diff --git a/src/common.h b/src/common.h
index 3fa0d4a..55feacb 100644
--- a/src/common.h
+++ b/src/common.h
@@ -5,16 +5,19 @@
 #include <gsl/gsl_errno.h>
 #include <math.h>
 
+#define FLAT 0
 #define PI 3.1415926535897932384626433832795028841971693993751058
 #define Rs 1
-#define M (0.5*Rs)
+#define M (FLAT ? 0 : 0.5*Rs)
 #define R0 (15*Rs)
-#define f(r) ((1.0-Rs/((double)r)))
-#define bmin (1.5*sqrt(3.0)+5e-10)
+#define f(r) ((1.0-(2*M)/((double)r)))
+#define bmin (FLAT ? 1e-10 : 1.5*sqrt(3.0)+5e-10)
 #define cotpsi_max (sqrt(R0*R0/(bmin*bmin*f(R0)) - 1))
 #define THETAERROR 100000
 #define color_index(i,j,c) (j*W*3+i*3+c)
 
+#define cutperc (FLAT ? 1.0 : 0.99)
+
 typedef struct {
   int W;
   int H;