Font Set Manipulation in Textmode.



1. Introduction
2. Enable/Disable Font Manipulation
3. Secondary Font Set
4. BW Dithering
5. Finito



1. Introduction

As many of you probably have noticed, Symantec uses a smooth mouse_arrow
in textmode. The trick is font set manipulation. The phong_face in
FAQSYS_about is also done with font set manipulation. How is it done?

Remember to be in textmode!




2. Enable/Disable Font Manipulation

Well, first you will have to enable font manipulation. This is done by
writing some word values to some ports:

  port(0x03c4) = 0x0402;  Wait;
  port(0x03c4) = 0x0704;  Wait;
  port(0x03ce) = 0x0204;  Wait;
  port(0x03ce) = 0x0005;  Wait;
  port(0x03ce) = 0x0006;  Wait;

After each port_writing you have to wait some ticks. I think you have to wait
6 cycles or something, so this will work:

  for (i=0; i<3; ++i);

Now your videocard has enabled the font set to be manipulated. The font set
has been copied to memory_address A000:0000. Each character (0 to 255) is
located at a 32-byte block. So if you wanna find the offset to the char 'A',
which has ASCII code 65, just:

  A_address = 32*65;

Only the first 16-byte block contains character-bitdata. As you know in
80x25x16 (mode 3) the font set is 8x16, this means each byte in the 16-byte
block presents one scanline, which means 8 pixels. The letter 'A' has this
bitdata: (or something like this (I am not a graphic_man)!)


 128 32   8   2
   64  16   4   1
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º                º = 00000000 = 0
º                º = 00000000 = 0
º                º = 00000000 = 0
º      ÛÛ        º = 00010000 = 16
º    ÛÛ  ÛÛ      º = 00101000 = 40
º  ÛÛ      ÛÛ    º = 01000100 = 68
º  ÛÛ      ÛÛ    º = 01000100 = 68
ºÛÛ          ÛÛ  º = 10000010 = 130
ºÛÛ          ÛÛ  º = 10000010 = 130
ºÛÛÛÛÛÛÛÛÛÛÛÛÛÛ  º = 11111110 = 254
ºÛÛ          ÛÛ  º = 10000010 = 130
ºÛÛ          ÛÛ  º = 10000010 = 130
º                º = 00000000 = 0
º                º = 00000000 = 0
º                º = 00000000 = 0
º                º = 00000000 = 0
ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ              
                                
                         ÚÄÄÄÄÄÄÙ
                   ÚÄÄÄÄÄÁÄÄÄÄÄ¿
Well, this is your 16-byte block for the beatiful letter 'A'. Just write it
at offset (32*65). As you have finished manipulated chars, you will have to
tell your videocard to save the font set and disable font manipulation.
This is done almost the same way as enabling it. Some word values to some
ports:

  port(0x03c4) = 0x0302;  Wait;
  port(0x03c4) = 0x0304;  Wait;
  port(0x03ce) = 0x0004;  Wait;
  port(0x03ce) = 0x1005;  Wait;
  port(0x03ce) = 0x0e06;  Wait;


If you have studied the Symantec_mousearrow you probably noticed the
arrow changs some chars by adding a 9th bit. Well, the answer is that you
have to use 9-bits chars (e.g Í ÙÄ ¿), so you don't get the arrow ugly_looking
with vertical-lines! (There is one pixel_space between the chars on screen,
except for 9-bit chars. The 9th bit is simply a copy of the first bit, so
that is why some chars looks ugly. I am not sure if you can choose which
char to be 9-bit, but I haven't done any research either.



3. Secondary Font Set

Another thing which is very interesting is to enable the secondary font-set,
which means you don't have to care about all the chars you mess with, but
there is one defect. As you enable the secondary font-set, you use one bit
more, which means you will only get 0..7 background colors. If you like this
and wanna enable 512-font set, do like this:

  AH = 0x11
  AL = 0x03
  BL = 0x12
  INT 0x10


BL selects the charactor sets VIA bit 3 in char attribute. BL must be loaded
so the video_controller knows which block to use. Depending on wether bit 3
of the charactor attribute is ON or OFF. The upper 4 bits selects a block
number to use for the ON state of bit 3, the lower four bits selects the OFF
state of bit 3.



4. BW Dithering

If you are thinking of doing demo_effects in textmode with manipulated
chars as the FAQSYS_vector, you will probably need a BW dither procedure.
I read a Microsoft_doc once, and they said that pattern-dithering was
very popular and classic. I tried it and it worked.
Imagine you use 64 colors of shading in your virtual screenbuffer, and
you wanna convert it to 2 colors, so it will look ok in textmode, you
can replace each pixel with the pixel in the dither_pattern for that color.

First precalc 64 patterns, 16x16 resolution works fine.
Pattern  0 is empty
Pattern 32 is half filled (every other pixel)
Pattern 63 is solid filled

So as you convert your virtual screenbuffer pixel by pixel, your dither_pixel
is found by:

                              Which pattern
                                   ³   Y-Coordinate in pattern
                                   ³          ³   X-Coordinate in pattern
                                                       

  dither_pixel = dither_pattern[shadecolor][y and 15][x and 15]

I don't know if this is the correct way, but it works ok.


5. Finito

I think that was all. Not much, but ok if you didn't already knew this.
Sorry, but my english could be much better.

- STEFF/Sorrox