes for all the
character cells it subsequently uses.

Try some of these out. You should now be able to see how the program
at the beginning of this section worked (remember that a space is a
character that has its ink and paper the same colour).

There are some ore numbers you can use in these statements that have
less direct effects.

8 can be used in all four statements, and means 'transparent' in the
same sense that the old attribute shows through. Suppose, for
instance, that you do...

	PAPER 8

No character position will ever have its paper colour set to 8 because
there is no such colour; what happens is that when a position is
printed on, its paper colour is left the same as it was before.
However, INK 8, BRIGHT 8 and FLASH 8 work the same way as for the
other attribute numbers.

9 can be used only with PAPER and INK, and means 'contrast'. The
colour (ink or paper) that you use it with is made to contrast with
the other by being made white if the other is a dark colour (black,
blue, red or magenta), or being made black if the other is a light
colour (green, cyan, yellow or white).

Try this by doing...

	INK 9: FOR c=0 TO 7: PAPER c: PRINT c: NEXT c

A more impressive display of its power is to run the program at the
beginning to make coloured stripes (again, making sure that you are in
the lower screen when you type RUN), and then doing...

	INK 9: PAPER 8: PRINT AT 0, 0;:
	FOR n=1 TO 1000: PRINT n;:NEXT n

The ink colour here is always made to contrast with the old paper
colour for each character cell.

Colour TV relies on the fact that the human eye need see only three
colours of light (red, green and blue) in various combinations and
intensities in order to perceive all the colours of the spectrum. The
+3 also displays its spectrum of colours by using mixtures of red,
green and blue. For instance, yellow is made by mixing red with green
- which is why the code, 6, is the sum of the codes for red and green.

To see how all eight colours fit together, imagine three rectangular
spotlights, coloured red, green and blue shining at not quite the same
place on a piece of white paper in the dark. Where they overlap you
will see mixtures of colours, as shown by the following program (note
that solid ink spaces are obtained by entering graphics mode (pressing
GRAPH) then holding down CAPS SHIFT while pressing '8'. To exit from
graphics mode, press '9'.)...

	  10 BORDER 0: PAPER 0: INK 7: CLS
	  20 FOR a=1 TO 6
	  30 PRINT TAB 6; INK 1;"##################": REM 18 ink
		squares
	  40 NEXT a
	  50 LET dataline=200
	  60 GO SUB 1000
	  70 LET dataline=210
	  80 GO SUB 1000
	  90 STOP
	 200 DATA 2,3,7,5,4
	 210 DATA 2,2,6,4,4
	1000 FOR a=1 TO 6
	1010 RESTORE dataline
	1020 FOR b=1 TO 5
	1030 READ c: PRINT INK c;"######";: REM 6 ink squares
	1040 NEXT b: PRINT: NEXT a
	1050 RETURN

There is a function called ATTR that finds out what the attributes ate
at a given position on the screen. It is a fairly complicated
function, so it has been relegated to the end of this section.

There are two more statements, INVERSE and OVER, which control not the
attributes, but the dot pattern that is printed on the screen. They
use the numbers 0 for off, and 1 for on. If you use 'INVERSE 1', then
each character cell's dot pattern will be the inverse of its usual
form, i.e. paper dots will be replaced by ink dots and vice
versa. Thus the character cell containing 'a' (shown previously) would
be printed as follow...

                    _______________________________
                   |###|###|###|###|###|###|###|###|
                   |###|###|###|###|###|###|###|###|
                   |###|###|###|###|###|###|###|###|
                   |###|###|###|###|###|###|###|###|
                   |###|###|   |   |   |###|###|###|
                   |###|###|___|___|___|###|###|###|
                   |###|###|###|###|###|   |###|###|
                   |###|###|###|###|###|___|###|###|
                   |###|###|   |   |   |   |###|###|
                   |###|###|___|___|___|___|###|###|
                   |###|   |###|###|###|   |###|###|
                   |###|___|###|###|###|___|###|###|
                   |###|###|   |   |   |   |###|###|
                   |###|###|___|___|___|___|###|###|
                   |###|###|###|###|###|###|###|###|
                   |###|###|###|###|###|###|###|###|


If (as at switch on) we have black ink on white paper, then the 'a'
will appear as white on black.

The statement...

	OVER 1

...sets into action a particular sort of overprinting. Normally when
something is written into a character position, it completely
obliterates what was there before; however, using 'OVER 1', the new
character is simply added on top of the old one. This can be
particularly useful for writing composite characters, like an
underlined letter, as in the following program. (Reset the computer
and select '+3 BASIC'. Note that the underline character is obtained
by pressing SYMB SHIFT together with '0'.)...

	10 OVER 1
	20 PRINT "w"; CHR$ 8;"_";

(Notice we have used the control character 'CHR$ 8' (backspace) before
overprinting the 'w' with '_'.)

There is another way of using INK, PAPER and so on which you will
probably find more useful than having them as statements. You can put
them as items in a PRINT statement (followed by ';'), and they then do
exactly the same as they would have done if they had been used as
statements on their own, except that their effect is only temporary,
lasting as far as the end of the PRINT statement that contains
them. Thus if you type...

	PRINT PAPER 6;"x";: PRINT "y"

...then only the 'x' will be on yellow paper.

PAPER, INK, etc. when used as statements do not affect the colour in
the bottom part of the screen (where INPUT data is typed in and
reports are displayed). The bottom screen uses the colour of the
border for its paper colour, code 9 (for contrast) for its ink colour,
has flashing off, and everything at normal brightness. You can change
the border colour to any of the eight normal colours (not 8 or 9)
using the statement...

	BORDER colour

When you type in INPUT data, it follows this rule of using contrasting
ink on border coloured paper, but you can change the colour of the
captions written by the +3 by using PAPER, INK, etc. items in the
INPUT statement, just as you would in a PRINT statement. Their effect
lasts either to the end of the statement, or until some INPUT data is
typed in, whichever comes soonest. Try...

	INPUT FLASH 1; INK 4;"Enter a number?";n

The +3 has a high regard for your sanity - no matter what combination
of effects and colours you manage to produce from a BASIC program, the
editor will always use black ink on white paper.

There is one more way of changing the colours by using control
characters - rather like the control characters for AT and TAB in part
15.

	CHR$ 16 corresponds to INK
	CHR$ 17 corresponds to PAPER
	CHR$ 18 corresponds to FLASH
	CHR$ 19 corresponds to BRIGHT
	CHR$ 20 corresponds to INVERSE
	CHR$ 21 corresponds to OVER

These are each followed by one character that shows a colour by its
code; so that (for instance)...

	PRINT CHR$ 16+ CHR$ 9;"item"

...has the same effect as...

	PRINT INK 9;"item"

On the whole, you would not bother to use these control characters
because you might just as well use the statements PAPER, INK,
etc. However, if you have some old 48K BASIC programs on cassette, you
may find such control characters embedded in the listing. In general,
the editor will actively ignore them, and remove them at the first
opportunity. It is not possible to insert them into listings as with
the old 48K Spectrum.

The ATTR function has the form...

	ATTR(line,column)

Its two arguments are the line and column numbers that you would use
in an AT item, and its result is a number that shows the colours and
so on at the corresponding character position on the TV screen. You
can use this as freely in expressions as you can any other function.

The number that is the result is the sum of four other numbers as
follows...

	128 - if the character cell is flashing, 0 if it is steady.
	64  - if the character cell is bright, 0 if it is normal.
	8   - multiplied by the code for the paper colour.
	1   - multiplied by the code for the ink colour.

For instance, if the character cell is flashing, normal brightness,
yellow paper and blue ink, then the four numbers that we have to add
together are 128, 0, 8x6=48 and 1, making 177 altogether. Test this
with...

	PRINT AT 0,0; FLASH 1; PAPER 6; INK 1;" "; ATTR (0,0)


Exercises...

1. Try...

	PRINT "B"; CHR$ 8; OVER 1;"/";

Where the '/' has cut through the 'B', it has left a white dot. This
is the way that overprinting works on the +3 - two papers or two inks
give a paper, one of each gives an ink. This has the interesting
property that if you overprint with the same thing twice you end up
with what you had at the beginning. If you now type...

	PRINT CHR$ 8; OVER 1;"/"

...why do you recover an unblemished 'B'?

2. Run this program...

	10 POKE 22528+ RND *704, RND *127
	20 GO TO 10

(Never mind how this program works.) The program is changing the
colours of squares on the TV screen and the RND should ensure that
this happens randomly. (The diagonal stripes that you eventually see
are a manifestation of the hidden pattern in RND, i.e. pseudo-random
instead of truly random.)