Latest version that mostly works.

terminal_char_load.c

/**
 * PLATOTerm64 - A PLATO Terminal for the Commodore 64
 * Based on Steve Peltz's PAD
 * 
 * Author: Thomas Cherryhomes <thom.cherryhomes at gmail dot com>
 *
 * terminal_char_load.c - Character set loading routine for 5x6 font.
 */

#include <cbm.h>
#include <c64.h>
#include <string.h>
#include <stdio.h>
#include <stdarg.h>
#include "../terminal.h"
#include "../protocol.h"

// Temporary PLATO character data, 8x16 matrix
static unsigned char char_data[]={0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
				  0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00};

static unsigned char BTAB[]={0x80,0x40,0x20,0x10,0x08,0x04,0x02,0x01}; // flip one bit on (OR)
static unsigned char BTAB_5[]={0x08,0x10,0x10,0x20,0x20,0x40,0x80,0x80}; // flip one bit on for the 5x6 matrix (OR)

static unsigned char TAB_0_5[]={0x05,0x05,0x05,0x04,0x04,0x04,0x03,0x03,0x02,0x02,0x01,0x01,0x01,0x00,0x00,0x00};
static unsigned char TAB_0_5i[]={0x00,0x00,0x00,0x01,0x01,0x01,0x02,0x02,0x03,0x03,0x04,0x04,0x04,0x05,0x05,0x05};

static unsigned char TAB_0_4[]={0x00,0x00,0x01,0x02,0x02,0x03,0x03,0x04}; // return 0..4 given index 0 to 7

static unsigned char PIX_THRESH[]={0x03,0x02,0x03,0x03,0x02, // Pixel threshold table.
				   0x03,0x02,0x03,0x03,0x02,
				   0x02,0x01,0x02,0x02,0x01,
				   0x02,0x01,0x02,0x02,0x01,
				   0x03,0x02,0x03,0x03,0x02,
				   0x03,0x02,0x03,0x03,0x02};

static unsigned char PIX_WEIGHTS[]={0x00,0x00,0x00,0x00,0x00, // Pixel weights
				    0x00,0x00,0x00,0x00,0x00,
				    0x00,0x00,0x00,0x00,0x00,
				    0x00,0x00,0x00,0x00,0x00,
				    0x00,0x00,0x00,0x00,0x00,
				    0x00,0x00,0x00,0x00,0x00};

static unsigned char TAB_0_25[]={0,5,10,15,20,25}; // Given index 0 of 5, return multiple of 5.

static unsigned char pix_cnt;     // total # of pixels
static unsigned char curr_word;   // current word
static unsigned char u,v;       // loop counters

extern unsigned char fontm23[768];
extern unsigned short fontptr[160];

/**
 * terminal_char_load - Store a character into the user definable
 * character set.
 */
void terminal_char_load(padWord charnum, charData theChar)
{
  // Clear char data. 
  memset(char_data,0,sizeof(char_data));
  memset(PIX_WEIGHTS,0,sizeof(PIX_WEIGHTS));
  memset(&fontm23[fontptr[charnum]],0,6);
  
  // Transpose character data.  
  for (curr_word=0;curr_word<8;curr_word++)
    {
      for (u=16; u-->0; )
	{
	  if (theChar[curr_word] & 1<<u)
	    {
	      pix_cnt++;
	      PIX_WEIGHTS[TAB_0_25[TAB_0_5[u]]+TAB_0_4[curr_word]]++;
	      char_data[u^0x0F&0x0F]|=BTAB[curr_word];
	    }
	}
    }

  // Determine algorithm to use for number of pixels.
  // Algorithm A is used when roughly half of the # of pixels are set.
  // Algorithm B is used either when the image is densely or sparsely populated (based on pix_cnt).
  if ((54 <= pix_cnt) && (pix_cnt < 85))
    {
      // Algorithm A - approx Half of pixels are set
      for (u=6; u-->0; )
  	{
  	  for (v=5; v-->0; )
  	    {
  	      if (PIX_WEIGHTS[TAB_0_25[u]+v] >= PIX_THRESH[TAB_0_25[u]+v])
  		fontm23[fontptr[charnum]+u]|=BTAB[v];
  	    }
  	}
    }
  else if ((pix_cnt < 54) || (pix_cnt >= 85))
    {
      // Algorithm B - Sparsely or heavily populated bitmaps
      for (u=16; u-->0; )
	{
	  for (v=8; v-->0; )
	    {
	      if (char_data[u] & (1<<v))
		{
		  fontm23[fontptr[charnum]+TAB_0_5i[u]]|=BTAB_5[v];
		}
	    }
	}
    }
  
}