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Model distortion

Distortions on 3D models may be desired for many use cases, such as to test the generalization of machine learning algorithms.

Prerequisites

These instructions have been tested in Blender 2.92, which has a revamped user interface from previous versions. The version in apt-get may be older. These instructions should work in newer versions. Install manually as needed. If newer versions do not work for you, Blender 2.92 can be found here.

Helpful tip for Blender Python development: To show Python API in the tooltips when the cursor is hovered over a button or field, go to Edit > Preferences, Interface tab, Display group, check Python Tooltips.

Usage

Launch Blender GUI.

Locate model file and mesh name

Before we run the script, we need to locate the path of the 3D model file and find the name of the part in the actual model file. These will be passed as parameters to the script.

First, open or import the 3D model file of the object you want to distort. In upper-left corner, File > Import. Navigate to the mesh file you want. There are two Coke can models as examples: models/dave_object_models/models/Coke/meshes/coke.obj and models/dave_object_models/models/Coke_Can/meshes/coke_can.obj. Choose Transform while importing: Z Up, X Forward.

Tip: To view the textured object in the viewport, click on the globe icon in the upper right of the viewport, which is for Viewport Shading: Material Preview.

If the texture is not loading correctly for you, check that you have the entire folder for the object, which includes the meshes and materials directories.

Look for the mesh name in the model. You can find this in the View Layer list in the upper-right panel. If the model has multiple meshes, you will need to merge the parts, so that the entire model is distorted, or choose the part that you wish to distort.

Tweak the model (optional)

Note that depending on how the polygons are arranged on a mesh, the results of the distortion will differ. Advanced users may choose to split up the polygons or otherwise change the polygon layout to get more even distortion results. This is a matter of trial and error until you are happy with the distortion result.

Run the script

At the top of Blender GUI, go to the Scripting tab. All commands will be executed in the Console panel in the middle-left of the screen.

Define the full path to the model file and the mesh name, which you found above. For example, the Coke model:

file_path = '/path/to/dave/models/dave_object_models/models/Coke/meshes/coke.obj'
object_prefix = 'LPCoke_Cube'

Or the Coke_Can model:

file_path = '/path/to/dave/models/dave_object_models/models/Coke_Can/meshes/coke_can.obj'
object_prefix = 'coke_can'

Set optional arguments. If not specified, the default will be used. Make sure method is specified as a list in brackets, even if it contains only one element.

distort_extent = 0.2
method = ['subdiv_mod', 'vert_rand', 'edge_subdiv']

Put the args into the input array:

import sys
sys.argv = ['distort.py', file_path, object_prefix, distort_extent, method]

Run the script, replacing its path with the one on your machine:

exec(open('/path/to/dave/urdf/scripts/mesh_distortion/distort.py').read());

This will execute the script with the command line arguments defined in sys.argv and export the result to file.

To add a new distortion method

There are many ways in Blender to modify a mesh. The example methods in the script may not have everything you need.

To add a new distortion method to the script, follow these steps:

  1. Experiment with the distortion method you want, manually in Blender, until you know the exact steps to reproduce a desired result. Find the Python API for the fields and buttons in the Blender user interface. You can find the Blender Python API name by hovering on most buttons, if you have Python Tooltips turned on in Edit > Preferences.
  2. In the Blender Python script, add a string parameter describing the distortion type to the METHODS list. A descriptive name would be one that closely resembles the core essential function name in Blender.
  3. Add a Python function to perform all the necessary steps programmatically, using the API you found above.
  4. Following the existing pattern in the Python script, add an if-statement to call the new method.
  5. Run it! Trial and error to make sure it works for different models. See troubleshoot section below.

Troubleshoot

Incorrect context

Blender might complain about incorrect context. One cause is that the object you are trying to modify is not the "active" object. (Note that this is different from the object being selected. An object can be selected, indicated by an orange outline, but not active, which would be indicated by a white outline.)

To make the object active, find the object programatically, then call

bpy.context.view_layer.objects.active = obj

There are example calls in the Python script.

Reference: Fouling rating scale

The script currently does not follow this scale. We do not have active plans to adhere to this scale. It is reproduced here for reference for possible adaptations.

Reproduced from Table 081-1-1 in Waterborne Underwater Hull Cleaning of Navy Ships, Chapter 081, Naval Ships' Technical Manual, S9086-CQ-STM-010, revision 5, 1 Oct 2006.

Fouling ratings (FR) in order of increasing severity

Type FR Description
Soft 0 A clean, foul-free surface; red and/or black AF paint or a bare metal surface.
Soft 10 Light shades of red and green (incipient slime). Bare metal and painted surfaces are visible beneath the fouling.
Soft 20 Slime as dark green patches with yellow or brown colored areas (advanced slime). Bare metal and painted surfaces may by obscured by the fouling.
Soft 30 Grass as filaments up to 3 inches (76 mm) in length, projections up to 1/4 inch (6.4 mm) in height; or a flat network of filaments, green, yellow, or brown in color; or soft non calcareous fouling such as sea cucumbers, sea grapes, or sea squirts projecting up to 1/4 inch (6.4 mm) in height. The fouling can not be easily wiped off by hand.
Hard 40 Calcareous fouling in the form of tubeworms less than 1⁄4 inch in diameter or height.
Hard 50 Calcareous fouling in the form of barnacles less than 1⁄4 inch in diameter or height.
Hard 60 Combination of tubeworms and barnacles, less than 1⁄4 inch (6.4 mm) in diameter or height.
Hard 70 Combination of tubeworms and barnacles, greater than 1⁄4 inch in diameter or height.
Hard 80 Tubeworms closely packed together and growing upright away from surface. Barnacles growing one on top of another, 1⁄4 inch or less in height. Calcareous shells appear clean or white in color.
Hard 90 Dense growth of tubeworms with barnacles, 1⁄4 inch or greater in height; Calcareous shells brown in color (oysters and mussels); or with slime or grass overlay.
Composite 100 All forms of fouling present, Soft and Hard, particularly soft sedentary animals without calcareous covering (tunicates) growing over various forms of hard growth.

Tips

  • Collision geometry

    When using the mesh with a physics engine, for example by way of an SDF file to be loaded into Gazebo, note that it is usually not a good idea to use the visual mesh for collision, because of the high polygon count. Typically, a simple primitive is used for collision, which makes computations much faster.

    However, if the goal is to have mesh deformation affect physical interactions like manipulation, using a simple primitive would defeat the purpose. In that case, a good compromise is to optimize (for example, using the Decimate modifier in Blender) the visual mesh down to a fraction of its polygon count, and use the optimized mesh for collision. There will be mismatches between collision and visual geometry, but collision computations would be much faster.

Known issues

  • Blender sometimes does not export the texture back to a COLLADA file correctly. Inputting OBJ and exporting COLLADA works fine.