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Optical Flow

First row: RGB images, second row: forward optical flow generated by Blender, third row: forward optical flow from a pretrained LiteFlowNet as comparison

In this example we demonstrate how to obtain forward / backward flow values between consecutive key frames.

Flow is visible if either the camera or objects move between frames. Here, the camera undergoes the following motion:

0 -10 4 1.3 0 0  # initial position
0 -12 4 1.3 0 0  # moving away from object
2 -12 4 1.3 0 0  # moving to the right
2 -12 6 1.3 0 0  # moving upwards
1 -11 5 1.3 0 0  # combined motion (to the left, towards object and downwards)

Usage

Execute in the BlenderProc main directory:

blenderproc run examples/advanced/optical_flow/main.py examples/advanced/optical_flow/camera_positions examples/resources/scene.obj examples/advanced/optical_flow/output
  • examples/advanced/optical_flow/main.py: path to the main python file to run.
  • examples/advanced/optical_flow/camera_positions: text file with parameters of camera positions.
  • examples/resources/scene.obj: path to the object file with the basic scene.
  • examples/advanced/optical_flow/output: path to the output directory.

Visualization

Visualize the generated data:

blenderproc vis hdf5 examples/advanced/optical_flow/output/1.hdf5

Implementation

# Render the optical flow (forward and backward) for all frames
bproc.renderer.render_optical_flow(get_backward_flow=True, 
                                   get_forward_flow=True, 
                                   blender_image_coordinate_style=False)
  • Go through all cam poses and renders forward and / or backward flow
  • The images are rendered using the .exr format which allows linear colorspace and higher precision, and then converted to numpy.float32 arrays
  • Per default, Blender uses the bottom left corner as coordinate system origin. OpenCV and popular Flow datasets use the upper left corner instead - change the flag "blender_image_coordinate_style": True if you want the default Blender behaviour. Note that the colors in the visualization will be different!