How to prevent things from out of sight rendering

Question

I was wondering if there was a way to only calculate the things that are currently visible in the frame. My scene has a lot of trees that are slowing down my render times, and these are mostly out of sight. This would be especially efficient especially in my scene.

TL;DR Is there a way to stop things in the background that are out of sight from being processed/ slowing down rendering?

Answer 1

and these are mostly out of sight

but we can still see them!

Please read How is Cycles different from Blender Internal first.

Most materials [and objects] scatter light, so Cycles needs to account for light bouncing in different directions. How this is done depends on which [Integrator][6] is used.

Objects scatter light even if they are outside of the visible frame.

If you remove them, you will likely receive a different result, even if they weren't directly visible. There are many ways to restrict rendering manually.

You could do a check with python to see if an object is in the cameras frustum. Adapting the code from this reddit post¹ we can select all the objects which don't contain points in the frustum. However for a production ready check, we would also have to take edges/faces modifiers and reflections into account.+

import bpy
import bmesh
from mathutils import Vector
from bpy_extras.object_utils import world_to_camera_view


def check_if_vertices_in_frustum(scene, cam, obj):
    mat_world = obj.matrix_world
    cs, ce = cam.data.clip_start, cam.data.clip_end

    for v in obj.data.vertices:
        co_ndc = world_to_camera_view(scene, cam, mat_world * v.co)
        #check wether point is inside frustum
        if (0.0 < co_ndc.x < 1.0 and
            0.0 < co_ndc.y < 1.0 and
             cs < co_ndc.z <  ce):
            return True
    return False



scene = bpy.context.scene
cam = scene.camera

for obj in scene.objects:
    if obj.type == 'MESH':
        if (check_if_vertices_in_frustum(scene, cam, obj)):
            obj.select = False
            # obj.hide_render = False
        else:
            obj.select = True
            # obj.hide_render = True

¹ Help with finding whether a vertex is visible from a camera or not