shithub: libgraphics

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clip: fix rectclipline.

mixed up CLIP[TB] and the slope was being computed as
an integer division, which caused artifacts.

--- a/clip.c

+++ b/clip.c

@@ -179,22 +179,25 @@

 /*

  * Cohen-Sutherland rectangle-line clipping

  */

-void

+int

 rectclipline(Rectangle r, Point *p0, Point *p1)

 {

 	int code0, code1;

-	int Δx;

+	int Δx, Δy;

 	double m;

 	Δx = p1->x - p0->x;

-	m = Δx == 0? 0: (p1->y - p0->y)/Δx;

+	Δy = p1->y - p0->y;

+	m = Δx == 0? 0: (double)Δy/Δx;

 	for(;;){

 		code0 = outcode(*p0, r);

 		code1 = outcode(*p1, r);

-		if(lineisinside(code0, code1) || lineisoutside(code0, code1))

-			break;

+		if(lineisinside(code0, code1))

+			return 0;

+		else if(lineisoutside(code0, code1))

+			return -1;

 		if(ptisinside(code0)){

 			swappt(p0, p1);

@@ -207,11 +210,11 @@

 		}else if(code0 & CLIPR){

 			p0->y += (r.max.x - p0->x)*m;

 			p0->x = r.max.x;

-		}else if(code0 & CLIPB){

+		}else if(code0 & CLIPT){

 			if(p0->x != p1->x && m != 0)

 				p0->x += (r.min.y - p0->y)/m;

 			p0->y = r.min.y;

-		}else if(code0 & CLIPT){

+		}else if(code0 & CLIPB){

 			if(p0->x != p1->x && m != 0)

 				p0->x += (r.max.y - p0->y)/m;

 			p0->y = r.max.y;

--- a/internal.h

+++ b/internal.h

@@ -52,7 +52,7 @@

 /* clip */

 int clipprimitive(Primitive*);

-void rectclipline(Rectangle, Point*, Point*);

+int rectclipline(Rectangle, Point*, Point*);

 /* util */

 int min(int, int);

--- a/render.c

+++ b/render.c

@@ -205,7 +205,8 @@

 		p0 = Pt(prim.v[0].p.x, prim.v[0].p.y);

 		p1 = Pt(prim.v[1].p.x, prim.v[1].p.y);

 		/* clip it against our wr */

-		rectclipline(task->wr, &p0, &p1);

+		if(rectclipline(task->wr, &p0, &p1) < 0)

+			break;

 		/* transpose the points */

 		if(abs(p0.x-p1.x) < abs(p0.y-p1.y)){