Guard --ire0_adjust hsync against a degenerate field (ZeroDivisionError aborts the decode) - #340
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harrypm merged 1 commit intoJul 20, 2026
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…generate fields When --ire0_adjust includes hsync, hz_to_output measures the per-field black level (backporch window) and sync level (hsync window). On a degenerate field (dropout / sync collapse) both windows read the same level, so ire0 == hsync_level, hz_ire = (ire0 - hsync_level)/-vsync_ire = 0, and hz_to_output_array divides out_scale by it -> uncaught ZeroDivisionError that aborts the entire decode. Guard the per-field hz_ire: if non-finite or zero, warn and fall back to the globally calibrated hz_ire for that field, so a bad field yields an imperfect frame instead of killing the decode. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Description
In
FieldShared.hz_to_output'shsyncsubmode, a degenerate field makes the backporch and hsyncwindows read the same level, so
hz_ire = (ire0 - hsync_level) / -vsync_irebecomes0(
field.py:1074).hz_to_output_arraythen dividesout_scaleby it(
utils.py:1111)and raises
ZeroDivisionError, aborting the decode. This guards the per-fieldhz_ire: if0ornon-finite, warn and fall back to the calibrated
DecoderParams["hz_ire"].Motivation
A single degenerate field currently discards an arbitrarily long decode with no partial output — mine
died at frame 94056 after ~14.6h. It's the exact tape condition the
hsyncsubmode exists to helpwith (out-of-spec / AGC tapes with dropouts; cf. #314, #318).
Related Issues
Fixes #339
Changes Made
vhsdecode/field.py: inhz_to_output'shsyncsubmode, if the computedhz_ireis non-finiteor
0, log a warning and fall back toself.rf.DecoderParams["hz_ire"]for that field.Testing
tests/unit/test_ire0_adjust.py— 3 tests; 2 fail unpatched (ZeroDivisionError athz_to_output), all pass with the guard. Also a full from-frame-0 decode past the failure point.The fallback reuses the global
hz_ire(the value used when the submode is off), so a degeneratefield degrades to "as if hsync adjustment weren't available here" rather than crashing. Happy to take
a different remedy. A no-capture repro script is inlined in #339. Written with AI assistance.