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# problem with filling polygon using bresenhams algorithm

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hi all, i have a very annoying problem when filling a four sided poly using bresenhams algorithm to create an array of scanline end points. Specifically, I'm getting the problem when interpolating vertex colours; the screen coordinates are coming out fine. The problem is that some polygons are getting 'stepped' results - the colours do not blend perfectly smoothly.(ie i have bands of colour) The wierd thing is that an adjacent polygon which (duh) shares the same edge is filled fine (perfectly smooth that is). I can post code if it helps, but thought maybe some of you had come across this before. I thought it may have something to do with rounding, as I cast my rgb values as floats as well as the gradient of each colour wrt the direction i'm moving in to fill the edge array. I then round the values back to ints using this: #define round(x) ((x)>=0?(long)((x)+0.5):(long)((x)-0.5)) when I assign them to the array. Oh, dammit i might as well post the code here now. I'm writing in C and using SDL fyi. Another thing I thought it might be is some difference between the Dx > Dy and Dx < Dy code, but that doesn't really make sense, given that an adjacent polygon fills ok. thanks for any help you can offer, i'm stumped on this one
void fill_edgearray(fill_line *pointarray,edge e, int miny, int maxy)
{
int x1,y1,x2,y2;
int bx, by, dx, dy, xsign, ysign, p, const1, const2;
float dr, dg, db; /*slopes of the rgb parts of a colour*/
float r,g,b; /*current rgb values */
colour current_col;

x1 = e.points[0].x;
x2 = e.points[1].x;
y1 = e.points[0].y;
y2 = e.points[1].y;

bx = x1;		/*b represents current value */
by = y1;
dx = (x2 - x1);
dy = (y2 - y1);
/*initialise colours*/
r = float(e.points[0].col.red);
g = float(e.points[0].col.green);
b = float(e.points[0].col.blue);
/*set up the intial colour*/
set_colour(¤t_col,round(r),round(g),round(b));

if(dy==0) /* horizontal*/
{
/* don't do anything at the moment as we don't add horizontal lines to scanline array */
}
else if (dx == 0) /* vertical line*/
{
dr = float(e.points[1].col.red - e.points[0].col.red)/float(dy);
dg = float(e.points[1].col.green - e.points[0].col.green)/float(dy);
db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dy);
ysign = dy /abs(dy);
add_fill_line(&pointarray[by-miny], bx,current_col);
while(by != y2)
{
by+=ysign;
r+=dr;
g+=dg;
b+=db;
set_colour(¤t_col,round(r),round(g),round(b));
add_fill_line(&pointarray[by-miny], bx,current_col);
}
}
else	/*bresenhal algo starts here*/
{
xsign = dx/abs(dx);
ysign = dy/abs(dy);

dx = abs(dx);
dy = abs(dy);

if (by != maxy)
add_fill_line(&pointarray[by-miny], bx,current_col); /*initial point on line*/
if (dx < dy) /*more vertical than horizontal*/
{
/*initialise color slope variables*/
dr = float(e.points[1].col.red - e.points[0].col.red)/float(dy);
dg = float(e.points[1].col.green - e.points[0].col.green)/float(dy);
db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dy);

p = 2*dx - dy;
const1 = 2*dx;
const2 = 2* (dx - dy);
while (by != y2)
{
by = by + ysign;
r+=dr;
g+=dg;
b+=db;
if(p < 0){
p = p + const1;
}
else
{
p = p + const2;
bx = bx + xsign;
set_colour(¤t_col,round(r),round(g),round(b));
}
if (by != maxy)
add_fill_line(&pointarray[by-miny], bx,current_col);
}

}
else /*more horizontal than vertical*/
{
/*initialise color slope variables*/
dr = float(e.points[1].col.red - e.points[0].col.red)/float(dx);
dg = float(e.points[1].col.green - e.points[0].col.green)/float(dx);
db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dx);

p = 2*dy - dx;
const1 = 2*dy;
const2 = 2*(dy - dx);
while (bx != x2)
{
bx = bx + xsign;
r+=dr;
g+=dg;
b+=db;
if(p < 0){
p = p + const1;
}
else{
p = p + const2;
by = by + ysign;
set_colour(¤t_col,round(r),round(g),round(b));
}
if (by != maxy)
add_fill_line(&pointarray[by-miny], bx,current_col);
}

}
}
}



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well isn't that funny, asking for help can be the best kind of help itself. silly me, I wasn't assigning the colour values to my colour struct unless the Y/X value had changed. sorry for wasting your time!

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for future reference, use the source tags instead of code tags for that much code.
#define round(x) ((x)&gt;=0?(long)((x)+0.5):(long)((x)-0.5))void fill_edgearray(fill_line *pointarray,edge e, int miny, int maxy){	int x1,y1,x2,y2;	int bx, by, dx, dy, xsign, ysign, p, const1, const2;	float dr, dg, db; /*slopes of the rgb parts of a colour*/	float r,g,b; /*current rgb values */	colour current_col;			x1 = e.points[0].x;	x2 = e.points[1].x;	y1 = e.points[0].y;	y2 = e.points[1].y;	bx = x1;		/*b represents current value */	by = y1;	dx = (x2 - x1);	dy = (y2 - y1);	/*initialise colours*/	r = float(e.points[0].col.red);	g = float(e.points[0].col.green);	b = float(e.points[0].col.blue);	/*set up the intial colour*/	set_colour(&current_col,round(r),round(g),round(b));	if(dy==0) /* horizontal*/	{		/* don't do anything at the moment as we don't add horizontal lines to scanline array */	}	else if (dx == 0) /* vertical line*/	{		dr = float(e.points[1].col.red - e.points[0].col.red)/float(dy);		dg = float(e.points[1].col.green - e.points[0].col.green)/float(dy);		db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dy);		ysign = dy /abs(dy);		add_fill_line(&pointarray[by-miny], bx,current_col);		while(by != y2)		{			by+=ysign;			r+=dr;			g+=dg;			b+=db;			set_colour(&current_col,round(r),round(g),round(b));			add_fill_line(&pointarray[by-miny], bx,current_col);		}	}	else	/*bresenhal algo starts here*/	{		xsign = dx/abs(dx);		ysign = dy/abs(dy);				dx = abs(dx);		dy = abs(dy);		if (by != maxy)			add_fill_line(&pointarray[by-miny], bx,current_col); /*initial point on line*/		if (dx &lt; dy) /*more vertical than horizontal*/		{			/*initialise color slope variables*/			dr = float(e.points[1].col.red - e.points[0].col.red)/float(dy);			dg = float(e.points[1].col.green - e.points[0].col.green)/float(dy);			db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dy);			p = 2*dx - dy;			const1 = 2*dx;			const2 = 2* (dx - dy);			while (by != y2)			{				by = by + ysign;				r+=dr;				g+=dg;				b+=db;				if(p &lt; 0){					p = p + const1;				}				else				{					p = p + const2;					bx = bx + xsign;					set_colour(&current_col,round(r),round(g),round(b));									}				if (by != maxy)					add_fill_line(&pointarray[by-miny], bx,current_col);			}					}		else /*more horizontal than vertical*/		{			/*initialise color slope variables*/			dr = float(e.points[1].col.red - e.points[0].col.red)/float(dx);			dg = float(e.points[1].col.green - e.points[0].col.green)/float(dx);			db = float(e.points[1].col.blue - e.points[0].col.blue)/float(dx);						p = 2*dy - dx;			const1 = 2*dy;			const2 = 2*(dy - dx);			while (bx != x2)			{				bx = bx + xsign;				r+=dr;				g+=dg;				b+=db;				if(p &lt; 0){					p = p + const1;				}				else{					p = p + const2;					by = by + ysign;					set_colour(&current_col,round(r),round(g),round(b));				}				if (by != maxy)					add_fill_line(&pointarray[by-miny], bx,current_col);			}					}	}}

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