vapoursynth-interlace

Progressive to interlaced conversion for VapourSynth and AviSynth+.

The filter implements the perceptually optimal interlacing filter of BBC R&D White Paper WHP 315 (K.C. Noland, "Optimal Interlacing using Human Sensitivity Measurements", 2016): a 19-line by 5-frame linear vertical-temporal low-pass applied to the progressive sequence before discarding alternate lines, with a pass-band shaped by the measured human sensitivity to interlacing aliases (WHP 230). Output behavior has been validated against the EBU TECH 3384 test patterns and measurement methodology.

Usage

clip = core.interlace.Interlace(clip, tff=True, filter=True)

AviSynth+ takes the same arguments:

Interlace(tff=true, filter=true)

2N progressive frames become N woven frames at half rate. _FieldBased is set and frame durations doubled.

• clip — constant-format progressive clip; 8–16-bit integer, 16-bit or 32-bit float, any subsampling with even plane heights. Half precision is a first-class citizen for memory-bound pipelines: it filters faster than single precision at half the frame memory (F16C conversion on load, f32 accumulation). Spatial resizing is out of scope: for e.g. 2160p→1080i, scale to 1080p with resize first.
• tff — field order of the output; the temporally first frame supplies the top field when true (default).
• filter — apply the WHP 315 vertical-temporal prefilter (default). With filter=False fields are woven with no filtering, which aliases. Only useful as a worst-case reference.
• cpu — cap the instruction set used, for testing and debugging: "none" (plain C), "sse2", "avx2" or "avx512". Default: everything the host supports. Levels the hardware lacks are simply not used.

Same filter on all planes (per WHP 230); zero phase, so no chroma/luma timing shift. SSE2/AVX2/AVX-512 selected at runtime. Integer output is bit-identical to the C implementation on every path. Float paths use FMA on AVX2 and above, so float output can differ from C by a few ulp (~1e-6 relative). The fused sum skips one rounding per tap and is the more accurate of the two.

4:2:0 and chroma siting

Progressive 4:2:0 sites chroma relative to luma per _ChromaLocation. Interlaced 4:2:0 sites each field's chroma at the ¼/¾ positions between the field's luma lines. The filter honors the input _ChromaLocation (left/center/topleft/top/bottomleft/bottom; assumed left when absent) and resamples vertically subsampled chroma onto the interlaced siting grid inside the same vertical-temporal convolution - fractionally delayed variants of the same kernel, at no extra cost.

Chroma bandwidth note: subsampled chroma gets the same filter at its own lattice (the cutoff sits at the same fraction of the halved chroma Nyquist). The WHP 315 / TECH 3384 references only cover 4:2:2; per-plane application is the natural reading of WHP 230's "same filter on all components".

Recommended workflows

Do all spatial scaling, color conversion and tonemapping progressive; interlace last. Never resize after interlacing.

# 1080p50 -> 1080i25, staying 4:2:0
out = core.interlace.Interlace(clip)      # chroma siting handled
# encode with an interlaced-aware encoder (e.g. x264 --tff)

# 1080p50 4:2:0 -> 1080i25 4:2:2
c = core.resize.Bicubic(clip, format=vs.YUV422P10)
out = core.interlace.Interlace(c)

# UHD 2160p50 -> 1080i25 (down-convert then interlace)
c = core.resize.Lanczos(clip, 1920, 1080, format=vs.YUV422P10, filter_param_a=4)
out = core.interlace.Interlace(c)

# HDR UHD -> SDR 1080i: tonemap/convert color first, progressive
c = your_hdr_to_sdr(clip)                 # e.g. resize/placebo chain
c = core.resize.Lanczos(c, 1920, 1080, format=vs.YUV422P10, filter_param_a=4)
out = core.interlace.Interlace(c)

# 1080p50 -> 576i25 (PAL SD, 16:9 anamorphic). The active picture is
# 702 wide (52us at 13.5MHz). Fudge the image to 704 (for mod 16 and even
# chroma) instead or don't fudge and crop the width 5.5 first.
c = core.resize.Lanczos(c, 704, 576, format=vs.YUV420P8, filter_param_a=4)
c = core.std.AddBorders(c, left=8, right=8, color=[16, 128, 128])
out = core.interlace.Interlace(c)

# 1080p59.94 -> 480i29.97 (NTSC SD, 16:9 anamorphic). SMPTE RP 187 puts the
# 525-line active picture at 708 samples; resize yields SAR 640/531.
c = core.std.SetFrameProps(clip, _SARNum=1, _SARDen=1)
c = core.resize.Lanczos(c, 708, 480, format=vs.YUV422P10, filter_param_a=4)
c = core.std.AddBorders(c, left=6, right=6, color=[64, 512, 512])
out = core.interlace.Interlace(c)

Choosing a downscaler for 2160p→1080i

EBU TECH 3384 ships a companion 2160p test pattern for exactly this chain. As shipped, its gratings sit at the same absolute frequencies as the 1080p pattern (13.5–80.8% of 1080p Nyquist after the 2:1 downscale) and its zone plate tops out at 1080p Nyquist — worst-case oversampled 1080p content. The pattern therefore measures how faithfully a 2160p→1080i chain reproduces the native 1080i response. It contains nothing above 1080p Nyquist, so it does not rank alias suppression.

Every zimg kernel, measured through resize + Interlace against the official file: horizontal grating MTF (the interlacer leaves horizontal untouched, so this is the resizer alone), worst deviation of the composed vertical gratings from the native-1080p chain over gratings 1–4 (the pass/knee region of the interlacing filter), and white-cross ringing (native chain: −11.8%).

Gratings 1–6 are 13.5/26.9/40.4/53.9/67.3/80.8% of 1080p Nyquist. Spline36, Spline64, Lanczos3 and Lanczos4 are one quality class: within 0.2 dB of the native chain where it matters, identical ringing. Lanczos4 retains the most detail near Nyquist; Bilinear, Mitchell and Spline16 measurably dull the chain. Composed ringing is lower than native because the down-conversion band-limits the edges. The full composed chain's vertical-temporal cutoff tracks the WHP 315 template within 0.021 normalized frequency over the entire temporal range.

Alias suppression

The official material carries nothing above 1080p Nyquist, so alias rejection is scored separately (tools/resizer_alias.py): synthetic 2160p gratings at 1.05–1.95× 1080p Nyquist, whose downscale residue lands at 2−f. Horizontal is the resizer alone; vertically the interlacing filter's stop band absorbs whatever lands above its cutoff — aliases from 1.05–1.35× Nyquist land at 0.95–0.65× and come out ≤ −37 dB for every kernel, so the vertical axis only discriminates at 1.5× and beyond. Response in dB relative to the source grating; −54 dB is the 10-bit measurement floor:

At 1.05× every kernel sits at −7…−10 dB — the transition width is inherent, and the soft kernels buy their ~1.5 dB there with 4–6 dB of passband loss. Lanczos4 leads from 1.2× on both axes and has the best composed-vertical worst case; its only gap versus the splines is side-lobe leakage around 1.5–1.65× at ≈ −45 dB, below every alternative's worst case. Lanczos3's side lobes never reach the floor. Taken together with the scorecard above: Lanczos4 is the measured pick for 2160p→1080i, with Spline36/Spline64 as slightly softer, marginally cleaner-far-field alternatives.

Building

meson setup build
`meson test -C build` runs the checkasm verification (fetched as a meson subproject)

Requires VapourSynth (V4 API), meson, and nasm on x86-64. meson test -C build runs the checkasm kernel verification (fetched as a meson subproject; -Dcheckasm=disabled to skip). Or pip install vapoursynth-interlace.

Add -Davisynth=true to also build the AviSynth+ frontend into the same module; both hosts load the same file and each finds its own entry point. The released wheels enable it, so AviSynth+ users can take the plugin straight out of the win64 wheel (also published as a standalone interlace.dll build artifact).

The AviSynth+ end-to-end test links libavisynth, which the repo does not vendor, so it joins meson test only when you point -Davisynth_lib_dir at the directory holding your libavisynth.so.N:

meson setup build -Davisynth=true -Davisynth_lib_dir=$LIBAVS
meson test -C build

License

GPL-2.0-or-later.