How to achieve the equivalent of ffmpeg's -colorspace command line option using PyAV's APIs?

[yosefk]

I have pretty basic PyAV-based code producing slightly wrong colors similarly to using ffmpeg with the default options. The code is:

class MP4:
    def __init__(self, fname, width, height, fps):
        self.output = av.open(fname, 'w', format='mp4')
        self.stream = self.output.add_stream('h264', str(fps))
        self.stream.width = width
        self.stream.height = height
    def write_frame(self, pixels):
        frame = av.VideoFrame.from_ndarray(pixels, format='rgb24')
        packet = self.stream.encode(frame)
        self.output.mux(packet)
    def close(self):
        packet = self.stream.encode(None)
        self.output.mux(packet)
        self.output.close()

The ffmpeg invocation producing similarly wrong colors is:

ffmpeg -y -r 12 -i 'frame%04d.png' -r 12 out.mp4

Using the -colorspace flag fixes the colors in this ffmpeg invocation:

ffmpeg -colorspace bt709 -y -r 12 -i 'frame%04d.png' -r 12 out.mp4

What is the equivalent in PyAV's APIs? (My apologies for what I'm guessing is the very basic question, I've spent more time trying to get this right than I'd like to admit... I did try setting setting {stream.codec_context,frame}.{colorspace,color_range,color_tsc,color_primaries} - I actually expected setting frame's attributes to work since the ffmpeg -colorspace option, according to ffmpeg's printouts, modifies the input PNGs' colorspace, not the output sequence's, which is confirmed by checking these attributes by opening the good & bad ffmpeg output files using PyAV & printing the values - both good & bad file have the same attributes and it's not bt709. But setting the input frame's colorspace does nothing, too.)

[yosefk]

Adding frame = frame.reformat(format='yuv420p', dst_colorspace=av.video.reformatter.Colorspace.ITU709) before the call to encode fixes the problem. I don't know why both the format and the dst_colorspace arguments are needed for this to work, I tried them separately to no effect and gave up on them but now they turn out to work when combined. (I would guess that only the dst_colorspace option would be needed based on the reasoning in the end of the question.)