Example: opencv_movieΒΆ


You can launch an interactive, editable version of this example without installing any local files using the Binder service (although note that at some times this may be slow or fail to open): launchbinder

An example that uses a function from external C library (OpenCV in this case). Works for all C-based code generation targets (i.e. for weave and cpp_standalone device) and for numpy (using the Python bindings).

This example needs a working installation of OpenCV2 and its Python bindings. It has been tested on Ubuntu 14.04 with OpenCV 2.4.8 (libopencv-dev and python-opencv packages).

import os
import urllib2
import cv2  # Import OpenCV2
import cv2.cv as cv  # Import the cv subpackage, needed for some constants

from brian2 import *

defaultclock.dt = 1*ms
prefs.codegen.target = 'weave'
prefs.logging.std_redirection = False
filename = os.path.abspath('Megamind.avi')

if not os.path.exists(filename):
    print('Downloading the example video file')
    response = urllib2.urlopen('http://docs.opencv.org/2.4/_downloads/Megamind.avi')
    data = response.read()
    with open(filename, 'wb') as f:

video = cv2.VideoCapture(filename)
width, height, frame_count = (int(video.get(cv.CV_CAP_PROP_FRAME_WIDTH)),
fps = 24
time_between_frames = 1*second/fps

# Links the necessary libraries
prefs.codegen.cpp.libraries += ['opencv_core',

# Includes the header files in all generated files
prefs.codegen.cpp.headers += ['<opencv2/core/core.hpp>',

# Pass in values as macros
# Note that in general we could also pass in the filename this way, but to get
# the string quoting right is unfortunately quite difficult
prefs.codegen.cpp.define_macros += [('VIDEO_WIDTH', width),
                                    ('VIDEO_HEIGHT', height)]
@implementation('cpp', '''
double* get_frame(bool new_frame)
    // The following initializations will only be executed once
    static cv::VideoCapture source("VIDEO_FILENAME");
    static cv::Mat frame;
    static double* grayscale_frame = (double*)malloc(VIDEO_WIDTH*VIDEO_HEIGHT*sizeof(double));
    if (new_frame)
        source >> frame;
        double mean_value = 0;
        for (int row=0; row<VIDEO_HEIGHT; row++)
            for (int col=0; col<VIDEO_WIDTH; col++)
                const double grayscale_value = (frame.at<cv::Vec3b>(row, col)[0] +
                                                frame.at<cv::Vec3b>(row, col)[1] +
                                                frame.at<cv::Vec3b>(row, col)[2])/(3.0*128);
                mean_value += grayscale_value / (VIDEO_WIDTH * VIDEO_HEIGHT);
                grayscale_frame[row*VIDEO_WIDTH + col] = grayscale_value;
        // subtract the mean
        for (int i=0; i<VIDEO_HEIGHT*VIDEO_WIDTH; i++)
            grayscale_frame[i] -= mean_value;
    return grayscale_frame;

double video_input(const int x, const int y)
    // Get the current frame (or a new frame in case we are asked for the first
    // element
    double *frame = get_frame(x==0 && y==0);
    return frame[y*VIDEO_WIDTH + x];
'''.replace('VIDEO_FILENAME', filename))
@check_units(x=1, y=1, result=1)
def video_input(x, y):
    # we assume this will only be called in the custom operation (and not for
    # example in a reset or synaptic statement), so we don't need to do indexing
    # but we can directly return the full result
    _, frame = video.read()
    grayscale = frame.mean(axis=2)
    grayscale /= 128.  # scale everything between 0 and 2
    return grayscale.ravel() - grayscale.ravel().mean()

N = width * height
tau, tau_th = 10*ms, time_between_frames
G = NeuronGroup(N, '''dv/dt = (-v + I)/tau : 1
                      dv_th/dt = -v_th/tau_th : 1
                      row : integer (constant)
                      column : integer (constant)
                      I : 1 # input current''',
                threshold='v>v_th', reset='v=0; v_th = 3*v_th + 1.0',
G.v_th = 1
G.row = 'i/width'
G.column = 'i%width'

G.run_regularly('I = video_input(column, row)',
mon = SpikeMonitor(G)
runtime = frame_count*time_between_frames
run(runtime, report='text')
device.build(compile=True, run=True)

# Avoid going through the whole Brian2 indexing machinery too much
i, t, row, column = mon.i[:], mon.t[:], G.row[:], G.column[:]

import matplotlib.animation as animation

# TODO: Use overlapping windows
stepsize = 100*ms
def next_spikes():
    step = next_spikes.step
    if step*stepsize > runtime:
        raise StopIteration()
    spikes = i[(t>=step*stepsize) & (t<(step+1)*stepsize)]
    next_spikes.step += 1
    yield column[spikes], row[spikes]
next_spikes.step = 0

fig, ax = plt.subplots()
dots, = ax.plot([], [], 'k.', markersize=2, alpha=.25)
ax.set_xlim(0, width)
ax.set_ylim(0, height)
def run(data):
    x, y = data
    dots.set_data(x, y)

ani = animation.FuncAnimation(fig, run, next_spikes, blit=False, repeat=True,