The depth map generated by the Cycles rendering engine seems to have (radial) distortion, while the Blender renderer does not.

However, I could not find anywhere information on the internal camera model used by the Cycles engine, leading to this very post.


With the Blender default scene and default parameters, I use a Viewer node connected to the Z Render layer and a python script to read the generated depth map, compute 3D point coordinates and export as OBJ.

The node graph is very simple:

enter image description here

The export python code too:

import bpy
import numpy as np

print("exporting depth pointcloud to OBJ...")

scale = bpy.context.scene.render.resolution_percentage / 100

pixels = bpy.data.images['Viewer Node'].pixels
img = np.array(pixels[:]).reshape(int(bpy.context.scene.render.resolution_y * scale), int(bpy.context.scene.render.resolution_x * scale),-1)

camdata = bpy.data.objects["Camera"].data

sensor_height = bpy.context.scene.render.resolution_y / bpy.context.scene.render.resolution_x * camdata.sensor_width

f = open("/tmp/pointcloud_blender.obj","w")

for u in range(0, img.shape[1]):
    for v in range(0, img.shape[0]):

        d = img[v, u][0]

        if d > 100.0:

        x = d * (0.5 - float(u) / float(img.shape[1])) * camdata.sensor_width / camdata.lens;
        y = d * (0.5 - float(v) / float(img.shape[0])) * sensor_height / camdata.lens;
        z = d;

        f.write("v " + str(x) + " " + str(y) + " " + str(z) + "\n")



When rendering with the Blender internal engine, the generated point cloud looks like the square as it should (white), but the Cycles rendering engine gives a distorted point cloud (red):

enter image description here

When looking at the depth maps, one can clearly see a radial distortion (Blender render vs Cycles render, exported with File Output node):

enter image description here

This is a cut of the depth maps (blue, green) and difference image (red) at row 240:

enter image description here

(the two depth maps being different are not really the problem here, as long as I have the correct unprojection model)


I looked in the documentation, in this answer stating that the Blender camera has no distortion coefficients, on this blog that is the correct model I use for the Blender camera.

I also studied the code for Cycles (src/render/camera.cpp, src/util/util_projection.h, src/util/util_transform.h) but could not find any trace of radial distortion.


Does anybody have an idea of the internal camera model used by the Cycles rendering engine ? Or how to compute correct camera intrinsic parameters ? I guess the lens has distortion, but I could not find parameters or even code running this.

I need to be able to use the point cloud from the depth maps from different views, but with distortion they are not usable.

Thanks !

  • 7
    $\begingroup$ Welcome to BSE. This is the best first post I have seen one this site. Very well done, the formatting, layout, sources, and research all excellent. I wish everybody put as much effort into their posts. Glad you found the answer (and then shared it here!), sorry we could not help. $\endgroup$ – David Feb 9 '19 at 3:53
  • 1
    $\begingroup$ This completely slipped under my radar, great post, welcome to the community! $\endgroup$ – Jaroslav Jerryno Novotny Mar 13 '19 at 0:01

Thanks to the Blender dev forum, I figured it out.

The Cycles camera is a pinhole model that "uses the distance between a given point and the pinhole as its Z depth". So we cannot apply the depth to points on the camera plane, but rather points that lie on the unit sphere, hence normalized.

Here is the correct formula to unproject a camera pixel into a 3D point :

    import math

    # coordinates on camera plane
    x = (0.5 - float(u) / float(img.shape[1])) * camdata.sensor_width / camdata.lens;
    y = (0.5 - float(v) / float(img.shape[0])) * sensor_height / camdata.lens;
    z = 1.0;

    norm = math.sqrt(x*x + y*y + z*z)

    # normalize = project point on unit sphere, then apply depth
    x = d * x / norm
    y = d * y / norm
    z = d * z / norm

The relevant code is in kernel_camera.h


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.