add usage

1 files changed, 27 insertions, 11 deletions

diff --git a/src/pnmcurve.c b/src/pnmcurve.c
index 7f22ba5..49adad2 100644
--- a/src/pnmcurve.c
+++ b/src/pnmcurve.c
@@ -23,6 +23,19 @@ buildCurve(const char *ctrl_points, int resolution, int maxval);
 #define GREEN 0x2
 #define BLUE 0x4
 
+static void
+usage() {
+        fprintf(stderr,
+                "usage: pnmcurve [-rgb] -c <curve>\n"
+                " -r apply curve to red channel.\n"
+                " -g apply curve to green channel.\n"
+                " -b apply curve to blue channel.\n"
+				" -c <curve> curve to apply. <curve> must be\n"
+				" of the form x1:y1,x2,y2,x3,y3 where xi,yi are\n"
+				" control points of a cubic spline curve.\n"
+				" xi,yi must be in the range [0.0,1.0].\n");
+}
+
 int
 main(int argc, char **argv) {
 	int c, i;
@@ -51,6 +64,11 @@ main(int argc, char **argv) {
 		}
 	}
 
+	if (!curve) {
+		usage();
+		exit(1);
+	}
+
 	if (readPnmHeader(stdin, &in_pnm) != 0) {
 		fprintf(stderr, "failed to read PNM file\n");
 		exit(1);
@@ -61,21 +79,19 @@ main(int argc, char **argv) {
 		exit(1);
 	}
 
-	if (curve) {
-		if (channels == 0) channels = RED | GREEN | BLUE;
+	if (channels == 0) channels = RED | GREEN | BLUE;
 
-		table = buildCurve(curve, in_pnm.maxval + 1, in_pnm.maxval);
+	table = buildCurve(curve, in_pnm.maxval + 1, in_pnm.maxval);
 
-		if (!table) {
-			fprintf(stderr, "could not build table.\n");
-			exit(1);
-		}
-	
-		gTable[0] = channels & RED ? table : NULL;	
-		gTable[1] = channels & GREEN ? table : NULL;	
-		gTable[2] = channels & BLUE ? table : NULL;	
+	if (!table) {
+		fprintf(stderr, "could not build table.\n");
+		exit(1);
 	}
 
+	gTable[0] = channels & RED ? table : NULL;	
+	gTable[1] = channels & GREEN ? table : NULL;	
+	gTable[2] = channels & BLUE ? table : NULL;	
+
 	writePnmHeader(stdout, &in_pnm);
 
 	pam_transform(stdin, stdout, &in_pnm, gTable);