Implementation of DDA Line Algorithm

Implementation of DDA Line Algorithm

I have tested all the cases of how a line could be 1. vertically 2.
horizontally 3. has a positive or less than 1 slope. The function works,
but I would like to review it, if there are overflows, lost test
cases..etc. I just read the algorithm in wikipedia, and tried to implement
it out of the wikipedia article.
// Draw line using DDA Algorithm
void Graphics::DrawLine( int x1, int y1, int x2, int y2, Color&color )
{
float xdiff = x1-x2;
float ydiff = y1-y2;
int slope = 1;
if ( y1 == y2 )
{
slope = 0;
}
else if ( x1 == x2 )
{
slope = 2; // vertical lines have no slopes...
}
else
{
slope = (int)xdiff/ydiff;
}
if ( slope <= 1 )
{
int startx = 0;
int endx = 0;
if ( x1 > x2 )
{
startx = x2;
endx = x1;
}
else
{
startx = x1;
endx = x2;
}
float y = y1; // initial value
for(int x = startx; x <= endx; x++)
{
y += slope;
DrawPixel(x, (int)abs(y), color);
}
}
else if ( slope > 1 )
{
float x = x1; // initial value
int starty = 0;
int endy = 0;
if ( y1 > y2 )
{
starty = y2;
endy = y1;
}
else
{
starty = y1;
endy = y2;
}
for(int y = starty; y <= endy; y++)
{
x += 1/slope;
DrawPixel((int)x, y, color);
}
}
}