"""
motion_capture/filter.py

Custom signal filter: convert pixel coordinates to millimetres.

Usage in Signals → Channels pipeline:
    type: pixel_to_mm
    px_per_mm: <calibration value>   pixels per millimetre

Calibration: measure a known distance on-screen (e.g. a ruler in frame)
and count how many pixels it spans. Divide pixels by mm → px_per_mm.
"""

from core.signal_processor import FilterBase


class PixelToMMFilter(FilterBase):
    """
    y = (x - zero_px) / px_per_mm

    Converts pixel position to millimetres. Optionally zero at a reference
    point (useful when origin matters more than absolute position).
    """
    name = "pixel_to_mm"

    def __init__(self, px_per_mm: float = 10.0, zero_px: float = 0.0):
        self.params = {"px_per_mm": px_per_mm, "zero_px": zero_px}

    def __call__(self, value: float) -> float:
        return (value - self.params["zero_px"]) / max(self.params["px_per_mm"], 1e-9)

    def reset(self):
        pass