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Choosing parameters

Five numbers control everything. This is what each does and how to pick it.

alpha — how much to amplify

Start with the preset, raise it until the change is clear, then stop.

Too high looks different by pipeline. Colour saturates: regions go flat and lose detail. Motion tears: edges break into ripples and haloes, because the method's assumption fails at edges first. There is no correct value — it depends on how large the real change is and how much noise the camera adds, and both vary by clip.

fl and fh — which frequencies to keep

In cycles per second; everything outside is discarded. Pick it from what you are looking for: a resting heart rate of 60 bpm is 1 cycle per second, a guitar's low E string is 82. Two constraints apply.

  • Below half the frame rate. A camera at 30 fps cannot represent anything faster than 15 cycles per second; above that limit the movement appears as a slower one that is not really there.
  • Wide enough for the clip. A clip of N frames at f fps resolves frequencies in steps of f / N. A band narrower than one step contains no frequency, the filter returns zeros, and the output equals the input. The library warns and says how many frames it needs. The pulse preset needs about 181 frames — six seconds at 30 fps — before its band contains anything.

level — how far to shrink, for colour

How many times to halve the resolution before filtering. More shrinking means less noise and a smoother result, at the cost of fine detail. The preset uses 4, so a 512-pixel-wide frame is filtered at 32 wide. Shrink too far and the region you care about disappears into one sample.

lambda_c — the spatial cutoff, for motion

In pixels; detail finer than this is amplified progressively less. This is the artefact control. Raise it when the result shimmers; lower it to amplify finer movement and accept more noise. The presets use 16.

chrom_attenuation — how much colour to amplify

A multiplier on the colour channels relative to brightness. For colour work, 1.0: the colour change is the signal. For motion, about 0.1: the movement is in brightness, and amplifying colour just amplifies colour noise, which is very visible.

Where the shipped values come from

Every preset records its own provenance:

from vidmag.presets import PRESETS

print(PRESETS["pulse"].params)
print(PRESETS["pulse"].source)

The pulse and motion presets are the authors' own values for their own clips. The vibration preset comes from an example in this repository, and its description says so — that is weaker evidence, and the reader should know.