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The backend uses the whole frame similarity to determine if the screen has been updated:
https://github.com/os-autoinst/os-autoinst/blob/master/backend/baseclass.pm#L366

    $sim = $lastscreenshot->similarity($image) if $lastscreenshot;
    $watch->lap("similarity");

    # we have two different similarity levels - one (slightly higher value, based
    # t/data/user-settings-*) to determine if it's worth it to recheck needles
    # and one (slightly lower as less significant) determining if we write the frame
    # into the video
    if ($sim <= 54) {
        $self->last_image($image);
    }

, i.e. if the average difference is about 0.5 bit or less (similarity is in dB), the frame is treated as unchanged.

This is problematic as a small change in the needle region may not trigger an update of $self->last_image