--- ray/src/util/dctimestep.c	2025/03/22 01:27:22	2.52
+++ ray/src/util/dctimestep.c	2025/06/06 19:11:21	2.54
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: dctimestep.c,v 2.52 2025/03/22 01:27:22 greg Exp $";
+static const char RCSid[] = "$Id: dctimestep.c,v 2.54 2025/06/06 19:11:21 greg Exp $";
 #endif
 /*
  * Compute time-step result using Daylight Coefficient method.
@@ -8,18 +8,18 @@ static const char RCSid[] = "$Id: dctimestep.c,v 2.52 
  */
 
 #include <ctype.h>
+#include "paths.h"
 #include "platform.h"
 #include "standard.h"
 #include "cmatrix.h"
 #include "platform.h"
 #include "resolu.h"
 
-char	*progname;			/* global argv[0] */
-
 /* Sum together a set of images and write result to fout */
 static int
 sum_images(const char *fspec, const CMATRIX *cv, FILE *fout)
 {
+	static int	runcnt = 0;
 	int	myDT = DTfromHeader;
 	COLR	*scanline = NULL;
 	CMATRIX	*pmat = NULL;
@@ -28,8 +28,9 @@ sum_images(const char *fspec, const CMATRIX *cv, FILE 
 
 	if (cv->ncols != 1)
 		error(INTERNAL, "expected vector in sum_images()");
-	for (i = 0; i < cv->nrows; i++) {
-		const COLORV	*scv = cv_lval(cv,i);
+	for (i = cv->nrows; i-- > 0; ) {
+		const int	r = runcnt&1 ? i : cv->nrows-1 - i;
+		const COLORV	*scv = cv_lval(cv,r);
 		int		flat_file = 0;
 		char		fname[1024];
 		FILE		*fp;
@@ -39,10 +40,10 @@ sum_images(const char *fspec, const CMATRIX *cv, FILE 
 		char		*err;
 							/* check for zero */
 		if ((scv[RED] == 0) & (scv[GRN] == 0) & (scv[BLU] == 0) &&
-				(myDT != DTfromHeader) | (i < cv->nrows-1))
+				(myDT != DTfromHeader) | (i > 0))
 			continue;
 							/* open next picture */
-		sprintf(fname, fspec, i);
+		sprintf(fname, fspec, r);
 		if ((fp = fopen(fname, "rb")) == NULL) {
 			sprintf(errmsg, "cannot open picture '%s'", fname);
 			error(SYSTEM, errmsg);
@@ -107,6 +108,7 @@ sum_images(const char *fspec, const CMATRIX *cv, FILE 
 	free(scanline);
 	i = cm_write(pmat, myDT, fout);			/* write picture */
 	cm_free(pmat);					/* free data */
+	++runcnt;					/* for zig-zagging */
 	return(i);
 }
 
@@ -177,8 +179,8 @@ main(int argc, char *argv[])
 	CMATRIX		*cmtx;		/* component vector/matrix result */
 	char		fnbuf[256];
 	int		a, i;
-
-	progname = argv[0];
+					/* set global progname */
+	fixargv0(argv[0]);
 					/* get options */
 	for (a = 1; a < argc && argv[a][0] == '-'; a++)
 		switch (argv[a][1]) {