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double-buffered drawing

	drawing is now done on a dedicated Image which is drawn on screen
	at the end of the setup() and draw() calls.

--- a/a.h

+++ b/a.h

@@ -16,8 +16,10 @@

 	Image *i;

 };

+void	drawcanvas(void);

 void	initstate(void);

 void	registerfuncs(lua_State*);

 Image*	getcolor(int);

+extern Image* canvas;

--- a/api.c

+++ b/api.c

@@ -1,5 +1,6 @@

 #include "a.h"

+Image *canvas;

 int nostroke;

 Image *stroke;

 int strokewidth;

@@ -9,6 +10,10 @@

 void

 initstate(void)

 {

+	Rectangle r;

+

+	r = rectsubpt(screen->r, screen->r.min);

+	canvas = allocimage(display, r, screen->chan, 0, DNofill);

 	nostroke = 0;

 	stroke = display->black;

 	strokewidth = 1;

@@ -16,6 +21,13 @@

 	fill = display->white;

 }

+void

+drawcanvas(void)

+{

+	draw(screen, screen->r, canvas, nil, ZP);

+	flushimage(display, 1);

+}

+

 int

 cbackground(lua_State *L)

 {

@@ -24,7 +36,7 @@

 	n = luaL_checkinteger(L, 1);

 	i = getcolor(n);

-	draw(screen, screen->r, i, nil, ZP);

+	draw(canvas, canvas->r, i, nil, ZP);

 	return 0;

 }

@@ -85,10 +97,10 @@

 	y1 = luaL_checkinteger(L, 2);

 	x2 = luaL_checkinteger(L, 3);

 	y2 = luaL_checkinteger(L, 4);

-	p1 = addpt(screen->r.min, Pt(x1, y1));

-	p2 = addpt(screen->r.min, Pt(x2, y2));

+	p1 = Pt(x1, y1);

+	p2 = Pt(x2, y2);

 	if(!nostroke)

-		line(screen, p1, p2, 0, 0, strokewidth, stroke, ZP);

+		line(canvas, p1, p2, 0, 0, strokewidth, stroke, ZP);

 	return 0;

 }

@@ -102,13 +114,13 @@

 	x = luaL_checkinteger(L, 1);

 	y = luaL_checkinteger(L, 2);

 	w = luaL_checkinteger(L, 3);

-	p1 = addpt(screen->r.min, Pt(x, y));

+	p1 = Pt(x, y);

 	p2 = addpt(p1, Pt(w, w));

 	r = Rpt(p1, p2);

 	if(!nofill)

-		draw(screen, r, fill, nil, ZP);

+		draw(canvas, r, fill, nil, ZP);

 	if(!nostroke)

-		border(screen, r, strokewidth, stroke, ZP);

+		border(canvas, r, strokewidth, stroke, ZP);

 	return 0;

 }

@@ -123,13 +135,13 @@

 	y = luaL_checkinteger(L, 2);

 	w = luaL_checkinteger(L, 3);

 	h = luaL_checkinteger(L, 4);

-	p1 = addpt(screen->r.min, Pt(x, y));

+	p1 = Pt(x, y);

 	p2 = addpt(p1, Pt(w, h));

 	r = Rpt(p1, p2);

 	if(!nofill)

-		draw(screen, r, fill, nil, ZP);

+		draw(canvas, r, fill, nil, ZP);

 	if(!nostroke)

-		border(screen, r, strokewidth, stroke, ZP);

+		border(canvas, r, strokewidth, stroke, ZP);

 	return 0;

 }

@@ -142,11 +154,11 @@

 	x = luaL_checkinteger(L, 1);

 	y = luaL_checkinteger(L, 2);

 	a = luaL_checkinteger(L, 3);

-	p = addpt(screen->r.min, Pt(x, y));

+	p = Pt(x, y);

 	if(!nofill)

-		fillellipse(screen, p, a, a, fill, ZP);

+		fillellipse(canvas, p, a, a, fill, ZP);

 	if(!nostroke)

-		ellipse(screen, p, a, a, strokewidth, stroke, ZP);

+		ellipse(canvas, p, a, a, strokewidth, stroke, ZP);

 	return 0;

 }

@@ -160,11 +172,11 @@

 	y = luaL_checkinteger(L, 2);

 	a = luaL_checkinteger(L, 3);

 	b = luaL_checkinteger(L, 4);

-	p = addpt(screen->r.min, Pt(x, y));

+	p = Pt(x, y);

 	if(!nofill)

-		fillellipse(screen, p, a, b, fill, ZP);

+		fillellipse(canvas, p, a, b, fill, ZP);

 	if(!nostroke)

-		ellipse(screen, p, a, b, strokewidth, stroke, ZP);

+		ellipse(canvas, p, a, b, strokewidth, stroke, ZP);

 	return 0;

 }

@@ -180,11 +192,11 @@

 	b = luaL_checkinteger(L, 4);

 	c = luaL_checkinteger(L, 5);

 	d = luaL_checkinteger(L, 6);

-	p = addpt(screen->r.min, Pt(x, y));

+	p = Pt(x, y);

 	if(!nofill)

-		fillarc(screen, p, a, b, fill, ZP, c, d);

+		fillarc(canvas, p, a, b, fill, ZP, c, d);

 	if(!nostroke)

-		arc(screen, p, a, b, strokewidth, stroke, ZP, c, d);

+		arc(canvas, p, a, b, strokewidth, stroke, ZP, c, d);

 	return 0;

 }

--- a/slug.c

+++ b/slug.c

@@ -11,7 +11,7 @@

 	if(!lua_isfunction(L, -1))

 		return;

 	lua_call(L, 0, 0);

-	flushimage(display, 1);

+	drawcanvas();

 }

 void

@@ -21,7 +21,7 @@

 	if(!lua_isfunction(L, -1))

 		return;

 	lua_call(L, 0, 0);

-	flushimage(display, 1);

+	drawcanvas();

 }

 Image*