SPyNet: Spatial Pyramid Network for Optical Flow

This code is based on the paper Optical Flow Estimation using a Spatial Pyramid Network.

[Unofficial Pytorch version] [Unofficial tensorflow version]

• First things first: Setting up this code
• Easy Usage: Compute Optical Flow in 5 lines
• Fast Performance Usage: Compute Optical Flow at a rocket speed
• Training: Train your own models using Spatial Pyramid approach on mulitiple GPUs
• End2End SPyNet: An easy trainable end-to-end version of SPyNet
• Optical Flow Utilities: A set of functions in lua for working around optical flow
• References: For further reading

First things first

You need to have Torch.

Install other required packages

cd extras/spybhwd
luarocks make
cd ../stnbhwd
luarocks make

For Easy Usage, follow this

Set up SPyNet

spynet = require('spynet')
easyComputeFlow = spynet.easy_setup()

Load images and compute flow

im1 = image.load('samples/00001_img1.ppm' )
im2 = image.load('samples/00001_img2.ppm' )
flow = easyComputeFlow(im1, im2)

To save your flow fields to a .flo file use flowExtensions.writeFLO.

For Fast Performace, follow this (recommended)

Set up SPyNet

Set up SPyNet according to the image size and model. For optimal performance, resize your image such that width and height are a multiple of 32. You can also specify your favorite model. The present supported modes are fine tuned models sintelFinal(default), sintelClean, kittiFinal, and base models chairsFinal and chairsClean.

spynet = require('spynet')
computeFlow = spynet.setup(512, 384, 'sintelFinal')    -- for 384x512 images

Now you can call computeFlow anytime to estimate optical flow between image pairs.

Computing flow

Load an image pair and stack and normalize it.

im1 = image.load('samples/00001_img1.ppm' )
im2 = image.load('samples/00001_img2.ppm' )
im = torch.cat(im1, im2, 1)
im = spynet.normalize(im)

SPyNet works with batches of data on CUDA. So, compute flow using

im = im:resize(1, im:size(1), im:size(2), im:size(3)):cuda()
flow = computeFlow(im)

You can also use batch-mode, if your images im are a tensor of size Bx6xHxW, of batch size B with 6 RGB pair channels. You can directly use:

flow = computeFlow(im)

Training

Training sequentially is faster than training end-to-end since you need to learn small number of parameters at each level. To train a level N, we need the trained models at levels 1 to N-1. You also initialize the model with a pretrained model at N-1.

E.g. To train level 3, we need trained models at L1 and L2, and we initialize it modelL2_3.t7.

th main.lua -fineWidth 128 -fineHeight 96 -level 3 -netType volcon \
-cache checkpoint -data FLYING_CHAIRS_DIR \
-L1 models/modelL1_3.t7 -L2 models/modelL2_3.t7 \
-retrain models/modelL2_3.t7

End2End SPyNet

The end-to-end version of SPyNet is easily trainable and is available at anuragranj/end2end-spynet.

Optical Flow Utilities

We provide flowExtensions.lua containing various functions to make your life easier with optical flow while using Torch/Lua. You can just copy this file into your project directory and use if off the shelf.

flowX = require 'flowExtensions'

[flow_magnitude] flowX.computeNorm(flow_x, flow_y)

Given flow_x and flow_y of size MxN each, evaluate flow_magnitude of size MxN.

[flow_angle] flowX.computeAngle(flow_x, flow_y)

Given flow_x and flow_y of size MxN each, evaluate flow_angle of size MxN in degrees.

[rgb] flowX.field2rgb(flow_magnitude, flow_angle, [max], [legend])

Given flow_magnitude and flow_angle of size MxN each, return an image of size 3xMxN for visualizing optical flow. max(optional) specifies maximum flow magnitude and legend(optional) is boolean that prints a legend on the image.

[rgb] flowX.xy2rgb(flow_x, flow_y, [max])

Given flow_x and flow_y of size MxN each, return an image of size 3xMxN for visualizing optical flow. max(optional) specifies maximum flow magnitude.

[flow] flowX.loadFLO(filename)

Reads a .flo file. Loads x and y components of optical flow in a 2 channel 2xMxN optical flow field. First channel stores x component and second channel stores y component.

flowX.writeFLO(filename,F)

Write a 2xMxN flow field F containing x and y components of its flow fields in its first and second channel respectively to filename, a .flo file.

[flow] flowX.loadPFM(filename)

Reads a .pfm file. Loads x and y components of optical flow in a 2 channel 2xMxN optical flow field. First channel stores x component and second channel stores y component.

[flow_rotated] flowX.rotate(flow, angle)

Rotates flow of size 2xMxN by angle in radians. Uses nearest-neighbor interpolation to avoid blurring at boundaries.

[flow_scaled] flowX.scale(flow, sc, [opt])

Scales flow of size 2xMxN by sc times. opt(optional) specifies interpolation method, simple (default), bilinear, and bicubic.

[flowBatch_scaled] flowX.scaleBatch(flowBatch, sc)

Scales flowBatch of size Bx2xMxN, a batch of B flow fields by sc times. Uses nearest-neighbor interpolation.

Timing Benchmarks

Our timing benchmark is set up on Flying chair dataset. To test it, you need to download

wget http://lmb.informatik.uni-freiburg.de/resources/datasets/FlyingChairs/FlyingChairs.zip

Run the timing benchmark

th timing_benchmark.lua -data YOUR_FLYING_CHAIRS_DATA_DIRECTORY

References

1. Our warping code is based on qassemoquab/stnbhwd.
2. The images in samples are from Flying Chairs dataset: Dosovitskiy, Alexey, et al. "Flownet: Learning optical flow with convolutional networks." 2015 IEEE International Conference on Computer Vision (ICCV). IEEE, 2015.
3. Some parts of flowExtensions.lua are adapted from marcoscoffier/optical-flow with help from fguney.
4. The unofficial PyTorch implementation is from sniklaus.

License

Free for non-commercial and scientific research purposes. For commercial use, please contact [email protected]. Check LICENSE file for details.

When using this code, please cite

Ranjan, Anurag, and Michael J. Black. "Optical Flow Estimation using a Spatial Pyramid Network." arXiv preprint arXiv:1611.00850 (2016).