From c081f1f87363ec756db2f2f2433b9174f94ae5a0 Mon Sep 17 00:00:00 2001 From: kyc001 <852117630@qq.com> Date: Thu, 26 Feb 2026 00:01:27 +0800 Subject: [PATCH] docs: clarify level 0/1 filter heuristic --- MANUAL.txt | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/MANUAL.txt b/MANUAL.txt index 78bab9af..3e4b77ad 100644 --- a/MANUAL.txt +++ b/MANUAL.txt @@ -13,8 +13,8 @@ Options: compression. Lower levels are faster, higher levels provide better compression, though with increasingly diminishing returns. - 0 => --zc 5 --fast (filter chosen heuristically) - 1 => --zc 10 --fast (filter chosen heuristically) + 0 => --zc 5 --fast (single filter, chosen heuristically) + 1 => --zc 10 --fast (single filter, chosen heuristically) 2 => --zc 11 -f 0,1,6,7 --fast 3 => --zc 11 -f 0,7,8,9 --brute-level 1 --brute-lines 3 4 => --zc 12 -f 0,7,8,9 --brute-level 1 --brute-lines 4 @@ -22,6 +22,13 @@ Options: 6 => --zc 12 -f 0-9 --brute-level 5 --brute-lines 8 max => (stable alias for the maximum level) + For levels 0 and 1, the heuristic currently chooses exactly one filter based on bit depth: + bit depth >= 8 => Bigrams (7) + bit depth < 8 => None (0) + + Since only one filter is enabled by default at these levels, '--fast' is usually a no-op + unless you manually enable additional filters, e.g. '-o0 -f 0,7'. + Manually specifying a compression option (zc, f, etc.) will override the optimization preset, regardless of the order you write the arguments. @@ -137,6 +144,9 @@ Options: Perform a fast compression evaluation of each enabled filter, followed by a single main compression trial of the best result. Recommended if you have more filters enabled than CPU cores. + + On preset levels 0 and 1 this is typically a no-op, because only one filter is enabled + by default at those levels. --zc Deflate compression level (0-12) for main compression trials. The levels here are defined