From 50f15bac90638da401d27229969dc502c75d1320 Mon Sep 17 00:00:00 2001
From: rodri <rgl@antares-labs.eu>
Date: Fri, 2 Aug 2024 17:57:42 +0000
Subject: 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.
---
 clip.c     | 34 +++++++++++++++++++++++++++++++---
 internal.h |  3 ++-
 render.c   |  6 +++---
 vertex.c   | 10 ++++++++++
 4 files changed, 46 insertions(+), 7 deletions(-)

diff --git a/clip.c b/clip.c
index a7750cf..9988ce1 100644
--- a/clip.c
+++ b/clip.c
@@ -193,11 +193,37 @@ outcode(Point p, Rectangle r)
 	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 @@ rectclipline(Rectangle r, Point *p0, Point *p1)
 		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){
diff --git a/internal.h b/internal.h
index f34776a..777ace8 100644
--- a/internal.h
+++ b/internal.h
@@ -45,6 +45,7 @@ void rmfbctl(Framebufctl*);
 
 /* 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 @@ void fprintvattrs(int, Vertex*);
 
 /* clip */
 int clipprimitive(Primitive*, Primitive*);
-int rectclipline(Rectangle, Point*, Point*);
+int rectclipline(Rectangle, Point*, Point*, Vertex*, Vertex*);
 
 /* util */
 int min(int, int);
diff --git a/render.c b/render.c
index 98dd0e9..549ee1a 100644
--- a/render.c
+++ b/render.c
@@ -128,7 +128,7 @@ rasterize(Rastertask *task)
 		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 @@ rasterize(Rastertask *task)
 
 		/* 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);
diff --git a/vertex.c b/vertex.c
index 970bbbc..92c6b0a 100644
--- a/vertex.c
+++ b/vertex.c
@@ -43,6 +43,16 @@ dupvertex(Vertex *v)
 	return nv;
 }
 
+void
+swapvertex(Vertex *a, Vertex *b)
+{
+	Vertex t;
+
+	t = *a;
+	*a = *b;
+	*b = t;
+}
+
 /*
  * linear attribute interpolation
  */
-- 
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