--- ray/src/rt/ambient.c	2005/08/22 16:07:26	2.62
+++ ray/src/rt/ambient.c	2007/09/15 02:47:30	2.65
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char	RCSid[] = "$Id: ambient.c,v 2.62 2005/08/22 16:07:26 greg Exp $";
+static const char	RCSid[] = "$Id: ambient.c,v 2.65 2007/09/15 02:47:30 greg Exp $";
 #endif
 /*
  *  ambient.c - routines dealing with ambient (inter-reflected) component.
@@ -192,8 +192,7 @@ setambient(void)				/* initialize calculation */
 		sprintf(errmsg, "cannot open ambient file \"%s\"", ambfile);
 		error(SYSTEM, errmsg);
 	}
-	nunflshed++;	/* lie */
-	ambsync();
+	ambsync();			/* load previous values */
 }
 
 
@@ -877,12 +876,12 @@ ambsync(void)			/* synchronize ambient file */
 	AMBVAL	avs;
 	register int  n;
 
-	if (nunflshed == 0)
+	if (ambfp == NULL)	/* no ambient file? */
 		return(0);
 	if (lastpos < 0)	/* initializing (locked in initambfile) */
 		goto syncend;
-				/* gain exclusive access */
-	aflock(F_WRLCK);
+				/* gain appropriate access */
+	aflock(nunflshed ? F_WRLCK : F_RDLCK);
 				/* see if file has grown */
 	if ((flen = lseek(fileno(ambfp), (off_t)0, SEEK_END)) < 0)
 		goto seekerr;
@@ -905,9 +904,10 @@ ambsync(void)			/* synchronize ambient file */
 			avinsert(avstore(&avs));
 			n -= AMBVALSIZ;
 		}
+		lastpos = flen - n;
 		/*** seek always as safety measure
 		if (n) ***/			/* alignment */
-			if (lseek(fileno(ambfp), (off_t)(flen-n), SEEK_SET) < 0)
+			if (lseek(fileno(ambfp), (off_t)lastpos, SEEK_SET) < 0)
 				goto seekerr;
 	}
 #ifdef  DEBUG
@@ -920,8 +920,11 @@ ambsync(void)			/* synchronize ambient file */
 #endif
 syncend:
 	n = fflush(ambfp);			/* calls write() at last */
-	if ((lastpos = lseek(fileno(ambfp), (off_t)0, SEEK_CUR)) < 0)
-		goto seekerr;
+	if ((n == EOF) | (lastpos < 0)) {
+		if ((lastpos = lseek(fileno(ambfp), (off_t)0, SEEK_CUR)) < 0)
+			goto seekerr;
+	} else
+		lastpos += (long)nunflshed*AMBVALSIZ;
 	aflock(F_UNLCK);			/* release file */
 	nunflshed = 0;
 	return(n);
@@ -935,7 +938,7 @@ seekerr:
 extern int
 ambsync(void)			/* flush ambient file */
 {
-	if (nunflshed == 0)
+	if (ambfp == NULL)
 		return(0);
 	nunflshed = 0;
 	return(fflush(ambfp));