I'm writing a program which will use scan conversion on triangles to fill in the pixels contained within the triangle.

One thing that has me confused is how to determine the x increment for the right edge of the triangle, or for slopes less than or equal to one.

Here is the code I have to handle left edges with a slope greater than one (obtained from Computer Graphics: Principles and Practice second edition):

for(y=ymin;y<=ymax;y++)
{
    edge.increment+=edge.numerator;
    if(edge.increment>edge.denominator)
    {
        edge.x++;
        edge.increment -= edge.denominator;
    }
}

The numerator is set from (xMax-xMin), and the denominator is set from (yMax-yMin)...which makes sense as it represents the slope of the line. As you move up the scan lines (represented by the y values). X is incremented by 1/(denomniator/numerator) ...which results in x having a whole part and a fractional part.

If the fractional part is greater than one, then the x value has to be incremented by 1 (as shown in edge.increment>edge.denominator).

This works fine for any left handed lines with a slope greater than one, but I'm having trouble generalizing it for any edge, and google-ing has proved fruitless.

Does anyone know the algorithm for that?