Processing/Line Graphing - Help

Question

I posted something similar yesterday, but got nothing. I spent a few hours today problem-solving, but didn't progress any.

I'm using Processing (the language) and trying to implement a method that draws a line between two points. (I don't want to use the library's line() method.)

My lineCreate method works great for positive slopes, but fails with negative slopes. Can you help figure out why?

Here's the lineCreate() code:

void createLine(int x0, int y0, int x1, int y1){
   //...
  // Handle slanted lines...
  double tempDX = x1 - x0;
  double tempDY = y1 - y0;            // Had to create dx and dy as doubles because typecasting dy/dx to a double data type wasn't working.
  double m = (-tempDY / tempDX);      // m = line slope. (Note - The dy value is negative 
  int deltaN = (2 * -dx);        // deltaX is the amount to increment d after choosing the next pixel on the line.
  int deltaNE = (2 * (-dy - dx));      // ...where X is the direction moved for that next pixel. 
  int deltaE = (2 * -dy);            // deltaX variables are used below to plot line.
  int deltaSE = (2 * (dy + dx));
  int deltaS = (2 * dx);
  int x = x0; 
  int y = y0;
  int d = 0;                            // d = Amount d-value changes from pixel to pixel. Depends on slope.
  int region = 0;                  // region = Variable to store slope region. Different regions require different formulas.
 if(m > 1){                            // if-statement: Initializes d, depending on the slope of the line.
      d = -dy - (2 * dx);                  // If slope is 1-Infiniti. -> Use NE/N initialization for d.
      region = 1;
  }
  else if(m == 1)
    region = 2;
  else if(m > 0 && m < 1){
      d = (2 * -dy) - dx;                  // If slope is 0-1 -> Use NE/E initialization for d.
      region = 3;
  }
  else if(m < 0 && m > -1){          
      d = (2 * dy) + dx;                  // If slope is 0-(-1) -> Use E/SE initliazation for d.
      region = 4;
  }
  else if(m == -1)
    region = 5;
  else if(m < -1){
      d = dy + (2 * dx);                  // If slope is (-1)-(-Infiniti) -> Use SE/S initialization for d.
      region = 6;
  }
  while(x < x1){                    // Until points are connected...
        if(region == 1){          // If in region one...
              if(d <= 0){                // and d<=0...
              d += deltaNE;            // Add deltaNE to d, and increment x and y.
              x = x + 1; 
              y = y - 1;
            }
            else{              
              d += deltaN;        // If d > 0 -> Add deltaN, and increment y.
              y = y - 1;
            }
        }
        else if(region == 2){
             x = x + 1;
             y = y - 1; 
        }
        else if(region == 3){      // If region two...
                if(d <= 0){              
              d += deltaE;
              x = x + 1; 
            }
            else{
              d += deltaNE;
              x = x + 1;
              y = y - 1;
            }
        }
        else if(region == 4){        // If region three...
              if(d <= 0){              
              d += deltaSE;
              x = x + 1; 
              y = y + 1;
            }
            else{
              d += deltaE;
              x = x + 1;
            }
        }
        else if(region == 5){
             x = x + 1;
             y = y + 1; 
        }
        else if(region == 6){        // If region four...
              if(d <= 0){              
              d += deltaSE;
              x = x + 1; 
              y = y + 1;
            }
            else{
              d += deltaS;
              y = y + 1;
            }
          }
        point(x, y);          // Paints new pixel on line going towards (x1,y1).
  }
  return;
}

Answer 1

Have a look at this page. It explains the whole theory behind line drawing with code examples.

There are a number of known algorithm for line drawing. Read about them here.