--- ray/src/rt/ambient.c	1996/07/10 13:33:56	2.41
+++ ray/src/rt/ambient.c	1999/01/10 12:45:19	2.46
@@ -56,7 +56,8 @@ static int  nunflshed = 0;	/* number of unflushed ambi
 #define MAX_SORT_INTVL	(SORT_INTVL<<6)
 #endif
 
-static COLOR  avsum = BLKCOLOR;		/* computed ambient value sum */
+static double  avsum = 0.;		/* computed ambient value sum (log) */
+static unsigned int  navsum = 0;	/* number of values in avsum */
 static unsigned int  nambvals = 0;	/* total number of indirect values */
 static unsigned int  nambshare = 0;	/* number of values from file */
 static unsigned long  ambclock = 0;	/* ambient access clock */
@@ -69,14 +70,16 @@ static long  sortintvl = SORT_INTVL;	/* time until nex
 	 * through memory on a multiprocessor, when we want to avoid
 	 * claiming our own memory (copy on write).  Go ahead anyway
 	 * if more than two thirds of our values are unshared.
+	 * Compile with -Dtracktime=0 to turn this code off.
 	 */
+#ifndef tracktime
 #define tracktime	(shm_boundary == NULL || nambvals > 3*nambshare)
+#endif
 
 #define	 AMBFLUSH	(BUFSIZ/AMBVALSIZ)
 
 #define	 newambval()	(AMBVAL *)bmalloc(sizeof(AMBVAL))
 
-extern long  ftell(), lseek();
 static int  initambfile(), avsave(), avinsert(), sortambvals(), avlmemi();
 static AMBVAL  *avstore();
 #ifdef  F_SETLKW
@@ -121,6 +124,7 @@ double  newa;
 setambient(afile)			/* initialize calculation */
 char  *afile;
 {
+	int	readonly = 0;
 	long  pos, flen;
 	AMBVAL	amb;
 						/* init ambient limits */
@@ -135,25 +139,37 @@ char  *afile;
 		return;
 	}
 						/* open ambient file */
-	if ((ambfp = fopen(afile, "r+")) != NULL) {
-		initambfile(0);
+	if ((ambfp = fopen(afile, "r+")) == NULL)
+		readonly = (ambfp = fopen(afile, "r")) != NULL;
+	if (ambfp != NULL) {
+		initambfile(0);			/* file exists */
 		pos = ftell(ambfp);
 		while (readambval(&amb, ambfp))
 			avinsert(avstore(&amb));
-						/* align */
+		nambshare = nambvals;		/* share loaded values */
+		if (readonly) {
+			sprintf(errmsg,
+				"loaded %u values from read-only ambient file",
+					nambvals);
+			error(WARNING, errmsg);
+			fclose(ambfp);		/* close file so no writes */
+			ambfp = NULL;
+			return;			/* avoid ambsync() */
+		}
+						/* align file pointer */
 		pos += (long)nambvals*AMBVALSIZ;
 		flen = lseek(fileno(ambfp), 0L, 2);
 		if (flen != pos) {
-			error(WARNING,
+			sprintf(errmsg,
 			"ignoring last %ld values in ambient file (corrupted)",
 					(flen - pos)/AMBVALSIZ);
+			error(WARNING, errmsg);
 			fseek(ambfp, pos, 0);
 			ftruncate(fileno(ambfp), pos);
 		}
-		nambshare = nambvals;
-	} else if ((ambfp = fopen(afile, "w+")) != NULL)
-		initambfile(1);
-	else {
+	} else if ((ambfp = fopen(afile, "w+")) != NULL) {
+		initambfile(1);			/* else create new file */
+	} else {
 		sprintf(errmsg, "cannot open ambient file \"%s\"", afile);
 		error(SYSTEM, errmsg);
 	}
@@ -190,7 +206,7 @@ register RAY  *r;
 FVECT  nrm;
 {
 	static int  rdepth = 0;			/* ambient recursion */
-	double	d;
+	double	d, l;
 
 	if (ambdiv <= 0)			/* no ambient calculation */
 		goto dumbamb;
@@ -228,12 +244,18 @@ FVECT  nrm;
 		return;
 dumbamb:					/* return global value */
 	copycolor(acol, ambval);
-	if (ambvwt <= 0 | nambvals == 0)
+	if (ambvwt <= 0 | navsum == 0)
 		return;
-	scalecolor(acol, (double)ambvwt);
-	addcolor(acol, avsum);			/* average in computations */
-	d = 1.0/(ambvwt+nambvals);
-	scalecolor(acol, d);
+	l = bright(ambval);			/* average in computations */
+	if (l > FTINY) {
+		d = (log(l)*(double)ambvwt + avsum) /
+				(double)(ambvwt + navsum);
+		d = exp(d) / l;
+		scalecolor(acol, d);		/* apply color of ambval */
+	} else {
+		d = exp( avsum / (double)navsum );
+		setcolor(acol, d, d, d);	/* neutral color */
+	}
 }
 
 
@@ -367,7 +389,7 @@ COLOR  cr;
 register AMBVAL	 *ap;
 FVECT  pv, nv;
 {
-	FVECT  v1, v2;
+	FVECT  v1;
 	register int  i;
 	double	d;
 
@@ -376,9 +398,8 @@ FVECT  pv, nv;
 	for (i = 0; i < 3; i++)
 		d += ap->gpos[i]*(pv[i]-ap->pos[i]);
 					/* gradient due to rotation */
-	VCOPY(v1, ap->dir);
-	fcross(v2, v1, nv);
-	d += DOT(ap->gdir, v2);
+	VCROSS(v1, ap->dir, nv);
+	d += DOT(ap->gdir, v1);
 	if (d <= 0.0) {
 		setcolor(cr, 0.0, 0.0, 0.0);
 		return;
@@ -442,14 +463,19 @@ avstore(aval)				/* allocate memory and store aval */
 register AMBVAL  *aval;
 {
 	register AMBVAL  *av;
+	double	d;
 
 	if ((av = newambval()) == NULL)
 		error(SYSTEM, "out of memory in avstore");
 	copystruct(av, aval);
 	av->latick = ambclock;
 	av->next = NULL;
-	addcolor(avsum, av->val);	/* add to sum for averaging */
 	nambvals++;
+	d = bright(av->val);
+	if (d > FTINY) {		/* add to log sum for averaging */
+		avsum += log(d);
+		navsum++;
+	}
 	return(av);
 }