shithub: libgraphics

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fix line interpolation during rasterization.

vertex attributes were not being updated after clipping
with their corresponding work rectangles, nor when
reordering the points before rasterization, which caused
serious artifacts.

--- a/clip.c

+++ b/clip.c

@@ -193,11 +193,37 @@

 	return code;

 }

+/* lerp vertex attributes to match the new positions */

+void

+adjustverts(Point *p0, Point *p1, Vertex *v0, Vertex *v1)

+{

+	Vertex v[2];

+	Point3 dp;

+	Point Δp;

+	double len, perc;

+

+	dp = subpt3(v1->p, v0->p);

+	len = hypot(dp.x, dp.y);

+

+	Δp = subpt(Pt(v0->p.x, v0->p.y), *p0);

+	perc = len == 0? 0: hypot(Δp.x, Δp.y)/len;

+	lerpvertex(&v[0], v0, v1, perc);

+

+	Δp = subpt(Pt(v0->p.x, v0->p.y), *p1);

+	perc = len == 0? 0: hypot(Δp.x, Δp.y)/len;

+	lerpvertex(&v[1], v0, v1, perc);

+

+	delvattrs(v0);

+	delvattrs(v1);

+	*v0 = dupvertex(&v[0]);

+	*v1 = dupvertex(&v[1]);

+}

+

 /*

  * Cohen-Sutherland rectangle-line clipping

  */

 int

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

+rectclipline(Rectangle r, Point *p0, Point *p1, Vertex *v0, Vertex *v1)

 {

 	int code0, code1;

 	int Δx, Δy;

@@ -211,14 +237,16 @@

 		code0 = outcode(*p0, r);

 		code1 = outcode(*p1, r);

-		if(lineisinside(code0, code1))

+		if(lineisinside(code0, code1)){

+			adjustverts(p0, p1, v0, v1);

 			return 0;

-		else if(lineisoutside(code0, code1))

+		}else if(lineisoutside(code0, code1))

 			return -1;

 		if(ptisinside(code0)){

 			swappt(p0, p1);

 			swapi(&code0, &code1);

+			swapvertex(v0, v1);

 		}

 		if(code0 & CLIPL){

--- a/internal.h

+++ b/internal.h

@@ -45,6 +45,7 @@

 /* vertex */

 Vertex dupvertex(Vertex*);

+void swapvertex(Vertex*, Vertex*);

 void lerpvertex(Vertex*, Vertex*, Vertex*, double);

 void berpvertex(Vertex*, Vertex*, Vertex*, Vertex*, Point3);

 void delvattrs(Vertex*);

@@ -52,7 +53,7 @@

 /* clip */

 int clipprimitive(Primitive*, Primitive*);

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

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

 /* util */

 int min(int, int);

--- a/render.c

+++ b/render.c

@@ -128,7 +128,7 @@

 		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 */

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

+		if(rectclipline(task->wr, &p0, &p1, &prim.v[0], &prim.v[1]) < 0)

 			break;

 		/* transpose the points */

@@ -140,8 +140,8 @@

 		/* make them left-to-right */

 		if(p0.x > p1.x){

-			swapi(&p0.x, &p1.x);

-			swapi(&p0.y, &p1.y);

+			swappt(&p0, &p1);

+			swapvertex(&prim.v[0], &prim.v[1]);

 		}

 		dp = subpt(p1, p0);

--- a/vertex.c

+++ b/vertex.c

@@ -43,6 +43,16 @@

 	return nv;

 }

+void

+swapvertex(Vertex *a, Vertex *b)

+{

+	Vertex t;

+

+	t = *a;

+	*a = *b;

+	*b = t;

+}

+

 /*

  * linear attribute interpolation

  */