Peggy2 0.30b line() improvement

[GoogleCodeExporter]

The Peggy2.cpp 0.30b line() function was returning some strange results.  
For example, (10,0) to (11,24) resulted in all points from (10,0) to 
(10,23), and point (11,24) being activated.  To me it would be better to 
have the 'split' show up in the middle - something like (10,0)-(10,12) and 
(11,13)-(11,24).

A simple change in the library function made this work.  See the "Count=
(dx-dy)/2" line and its counterpart in the snippet below.

Cheers,
Tim

void Peggy2::Line(int8_t x1, int8_t y1, int8_t x2, int8_t y2)
{
  if ( (x1>=25 && x2>=25) || (y1>=25 && y2>=25) ) return;
  int8_t dx = abs(x2 -x1);
  int8_t dy = abs(y2 -y1);

  int8_t p1x,p1y,p2x,p2y,i;

  if (dx > dy)
  {
    if (x2>x1) {
      p1x=x1;
      p1y=y1;
      p2x=x2;
      p2y=y2;
    } 
    else {
      p1x=x2;
      p1y=y2;
      p2x=x1;
      p2y=y1;
    }

    int8_t y = p1y;
    int8_t x = p1x;
    int8_t count = 0;
    int8_t increment = p2y > p1y ? 1 : -1;
count=(dx-dy)/2;
    for (i=0; i<=dx; i++)
    {   
      count += dy;
      if (count > dx)
      {
        count -= dx; 
        y+= increment;
      }             
      if (y>=0 && y<25 && x>=0 && x<25) 
        SetPoint(x,y);
        x++; 
      if (x>=25) 
        break;
    }
  }
  else
  {
    if (y2>y1) {
      p1x=x1;
      p1y=y1;
      p2x=x2;
      p2y=y2;
    } 
    else {
      p1x=x2;
      p1y=y2;
      p2x=x1;
      p2y=y1;
    }
    int8_t y = p1y;
    int8_t x = p1x;
    int8_t count = 0;
    int8_t increment = p2x > p1x ? 1 : -1;
count=(dy-dx)/2;
    for (i=0; i<=dy; i++)
    {   
      count += dx;
      if (count > dy)
      {
        count -= dy; 
        x+= increment;
      }             
      if (y>=0 && y<25 && x>=0 && x<25) SetPoint(x,y);
      y+=1; 
      if (y>=25) break;
    }
  }
}

Original issue reported on code.google.com by [email protected] on 5 Jan 2010 at 4:36

[GoogleCodeExporter]

I agree that this looks better.  One improvement to this code would be to use a 
bit shift instead of division-- it's 
much faster on the AVR.   The assignment can also be combined with declaration:

  int8_t count = (dx-dy) >> 1; 

Perhaps surprising, but just switching from division to a bit shift actually 
saves 86 bytes of program space.

Original comment by [email protected] on 7 Jan 2010 at 5:06

• Changed state: Started