A10 Designing New Character Sets (Fonts)¶
A font is a character set used by the computer to display characters (normally letters and numbers) on screen. A wide range of fonts can be used, which allow you to make letters appear differently on screen for different programs.
A10.1 Fonts on the QL¶
On the QL, each window can have two fonts attached. There are actually two default fonts, the first of which defines the characters from 32 to 127 and the second of which defines the characters from 127 to 255.
The reason that CHR$(127) is defined twice is that the definition in the first font is the definition used for the copyright symbol, whereas the definition in the second font is that used if a character is undefined, such as CHR$(2).
Normally this is not noticeable and when you open a new window (scr_ or con_), the standard QL fonts are attached to that channel.
Minerva users can use the commands:
POKE_L !124!40, font1
POKE_L !124!44, font2
to alter these default fonts and thereby attach user-defined fonts to every single window that is opened after this command.
SMSQ/E users can use the command CHAR_DEF to achieve a similar result.
A10.2 Changing Fonts in Programs¶
When a character is PRINTed, the QL looks up the code in the first font to find the binary definition of the character (see below). If the code is found in this first font then it is PRINTed, otherwise, the QL looks for the code in the second font and if found PRINTs this out.
If however the character is still not found, then the first character of the second font is printed out.
This therefore means that although substitute fonts need not have the same range as the QL standard fonts, it is important that a font contains all of the characters which will be used by a program!
On a QL, a font is stored in the following format:
|$0||Byte giving code of first character defined in the font|
|$1||Byte giving the number of characters defined (minus 1)|
|$2…$A||Nine bytes defining the first character|
|$B…||Nine bytes for each subsequent character…|
Each character contained in the font is designed on a grid 8 pixels wide by 9 pixels high, and therefore the easiest way of calculating the nine bytes which make up each character is to design the character on a sheet of graph paper (if you do not have a font design program) and then each row must be converted from binary to decimal.
Each square in the grid which contains a 1 will be shown as a pixel in the current INK colour when PRINTed to the screen. For instance, this is the binary representation of the character k:
| BIT 7 6 5 4 3 2 1 0 --+------------------------------- |0 0 1 0 0 0 0 0 0 = 64 |1 0 1 0 0 0 0 0 0 = 64 |2 0 1 0 0 0 1 0 0 = 68 R |3 0 1 0 0 1 0 0 0 = 72 O |4 0 1 1 1 0 0 0 0 = 112 W |5 0 1 0 0 1 0 0 0 = 72 |6 0 1 0 0 0 1 0 0 = 68 |7 0 0 0 0 0 0 0 0 = 0 |8 0 0 0 0 0 0 0 0 = 0 --+-------------------------------
Therefore a small program to set the character ‘a’ to the same as the character ‘k’ in channel #1 would be:
10 a=RESPR(11) 20 POKE a,97 : REMark 97=CODE('a') - first character in font 30 POKE a+1,0: REMark Only one character is being redefined 40 RESTORE 50 FOR i=0 TO 8:READ bit:POKE a+2+i,bit 60 DATA 64,64,68,72,112,72,68,0,0 70 CHAR_USE #1,a,0
Note that unfortunately no other characters will be printed correctly! This is because, when re-designing a font, it needs to include all of the characters which may be used in the window to which the font is attached.
When designing fonts for use on the QL, it is important that certain rules are followed to ensure that when characters are PRINTed on screen, they have the desired appearance in all screen modes. The QL converts a given character into the desired mode by doubling up various rows or columns (depending on the size set using the CSIZE command). If you intend to re-design letters, the rightmost two columns (bits 0 and 1) should always be left empty as these are ignored in current versions of the QL ROM.
The leftmost column (bit 7) is normally left blank to ensure a gap between adjacent characters, however, should you want to use this column also, you must be wary of the fact that pre-JS ROMs cannot display fonts correctly which use this column (bit 7) and so this should always be left blank unless you know that your font is going to be used specifically on later ROMs (see CSIZE).
In fact, if you are clever, you can use this bit 7 to obtain interesting effects on pre-JS ROMs, whilst still retaining the full 8 pixel wide characters (the effects vary depending on the ROM version and cannot be guaranteed!).
You should also be aware of the fact that an extra blank row is generally left between each line of characters. Toolkit II users can however prevent this by using CHAR_INC.