Flask: change content size - flask

Question

I have a Flask route which returns a video feed. I would like to be able to change the video frame size. How can I do this?
def gen(stream):
while True:
try:
frame = stream.get_last()
if frame is not None:
yield (b'--frame\r\n'
b'Pragma-directive: no-cache\r\n'
b'Cache-directive: no-cache\r\n'
b'Cache-control: no-cache\r\n'
b'Pragma: no-cache\r\n'
b'Expires: 0\r\n'
b'Content-Type: image/jpeg\r\n\r\n' + frame + b'\r\n\r\n')
except Exception as exception:
# Output unexpected Exceptions.
logging.error("Error occurred", exc_info=True)
#app.route('/video')
def video_feed():
return Response(gen(RedisImageStream(conn, args)),
mimetype='multipart/x-mixed-replace; boundary=frame')

Answer 1

If you need to change the size of the image only on the viewport, you may be able to edit your code that displays the image. If this is a website, maybe you can use some CSS configuration.
If you really need to change the size of the images you send out from your server, you will need to load each image into memory, then apply the conversion you want, and then re-encode it as JPEG. This is computationally expensive, and this is one the main sources of latency in video streaming; in fact, the main reason the streaming service of YouTube and Twitch and the usual suspects is expensive to run is because they need to re-encode the incoming video into many resolutions and send it out in real time.
For your case of Python and JPEG images, you can use PIL / Pillow. Here's an example:
import io
import PIL
def downscale(image, size):
'''
Accept a JPEG binary representation of an image,
and return the JPEG of a smaller version of the image
that has the same aspect ratio and is not larger than size.
'''
fp = io.BytesIO(image) # create a file-like object from the supplied buffer
im = PIL.Image(fp)
im_downscale = im.thumbnail(size) # Image.thumbnail creates a smaller version of the image no larger than size.
# If this is not what you want, take a look at Image.transform
outp = io.BytesIO() # create empty buffer for output
im_downscale.save(outp, "JPEG")
bytestring = outp.getvalue()
return bytestring
Then, before your yield line, call:
frame = downscale(frame, (400, 300))