How to effectively share a smaller area of a huge display

sharescreen.sh

#!/bin/bash

set -o errexit
set -o nounset
set -o pipefail
set -o monitor
set -o noglob

# https://ffmpeg.org/ffmpeg-devices.html#x11grab
# https://ffmpeg.org/ffmpeg-filters.html#eq
# https://ffmpeg.org/ffmpeg-utils.html#Color

# https://trac.ffmpeg.org/wiki/Capture/Desktop

# https://superuser.com/questions/1776901/streaming-video-over-udp-with-ffmpeg-h264-low-latency
# https://stackoverflow.com/questions/16658873/how-to-minimize-the-delay-in-a-live-streaming-with-ffmpeg

# https://zayne.io/articles/vintage-camera-filters-with-ffmpeg

V_WIDTH=1920
V_HEIGHT=1080
V_FORMAT=apng
V_FPS=10

V_FILTER_FONT="/usr/share/fonts/truetype/ubuntu/Ubuntu-B.ttf"
V_FILTER_EXPR="drawtext=fontfile=${V_FILTER_FONT}:text='Nick Mazurkin':fontcolor=CCCCCC:fontsize=24:box=1:[email protected]:boxborderw=10:x=(w-text_w-20):y=(h-text_h-20)"
#V_FILTER_EXPR="${V_FILTER_EXPR}, eq=brightness=0.0:saturation=0.7"
#V_FILTER_EXPR="${V_FILTER_EXPR}, colortemperature=temperature=5000:mix=0.5"

echo ">>> Press 'q' to exit the streamer"

/usr/bin/stdbuf -i0 -o0 -e0 \
    ffmpeg \
        -hide_banner -loglevel info -nostats \
        -video_size "${V_WIDTH}x${V_HEIGHT}" \
        -framerate "${V_FPS}" \
        -follow_mouse 200 -draw_mouse 1 -show_region 1 -region_border 2 \
        -f x11grab \
        -i "${DISPLAY}" \
        -vf "${V_FILTER_EXPR}" \
        -f "${V_FORMAT}" \
        - \
    | ffplay \
        -hide_banner -loglevel info -nostats \
        -x "${V_WIDTH}" -y "${V_HEIGHT}" \
        -window_title "screen" \
        -autoexit -fflags nobuffer -flags low_delay -probesize 32 -analyzeduration 1 -strict experimental -framedrop \
        -an \
        -sn \
        -vf setpts=0 \
        -sync ext \
        -f "${V_FORMAT}" \
        -i -