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Date: Sun, 16 Feb 2020 21:01:02 +0100
From: rgl@antares-labs.eu
To: 9front@9front.org
Subject: Re: [9front] rotate: perform arbitrary rotations
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this one is better.

i'll try to come up with a crop-free method.

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diff -r 287d82efe5f2 sys/src/cmd/rotate.c
--- a/sys/src/cmd/rotate.c	Mon Jan 13 23:22:35 2020 +0000
+++ b/sys/src/cmd/rotate.c	Sun Feb 16 20:55:33 2020 +0100
@@ -3,6 +3,49 @@
 #include <draw.h>
 #include <memdraw.h>
 
+double
+round(double n)
+{
+	return floor(n + 0.5);
+}
+
+Point
+rotatept(Point p, double θ, Point c)
+{
+	Point r;
+
+	p = subpt(p, c);
+	r.x = round(p.x*cos(θ) - p.y*sin(θ));
+	r.y = round(p.x*sin(θ) + p.y*cos(θ));
+	r = addpt(r, c);
+	return r;
+}
+
+Memimage*
+rotate(Memimage *m, double θ)
+{
+	Memimage *w;
+	Rectangle r;
+	Point p, c;
+
+	if((w = allocmemimage(m->r, m->chan)) == nil)
+		sysfatal("allocmemimage: %r");
+	memfillcolor(w, DTransparent);
+	c = divpt(addpt(m->r.min, m->r.max), 2);
+	for(r = m->r; r.min.y < m->r.max.y; r.min.y++){
+		r.min.x = m->r.min.x;
+		r.max.y = r.min.y+1;
+		for(; r.min.x < m->r.max.x; r.min.x++){
+			r.max.x = r.min.x+1;
+			p = rotatept(r.min, θ, c);
+			if(ptinrect(p, m->r))
+				memimagedraw(w, r, m, p, nil, ZP, S);
+		}
+	}
+	freememimage(m);
+	return w;
+}
+
 Memimage*
 rot90(Memimage *m)
 {
@@ -124,15 +167,10 @@
 		if(f == 'l')
 			r = 180;
 	}
-	switch(r % 360){
-	case 270:
+	for(r %= 360; r >= 90; r -= 90)
 		m = rot90(m);
-	case 180:
-		m = rot90(m);
-	case 90:
-		m = rot90(m);
-		break;
-	}
+	if(r > 0)
+		m = rotate(m, -r*PI/180);
 	if(writememimage(1, m) < 0)
 		sysfatal("writememimage: %r");
 	exits(0);
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