--- ray/src/px/mt160r.c	1990/01/26 08:17:31	1.6
+++ ray/src/px/mt160r.c	1991/05/06 13:14:21	1.8
@@ -49,7 +49,10 @@ char  *fname;
 		return(-1);
 	}
 				/* discard header */
-	getheader(input, NULL);
+	if (checkheader(input, COLRFMT, NULL) < 0) {
+		fprintf(stderr, "%s: not a Radiance picture\n", fname);
+		return(-1);
+	}
 				/* get picture dimensions */
 	if (fgetresolu(&xres, &yres, input) != (YMAJOR|YDECR)) {
 		fprintf(stderr, "%s: bad picture size\n", fname);
@@ -115,14 +118,16 @@ COLR  clr;
 register int  x;
 {
 	static int  cerr[NCOLS];
-	static int  err;
+	static int  err, errp;
 	int  b;
 	register int  isblack;
 
 	b = normbright(clr);
+	errp = err;
 	err += b + cerr[x];
 	isblack = err < 128;
 	if (!isblack) err -= 256;
-	cerr[x] = err /= 2;
+	err /= 3;
+	cerr[x] = err + errp;
 	return(isblack);
 }