If you want to add to the page content, you can add to the layout. All Vizy programs have a section where they set the layout:
kapp.layout = html.Div([video, ...etc...]
You can add ‘hello world’ to the page pretty simply:
kapp.layout = html.Div(["hello world", video, ...etc...]
Vizy is based on Dash – there is some information on layouts here.
Adding overlay text on the video is a but more involved, but here are some simple things you can do:
When instantiating Kvideo, set overlay to True:
video = kritter.Kvideo(overlay=True)
then at anytime (probably within a rendering thread) you can call draw_text and push the overlay changes to the client’s browser:
video.draw_text(100, 100, "hello world) # text at x=100, y=100 video pixels
Take a look at the video example (/home/pi/vizy/examples/video) – it’s a good example to modify.
Also, in case you haven’t seen, this tutorial has some useful information.