diff --git a/camera.py b/camera.py
index 4125842..da33a98 100644
--- a/camera.py
+++ b/camera.py
@@ -223,7 +223,7 @@ def angle_to_rotation_matrix(a,axis):
     M = M.roll((roll,roll),dims=(-2,-1))
     return M
 
-def get_center_and_ray(opt,pose,intr=None): # [HW,2]
+def get_center_and_ray(opt,pose,intr=None,ray_idx=None): # [HW,2]
     # given the intrinsic/extrinsic matrices, get the camera center and ray directions]
     assert(opt.camera.model=="perspective")
     with torch.no_grad():
@@ -232,6 +232,9 @@ def get_center_and_ray(opt,pose,intr=None): # [HW,2]
         x_range = torch.arange(opt.W,dtype=torch.float32,device=opt.device).add_(0.5)
         Y,X = torch.meshgrid(y_range,x_range) # [H,W]
         xy_grid = torch.stack([X,Y],dim=-1).view(-1,2) # [HW,2]
+    if ray_idx is not None:
+        # consider only subset of rays
+        xy_grid = xy_grid[ray_idx]
     # compute center and ray
     batch_size = len(pose)
     xy_grid = xy_grid.repeat(batch_size,1,1) # [B,HW,2]
diff --git a/model/nerf.py b/model/nerf.py
index b0dcb2c..efd9b6d 100644
--- a/model/nerf.py
+++ b/model/nerf.py
@@ -231,12 +231,9 @@ def get_pose(self,opt,var,mode=None):
 
     def render(self,opt,pose,intr=None,ray_idx=None,mode=None):
         batch_size = len(pose)
-        center,ray = camera.get_center_and_ray(opt,pose,intr=intr) # [B,HW,3]
+        center,ray = camera.get_center_and_ray(opt,pose,intr=intr,ray_idx=ray_idx) # [B,HW,3]
         while ray.isnan().any(): # TODO: weird bug, ray becomes NaN arbitrarily if batch_size>1, not deterministic reproducible
-            center,ray = camera.get_center_and_ray(opt,pose,intr=intr) # [B,HW,3]
-        if ray_idx is not None:
-            # consider only subset of rays
-            center,ray = center[:,ray_idx],ray[:,ray_idx]
+            center,ray = camera.get_center_and_ray(opt,pose,intr=intr,ray_idx=ray_idx) # [B,HW,3]
         if opt.camera.ndc:
             # convert center/ray representations to NDC
             center,ray = camera.convert_NDC(opt,center,ray,intr=intr)