From 370ac137ff0ce9dfc2be54540911fc772ffabdce Mon Sep 17 00:00:00 2001
From: Antonio Carzaniga <antonio.carzaniga@usi.ch>
Date: Mon, 29 Jul 2019 15:48:15 +0200
Subject: [PATCH] added clear (alpha) parameter to draw animation trails.

---
 balls.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/balls.c b/balls.c
index 9a71616..9f1967c 100644
--- a/balls.c
+++ b/balls.c
@@ -39,6 +39,8 @@ unsigned int n_balls = 50;
 static double g_y = 20;
 static double g_x = 0;
 
+static double clear_alpha = 1.0;
+
 void random_velocity(struct ball * p) {
     double r2;
     do {
@@ -154,7 +156,7 @@ static void draw_balls_onto_window () {
 	cr = gdk_cairo_create(window->window);
 
     /* clear pixmap */
-    cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0);
+    cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, clear_alpha);
     cairo_paint(cr);
 
     draw_gravity_vector();
@@ -239,7 +241,8 @@ void print_usage (const char * progname) {
 	    "\tfy=<y-force>\n"
 	    "\tradius=<min-radius>-<max-radius>\n"
 	    "\tv=<min-velocity>-<max-velocity>\n"
-	    "\tdelta=<frame-delta-time>\n",
+	    "\tdelta=<frame-delta-time>\n"
+	    "\tclear=<clear-alpha>\n"
 	    progname);
 }
 
@@ -283,6 +286,8 @@ int main (int argc, const char *argv[]) {
 	    continue;
 	if (sscanf(argv[i], "delta=%lf", &delta) == 1)
 	    continue;
+	if (sscanf(argv[i], "clear=%lf", &clear_alpha) == 1)
+	    continue;
 	print_usage(argv[0]);
 	return 1;
     }