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Chapter 7 - Coloured Text




The earlier chapters gave quite a lot of information if you've never encountered this sort of thing before. If you're suffering from mental indigestion, this part is a little more light hearted, just to give time for the rest of it to become comfortable. Eventually, it'll become second nature.

In this section I want to extend PRINT a little further and show you how to print in glorious technicolour instead of black and white as we've been doing so far. To change the colour of the text is easy and involves one new keyword: COLOUR (BBC BASIC will also accept COLOR if you want.)

In general, there are 16 colours we can use to liven up our text, but be wary because exactly how many of these colours are available depends on which MODE you are in. Due to restraints in the original micros like the Acorn BBC, different screen modes had different resolutions and, to conserve memory, different numbers of colours. To change modes you use the command MODE followed by the number of the mode you wish to invoke. Each mode has different resolutions in terms of number of columns wide, rows high and amount of colours available. For example, MODE 4 has 40 columns by 32 rows and only two colours.

There are many different screen modes available and you can find out all about them under MODE in the help files. Most of the examples here are in the default mode unless stated otherwise and so all 16 colours are available. The colours are invoked by using a number to represent each colour. The numbers are:

0 Black
1 Red
2 Green
3 Yellow
4 Blue
5 Magenta (blue-red)
6 Cyan (blue-green)
7 White
8 Intensified Black (grey)
9 Intensified Red
10 Intensified Green
11 Intensified Yellow
12 Intensified Blue
13 Intensified Magenta
14 Intensified Cyan
15 Intensified White

If you think of ink and paper, the colour of the ink is often referred to as the foreground colour and the colour of the paper is known as the background. To change the colour of the foreground, we merely call COLOUR X, where X is one of the numbers above - 0 to 15. The next time PRINT is called, the text will be printed in that colour. Easy.

REM Foreground colour
COLOUR 4
PRINT "I'm blue!"
END
When the program finishes, it leaves the foreground and background colours last selected. It's normally good etiquette to put things back as you found them, so perhaps we should modify the above to do this.
REM Foreground colour
COLOUR 4
PRINT "I'm blue!"
COLOUR 0
END
We can use COLOUR to change the background colour too. To do this, we add 128 to the above table and any text printed after will have the colour background given. Some programmers actually present this as a sum so it's slightly easier to read as it saves mental arithmetic. I like this but then my mental arithmetic was never too hot: it's up to you.
REM Background colour
COLOUR 128+6
PRINT "Hello, world"
COLOUR 128+15
END
You have to use a different COLOUR statement each time you change the foreground or background.
REM Foreground and background colour
COLOUR 4
COLOUR 128+6
PRINT "Hello, world"
COLOUR 0
COLOUR 128+15
END
If you are changing screens in your program, you frequently want to wipe the slate clean so information from the previous screen is not interfering with the current one. BASIC has a command to do this: CLS. This stands for CLear Screen, but takes less typing. To use it is quite simple, three letters and the previous screen is consigned to history:
REM CLS demo
PRINT TAB(35,10);"BBC BASIC"
WAIT 200
CLS
PRINT TAB(36,10);"Rules!!"
END
Should we want to make the whole of the screen change colour, first set the background colour then call CLS, like this:
REM Setting the screen colour
COLOUR 128+7
CLS
COLOUR 2
PRINT TAB(10,10);"Hello, world"
COLOUR 0
END
For a mode with limited colours it is possible to use the COLOUR command and substitute the defaults with others from the above table. The default colours for mode 4 are a black background with white text. To change the text colour to magenta call COLOUR 1,5 like this:
REM Setting the screen colour
MODE 4
PRINT "Default colour"
COLOUR 1,5
PRINT "Substituted colour"
END
This raises an interesting point. Each mode has a varying number of colours available. Mode 4 has two colours: 0 and 1, mode 5 has four: 0 to 3 and so on. When using COLOUR X%, the actual colour of the text may not directly correspond to the table above. Rather, each mode has a number of boxes into which one of the sixteen colours is slotted. If we don't like the colour BASIC chooses as a default for that box, we can change it for another as in the previous example. The box is referred to as the logical number whilst the colour is referred to as the physical number.

There are times when the default sixteen colours just aren't enough. Like when your mother just has to have Seagull Sunset Red for the bathroom. As we have just seen it is possible to make BASIC swap the colours we don't want for others from the table of sixteen, but we can also create our own and substitute these into the logical boxes. We can think of each colour for what it is: a mixture of red, green and blue and tell BASIC for colour 3 don't print yellow, print my new colour with this mix of red, green and blue. To do this, we invoke COLOUR with four parameters. The first is the colour number we wish to change, the second, third and fourth are the red, green and blue components respectively each with a range of 0 - 255. Example, to set colour 3 to orange (red = 255, green = 128, blue = 0), we would do the following:

REM Changing colours
REM Change colour 3 to orange
COLOUR 3,255,128,0
REM Now we've set the colour change to it
COLOUR 3
PRINT "Hello, world"
COLOUR 0
END

Even when the program stops, the colour will retain its new value. You can also change the colour as many times as you want, each time subsequent PRINTs will use the new colour without affecting anything previously printed with that colour index:

REM Changing colours
REM Change colour 3 to orange
COLOUR 3,255,128,0
REM Now we've set the colour change to it
COLOUR 3
PRINT "Hello, world"
REM Change colour 3 to brown
COLOUR 3,128,64,0
PRINT "Hello, world"
COLOUR 0
END
Once we've redefined a colour, it is quite easy to get back to the original colour for that particular box. If we are in a sixteen colour mode, just substitute the correct colour number from our little table and the default colour is restored again. For other modes, look in the help and it will tell you which are the defaults for that mode.

If you've got really carried away, changed lots of colours and now want to restore the defaults, BBC BASIC has a special command that will do just this. The command belongs to a family of commands that all affect the output to the screen in some way and are hence called VDU commands. To invoke it we type VDU and one or more numbers. The effects of the VDU command depend on the number that follows and are fully described in the help, the one we require is VDU 20. Here's how we do it:

REM Restoring colours
MODE 6
REM Change colour 3 to orange
COLOUR 3,255,128,0
REM Now we've set the colour change to it
COLOUR 3
PRINT "Hello, world"
REM Reset the colours back to the default
VDU 20
REM Reselect the foreground colour
COLOUR 3
PRINT "Hello, world"
END
Resetting the colours with VDU 20 also restores the default choice for the background and foreground colours.

Tip: Changing colours whilst printing
We can embed colour commands into a string variable. This way we can change colours on the fly whilst printing. To do this, we use CHR$(17) followed by CHR$(Colour_Number) before the string we want to print:
REM Changing colours in mid stream
PRINT CHR$(17)+CHR$(4)+"Hello";
PRINT CHR$(17)+CHR$(3)+" World"
COLOUR 0
END
CHR$(17) in this instance acts like the COLOUR command. Obviously, these can be hard to read, so if you're using this sort of thing a lot, you can make some strings up with this information pre-declared:
REM Changing colours in mid stream
Blue$=CHR$(17)+CHR$(4)
Yellow$=CHR$(17)+CHR$(3)
PRINT Blue$+"Hello"+Yellow$+" World"
END

Exercises

1) Examine the different MODEs in the help file, try printing text in MODEs 1 to 6 to get the feel for what they look like.
2) Modify the Changing colours program to produce new random colours by using three RND(255) statements in the call on lines 2 and 5.

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© Peter Nairn 2006