DIRT: a fast differentiable renderer for TensorFlow
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README.md

DIRT: a fast Differentiable Renderer for TensorFlow

DIRT is a library for TensorFlow, that provides operations for rendering 3D meshes. It supports computing derivatives through geometry, lighting, and other parameters. DIRT is very fast: it uses OpenGL for rasterisation, running on the GPU, which allows lightweight interoperation with CUDA.

Citation

If you use DIRT in your research, you should cite: Learning to Generate and Reconstruct 3D Meshes with only 2D Supervision (P. Henderson and V. Ferrari, BMVC 2018).

The appropriate bibtex entry is:

@inproceedings{henderson18bmvc,
  title={Learning to Generate and Reconstruct 3D Meshes with only 2D Supervision},
  author={Paul Henderson and Vittorio Ferrari},
  booktitle={British Machine Vision Conference (BMVC)},
  year={2018}
}

Why is DIRT useful?

Drawing 3D (or 2D) shapes differentiably is challenging in TensorFlow. For example, you could create a tensor containing a white square on a black background using the following:

import tensorflow as tf

canvas_width, canvas_height = 128, 128
centre_x, centre_y = 32, 64
square_size = 16

xs, ys = tf.meshgrid(tf.range(canvas_width), tf.range(canvas_height))

x_in_range = tf.less_equal(tf.abs(xs - centre_x), square_size / 2)
y_in_range = tf.less_equal(tf.abs(ys - centre_y), square_size / 2)
pixels = tf.cast(tf.logical_and(x_in_range, y_in_range), tf.float32)

However, if you calculate gradients of the pixels with respect to centre_x and centre_y, they will always be zero -- whereas for most use-cases, they should be non-zero at the boundary of the shape.

DIRT provides a single TensorFlow operation, rasterise, that renders shapes differentiably. Moreover, it includes helper code that supports 3D projection, lighting, etc. This allows full 2D or 3D scenes to be assembled directly in TensorFlow, with gradients flowing through the geometry, lighting and surface parameters.

Using DIRT, the above example becomes:

import tensorflow as tf
import dirt

canvas_width, canvas_height = 128, 128
centre_x, centre_y = 32, 64
square_size = 16

# Build square in screen space
square_vertices = tf.constant([[0, 0], [0, 1], [1, 1], [1, 0]], dtype=tf.float32) * square_size - square_size / 2.
square_vertices += [centre_x, centre_y]

# Transform to homogeneous coordinates in clip space
square_vertices = square_vertices * 2. / [canvas_width, canvas_height] - 1.
square_vertices = tf.concat([square_vertices, tf.zeros([4, 1]), tf.ones([4, 1])], axis=1)

pixels = dirt.rasterise(
    vertices=square_vertices,
    faces=[[0, 1, 2], [0, 2, 3]],
    vertex_colors=tf.ones([4, 1]),
    background=tf.zeros([canvas_height, canvas_width, 1]),
    height=canvas_height, width=canvas_width, channels=1
)[:, :, 0]

Requirements

  • an Nvidia GPU; the earliest drivers we have tested with are v367
  • Linux; we have only tested on Ubuntu, but other distributions should work
  • a GPU-enabled install of TensorFlow, version 1.4 or later recommended
  • python 2.7.9 or newer (python3 has not been tested)
  • cmake 3.8 or newer
  • gcc 4.9 or newer

Installation

Before installing, you should activate a virtualenv with tensorflow-gpu installed (or ensure your system python has that package), as DIRT will use this to search for appropriate TensorFlow headers during installation.

Simply clone this repository, then install with pip:

git clone https://github.com/pmh47/dirt.git
cd dirt
pip install .

If you plan to modify the DIRT code, you may prefer to install in development mode:

cd dirt
mkdir build ; cd build
cmake ../csrc
make
cd ..
pip install -e .

To sanity-check your build, run python tests/square_test.py, which should produce the output successful: all pixels agree.

Troubleshooting:

  • If the build cannot find GL/gl.h and GL/glext.h, you can get suitable versions of these by running the following from the dirt directory:

    mkdir external/GL ; cd external/GL
    wget https://raw.githubusercontent.com/mesa3d/mesa/master/include/GL/gl.h
    wget https://raw.githubusercontent.com/mesa3d/mesa/master/include/GL/glext.h
    cd ../..
    export INCLUDE=$PWD/external:$INCLUDE
    
  • If the build cannot find X11/Xlib.h, install the system package libx11-dev or libX11-devel

  • You should ensure that libGL and libEGL are in a location on LD_LIBRARY_PATH, and that these are the versions shipped with your Nvidia driver. In particular, if you have installed Mesa, this may have overwritten libGL with its own version, which will not work with DIRT

  • You should ensure that compute + graphics mode is enabled (through nvidia-smi) for your GPU

Usage

A simple, 2D example was given above. More sophisticated examples rendering 3D meshes are in the samples folder.

DIRT uses OpenGL for rasterisation, and uses OpenGL conventions for coordinate systems. In particular, the coordinates passed to rasterise are in OpenGL clip space, and the matrix helper functions assume that the camera points along the negative z-axis in world space. The only exception is that rasterised images follow the TensorFlow convention of having the top row first.

DIRT can be used in direct or deferred shading modes. Direct uses the rasterise operation directly to produce the final pixels, with simple Gouraud shading. Lighting calculations are performed per-vertex before rasterisation, and colours are interpolated between vertices linearly in 3D space). This is very efficient and simple to work with, but limits certain lighting effects (e.g. specular highlights) and does not allow texturing. Deferred uses the rasterise operation to generate a G-buffer, that captures the scene geometry at each pixel (typically the underlying vertex location and normal). Then, lighting calculations are performed per-pixel in a second pass.

How does DIRT work?

Theory

DIRT uses filter-based derivatives, inspired by OpenDR (Loper and Black, ECCV 2014). It makes considerable effort to return correctly-behaving derivatives even in cases of self-occlusion, where other differentiable renderers can fail.

Implementation

DIRT uses OpenGL for rasterisation, as this is fast, accurate, and very mature. We use Nvidia's OpenGL / CUDA interop to allow the vertices and pixels to remain on the same GPU both for processing by TensorFlow and for rasterisation, thus minimising copying overhead compared with other approaches. To avoid having to create an on-screen context for rendering, we use an Nvidia extension to EGL, that allows creating an OpenGL context bound to a GPU but not a physical display.

Alternatives to DIRT

Several other differentiable renderers have been described and released in recent years:

  • OpenDR (Loper and Black, ECCV 2014) supports Gouraud shading using Mesa CPU-based rendering, and uses filter-based derivatives similar to DIRT. It uses its own custom automatic differentiation framework written in python, hence does not integrate smoothly with TensorFlow

  • Neural 3D Mesh Renderer (Kato et al., CVPR 2018) supports similar functionality to DIRT, using a slightly different formulation for the approximate derivatives, but implements a custom rasterisation operation, rather than using OpenGL. It integrates with Chainer, but not TensorFlow (a PyTorch re-implementation is also available)

  • tf_mesh_renderer (Genova et al., CVPR 2018) similarly uses custom rendering (on the CPU in this case), but integrates directly with TensorFlow

  • tensorflow_mesh_renderer (Palazzi et al., ECCV Workshops 2018) renders silhouettes using built-in TensorFlow ops, but does not support shading

Contributing

Pull requests welcome!