I85 Answer ― Draw Line

/* Draw a line in the image */
void drawColorLine( colorImage image, int row0, int col0,
                    int row1, int col1, pixel pix)
{
  int r, c;

  /* For lines less than 45deg with x axis */
  /* move from column to column and plot the row on the line */
  if ( abs(col1-col0) > abs(row1-row0) )
  {
    /* make col0 left of col1 */
    if ( col0>col1 )
    {
      int temp = col0; col0 = col1; col1 = temp;
      temp = row0; row0 = row1; row1 = temp;
    }
    for ( c=col0; c<col1; c++ )
    {
      r = row0 + (c-col0)*(row1-row0)/(col1-col0) ;
      setColorPixel( image, r, c, pix );
    }
  }

  /* For lines greater or equal to 45deg with x axis */
  /* move from row to row and plot the col on the line */
  else
  {
    /* make row0 above row1 */
    if ( row0>row1 )
    {
      int temp = col0; col0 = col1; col1 = temp;
      temp = row0; row0 = row1; row1 = temp;
    }
    for ( r=row0; r<row1; r++ )
    {
      c = col0 + (r-row0)*(col1-col0)/(row1-row0) ;
      setColorPixel( image, r, c, pix );
    }
  }
}

Comments: For speed (important in graphics operations), move the calculation of the slope outside of the loop. An optimizing compiler might do this automatically.