Pipeline from matplotlib to gstreamer (hardware encode)

[gaggio]

Hi everyone!
I'm trying to produce a video file from a series of matplotlib plots.
I successfully produced the video with the animation module of matplotlib, but it was far too slow for my application (it was saving temporary png files to make the animation) so I ended up feeding the raw images to ffmpeg through stdin, see my code below. This already gives a nice speedup, but further improvement should be possible using the hardware encoder through gstreamer.
I'm stuck trying to figure out how to modify the following code to build a proper gstreamer pipeline...

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import numpy as np
import matplotlib.pylab as plt
import time
import subprocess

# Number of frames
nframes = 200

# Generate data
x = np.linspace(0, 100, num=nframes)
y = np.random.random_sample(np.size(x))

def testSubprocess(x, y):
    
    start_time = time.time()
    
    #set up the figure
    fig = plt.figure(figsize=(15, 9))
    canvas_width, canvas_height = fig.canvas.get_width_height()
        
    # First frame
    ax0 = plt.plot(x,y)
    pointplot, = plt.plot(x[0], y[0], 'or')
    
    def update(frame):
        # your matplotlib code goes here
        pointplot.set_data(x[frame],y[frame])
    
    # Open an ffmpeg process
    outf = 'testSubprocess.mp4'
    cmdstring = ('ffmpeg', 
                 '-y', '-r', '1', # overwrite, 1fps
                 '-s', '%dx%d' % (canvas_width, canvas_height), # size of image string
                 '-pix_fmt', 'argb', # format
                 '-f', 'rawvideo',  '-i', '-', # tell ffmpeg to expect raw video from the pipe
                 '-vcodec', 'mpeg4', outf) # output encoding
    p = subprocess.Popen(cmdstring, stdin=subprocess.PIPE)
    
    # Draw frames and write to the pipe
    for frame in range(nframes):
        # draw the frame
        update(frame)
        fig.canvas.draw()

        # extract the image as an ARGB string
        string = fig.canvas.tostring_argb()

        # write to pipe
        p.stdin.write(string)

    # Finish up
    p.communicate()
    
    print("Movie written in %s seconds" % (time.time()-start_time))

if __name__ == "__main__":
    testSubprocess(x, y)

I think the source for the pipeline should be

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filesrc location=/dev/stdin

but how to define the format of the input stream?

Thanks,
Federico