Function nl4d_default_lambda_ht 

pub fn nl4d_default_lambda_ht(channels: ChannelMode) -> f32

The default lambda_ht for nl4d’s hard-threshold stage, per plane.

lambda_ht is how many standard deviations of estimated noise a transform coefficient has to clear to survive. Raising it removes more noise and more fine detail with it, so the value is a trade rather than an optimum.

Luma and chroma values are picked by eye from a ladder of renders against real film grain, accepting more lost detail in exchange for less remaining noise on heavy grain. Separately confirmed not to over-filter near-clean animation. The reason why we’re going a bit heavier on high noise is because the encoders end up reducing that detail more than the denoiser does if that extra entropy remains in and overall produces a worse final image.

ChannelMode::Yuv reads the luma value, on the same “a fused pass is dominated by luma” assumption [hq_default_strength] makes for its own Yuv case.

Luma and the fused Yuv mode use 5.2, and chroma uses 3.4.