diff --git a/src/common.h b/src/common.h
index 1301b3c..4d74d22 100644
--- a/src/common.h
+++ b/src/common.h
@@ -11,3 +11,10 @@
 #define bmin (1.5*sqrt(3.0)+5e-10)
 #define THETAERROR 100000
 
+typedef struct {
+  int W;
+  int H;
+  double w; //width of the sensor. focal length=1
+  int (*angle_to_pixel)(double *, double *);
+} System;
+
diff --git a/src/render.c b/src/render.c
new file mode 100644
index 0000000..1dc2c09
--- /dev/null
+++ b/src/render.c
@@ -0,0 +1,43 @@
+#include <gsl/gsl_interp.h>
+#include <gsl/gsl_spline.h>
+#include "common.h"
+#include "init.h"
+
+int MC_pixel_render(const System *system, int i, int j, const gsl_spline *spline, double *rgb, int pixel_render_max, double pixel_render_err){
+  double dw = (system->w)/(system->W); // width and hight of one pixel
+  double x_lu = i*dw - (system->w)/2;
+  double y_lu = -j*dw + (system->H)*dw/2;
+
+  //TODO: fill the MC intergral
+
+  return 0;
+}
+
+int pixel_render(const System *system, int i, int j, const gsl_spline *spline, double *rgb, int pixel_render_max, double pixel_render_err){
+  return MC_pixel_render(system, i, j, spline, rgb, pixel_render_max, pixel_render_err);
+}
+
+int render(System *system, double **buffer, int pixel_render_max, double pixel_render_err, double chi_rela_err) {
+  double h = (system->H)*(system->w)/(system->W);
+  double tanpsi2 = (system->w)*(system->w)+h*h;
+  double cotpsi2 = 1/tanpsi2;
+  double bmax = R0/sqrt(f(R0)*(1+cotpsi2));
+ 
+  double *x=NULL;
+  double *y = NULL;
+  int size;
+
+  init(bmax, chi_rela_err, &size, &x, &y);
+
+  gsl_interp_accel *acc = gsl_interp_accel_alloc();
+  gsl_spline *spline = gsl_spline_alloc(gsl_interp_steffen, size);
+  gsl_spline_init(spline, x, y, size);
+
+  for(int j = 0; j < system->H; j++) {
+    for (int i = 0; i < system->W; i++) {
+      pixel_render(system, i, j, spline, buffer[j*(system->W) + i], pixel_render_max, pixel_render_err);
+    }
+  }
+
+  return 0;
+}