This code generates a very short video in the format of Yuv4mpeg2. It should be easily exendible to accept a series om bitmap images and create a video out of them.

yuv4mpeg2_sample.py

WIDTH = 100
HEIGHT = 100
DURATION = 1 # seconds
FRAMES = 25*DURATION

def rgb2ycrcb(r,g,b):
    """ ITU-R BT.601 (formerly CCIR 601) transformation
    input in range 0-255
    output in range 16-235 (headroom/footroom) for Y'
    output in range 16-240 (headroom/footroom) for Cr/Cb

    The man pages describes one shold use CCIR-601 colorspace for yuv4mpeg2 https://linux.die.net/man/5/yuv4mpeg
    The transformation from 8-bit rgb to that space is described at wikipedia under  https://en.wikipedia.org/wiki/YCbCr
    """
    y =  int( 16 +  65.841*r/255 + 128.553*g/255 +  24.966*b/255)
    cb = int(128 -  37.797*r/255 -  74.203*g/255 + 112.000*b/255)
    cr = int(128 + 112.000*r/255 -  93.786*g/255 -  18.214*b/255)
    return y, cr, cb

with open("test.y4m", 'wb') as f:
    # the simples possible header. more or less...
    # See the man-pages https://linux.die.net/man/5/yuv4mpeg for an explanation
    f.write(f"YUV4MPEG2 W{WIDTH} H{HEIGHT} F25:1 Ip A1:1 C444\n".encode())

    for frame in range(FRAMES):
        
        # each frame begins with a header. this is the simplest one
        f.write(b'FRAME\n')

        # construct image data pixel-per-pixel
        # ... but reorganize it so it comes channel-wise
        # in this setting each channel is called a 'plane'
        # one may use subsampling, so one only keeps every 2nd or every 4th Cr and Cb value
        # but in this example, I use C444, so we have the same number or chroma (Cr/Cb) values as luminosities (Y)
        yplane = []
        crplane = []
        cbplane = []
        for row in range(HEIGHT):
            for col in range(WIDTH):
                r,g,b = int(255*frame/FRAMES),int(row*255/HEIGHT),int(255*col/WIDTH)
                y,cr,cb = rgb2ycrcb(r,g,b)
                yplane.append(y)
                crplane.append(cr)
                cbplane.append(cb)

        # finally write the planes.
        # first the Y' plane
        f.write(bytes(yplane))
        # then the chroma-planes
        # if not using the C444 options, the Cr and Cb planes could be shorter
        f.write(bytes(crplane))
        f.write(bytes(cbplane))