#!/usr/bin/env python3
"""Find where two captures of the same screen differ, in CSS pixels.

Capture a screen twice, once with one value changed (a price, a count, a
status), then use the box for the template's proof.box so the ring and the
callout land exactly on the change.

    python3 diff-box.py a/phone.png a/phone-after.png --dpr 3
    -> image px [1091, 1610, 1115, 1648]   css px [364, 537, 8, 13]  (x, y, w, h)

The box covers only changed pixels (often just the digits that differ), so
widen it by hand to the whole value if the ring should frame "$18", not "8".
Needs Pillow (pip install pillow).
"""
import argparse

from PIL import Image, ImageChops

ap = argparse.ArgumentParser()
ap.add_argument("before")
ap.add_argument("after")
ap.add_argument("--dpr", type=float, default=1.0, help="device pixel ratio the captures were taken at")
ap.add_argument("--threshold", type=int, default=40, help="per-channel difference that counts as a change")
a = ap.parse_args()

before = Image.open(a.before).convert("RGB")
after = Image.open(a.after).convert("RGB")
if before.size != after.size:
    h = min(before.height, after.height)
    print(f"note: sizes differ {before.size} vs {after.size}; comparing the top {h}px")
    before, after = before.crop((0, 0, before.width, h)), after.crop((0, 0, after.width, h))
mask = ImageChops.difference(before, after).convert("L").point(lambda v: 255 if v > a.threshold else 0)
box = mask.getbbox()
if not box:
    raise SystemExit("no difference found")
x0, y0, x1, y1 = box
css = [round(x0 / a.dpr), round(y0 / a.dpr), round((x1 - x0) / a.dpr), round((y1 - y0) / a.dpr)]
print(f"image px {list(box)}   css px {css}  (x, y, w, h)")