reading middlebury 'flow' files with python (bytes array & numpy)

Question

I'm trying to read a .flo file as a numpy 2Channels image.

The format is described like this:

".flo" file format used for optical flow evaluation

Stores 2-band float image for horizontal (u) and vertical (v) flow components.
Floats are stored in little-endian order.
A flow value is considered "unknown" if either |u| or |v| is greater than 1e9.

bytes  contents

0-3     tag: "PIEH" in ASCII, which in little endian happens to be the float 202021.25
        (just a sanity check that floats are represented correctly)
4-7     width as an integer
8-11    height as an integer
12-end  data (width*height*2*4 bytes total)
        the float values for u and v, interleaved, in row order, i.e.,
        u[row0,col0], v[row0,col0], u[row0,col1], v[row0,col1], ...

(Excerpt from this readme)

Here's my code but I'm kind of stuck, I don't know how to read the file as a 2 Channels numpy 2D array.

import numpy as np
import os

# test.flo, 512*512 optical flow file

f = open('test.flo', 'rb')
f.seek(11, os.SEEK_SET)  # skip header bytes


data_array = np.fromfile(f, np.float16)
data_2D = np.resize(data_array, (512, 512))

Maybe someone knows how to do that ?

Answer 1

Try this. I've tested it on one .flo file so far.

import numpy as np
import sys

if __name__ == '__main__':
    if len(sys.argv) <= 1:
        print('Specify a .flo file on the command line.')
    else:
        with open(sys.argv[1], 'rb') as f:
            magic, = np.fromfile(f, np.float32, count=1)
            if 202021.25 != magic:
                print('Magic number incorrect. Invalid .flo file')
            else:
                w, h = np.fromfile(f, np.int32, count=2)
                print(f'Reading {w} x {h} flo file')
                data = np.fromfile(f, np.float32, count=2*w*h)
                # Reshape data into 3D array (columns, rows, bands)
                data2D = np.resize(data, (w, h, 2))
                print(data2D)

Answer 2

bsa's answer does not work for python 3.5 onwards. The small modification shown below, e.g. np.fromfile(f, np.int32, count=1)[0], will.

import numpy as np
import os
import sys

# WARNING: this will work on little-endian architectures (eg Intel x86) only!
if '__main__' == __name__:
    if len(sys.argv) > 1:
        with open(sys.argv[1], 'rb') as f:
            magic = np.fromfile(f, np.float32, count=1)
            if 202021.25 != magic:
                print('Magic number incorrect. Invalid .flo file')
            else:
                w = np.fromfile(f, np.int32, count=1)[0]
                h = np.fromfile(f, np.int32, count=1)[0]
                print('Reading %d x %d flo file' % (w, h))
                data = np.fromfile(f, np.float32, count=2*w*h)
                # Reshape data into 3D array (columns, rows, bands)
                data2D = np.resize(data, (h, w, 2))
    else:
        print('Specify a .flo file on the command line.')