See a pulse

Blood arriving in the skin with each heartbeat changes its colour by a fraction of one step in 255. The colour pipeline amplifies exactly that.

import vidmag

vidmag.magnify("face.mp4", preset="pulse", out="pulse.mp4")

What the preset does

Parameter	Value	Controls
alpha	50	How much the change is amplified
level	4	How far the image is shrunk before filtering
fl, fh	0.833–1.0 Hz	The band kept: 50–60 beats per minute
chrom_attenuation	1.0	How much colour, versus brightness, is amplified

These are the authors' own values for their face clip. They live in vidmag.presets.PRESETS, each with a note on where it came from.

Adjusting it for your clip

The heart rate is not 50–60 bpm. Divide beats-per-minute by 60 to get the band. A resting adult at 72 bpm is 1.2 cycles per second, so roughly 1.0 to 1.4:

vidmag.magnify("face.mp4", preset="pulse", fl=1.0, fh=1.4, out="pulse.mp4")

The clip is short. A clip of N frames at f frames per second resolves frequencies in steps of f / N; a band narrower than one step selects nothing, and the result equals the input. The library warns and says how many frames it needs. Record longer or widen the band.

It flickers or looks blotchy. alpha is too high for the noise in the clip — halve it. Amplification cannot tell signal from sensor noise, and the noise usually breaks first.

Nothing happens even with the right band. Check the lighting is steady. Mains-powered light flickers at 50 or 60 cycles per second, and a camera exposure beating against it makes a slow brightness ripple that swamps a pulse.

Reading a heart rate out

The amplified result is an array, so you can take a number from it instead of watching a video. Average each frame over a patch of skin, then find the strongest frequency:

import numpy as np
import vidmag

amplified = vidmag.magnify("face.mp4", preset="pulse")
patch = amplified[:, 100:200, 100:200, :].mean(axis=(1, 2, 3))

fps = 30.0
spectrum = np.abs(np.fft.rfft(patch - patch.mean()))
freqs = np.fft.rfftfreq(len(patch), 1 / fps)
band = (freqs > 0.7) & (freqs < 3.0)  # 42 to 180 beats per minute
print(f"{freqs[band][spectrum[band].argmax()] * 60:.0f} beats per minute")

Pick the patch over skin, away from hair and the edges of the face. This shows the idea; it is not a medical measurement.