Game hard and soft knockdown systems explained

Hard vs soft vs forced-stand: the core taxonomy

Most fighters use at least two knockdown classes. Names vary by franchise (HKD/SKD, blue/yellow, technical/spike) but the mechanical contract is consistent:

Document these types in your combat bible with frame counts, not just labels. A soft knockdown that lasts 18 frames behaves differently from one that lasts 8 frames even if both allow OTG. Pair knockdown duration with hitstun on the OTG hit so follow-up links remain possible or deliberately impossible.

OTG (on-the-ground) attack rules

OTG moves are attacks flagged to hit a grounded hurtbox during soft knockdown. Not every low attack is OTG-capable; the move must explicitly opt in. Common OTG design patterns:

• OTG-only flag — move connects on grounded state only; whiffs if defender already stood. Cleanest for balance (sweeps, dive kicks).
• OTG + standing — same hitbox works on ground and standing; simpler for players but harder to tune (risk of double-purpose oppressive lows).
• OTG bounce — OTG hit relaunches defender into air juggle; ties grounded and air routes together.
• OTG once per combo — prevents infinite sweep loops; common in tag fighters and anime fighters with long juggles.

Hard knockdown typically sets ground_invuln = true or otg_immune = true on the defender until the first wake-up frame. Soft knockdown clears those flags after a short vulnerable window (often frames 4–12 of the lie-down animation). Expose OTG windows in training mode alongside frame data so players learn which enders grant oki-only payoff vs full OTG extensions.

How knockdown type shapes okizeme payoff

Knockdown type determines what the attacker can do before the defender reaches wake-up options:

• Hard knockdown oki — attacker has time to jump in for safe jump, dash meaty, or set projectile trap. No OTG damage during lie-down; payoff is entirely meaty and frame-trap pressure on rise.
• Soft knockdown oki — attacker chooses OTG now (guaranteed damage if unread) or hold pressure for bigger wake mixup (risk defender techs or reverses). Creates a damage-now vs damage-later branch.
• Forced-stand oki — no setup time; attacker must already be in position. Favors strike-throw at close range or immediate mixup. Often follows juggle limit hits.

Balance tip: assign hard knockdown to high-damage combo enders and soft knockdown to mid-damage drops. If every launcher hard-knocks, OTG moves become dead weight in movelists. If every ender soft-knocks, defense has no reset breathing room and round length balloons.

Implementation: knockdown state machine

A minimal production model attaches knockdown metadata at hit resolution:

enum KnockdownType { None, Hard, Soft, ForcedStand }

struct KnockdownEvent {
  KnockdownType type;
  int lie_down_frames;      // 0 for forced stand
  bool otg_vulnerable;      // true during soft lie-down window
  int otg_window_start;     // frame offset when OTG opens
  int otg_window_end;       // frame offset when OTG closes
  bool otg_used_this_combo; // per-combo OTG cap
}

// On hit that causes knockdown:
event.type = MOVE_TABLE[move.id].knockdown_type;
event.lie_down_frames = TYPE_TABLE[event.type].duration;
event.otg_vulnerable = (event.type == Soft);

// On OTG hit attempt:
if (!defender.otg_vulnerable || defender.otg_immune) reject_hit();
if (attacker.combo.otg_used && OTG_ONCE_PER_COMBO) reject_hit();
attacker.combo.otg_used = true;

Engineering details that cause live bugs:

• Knockdown override stacking — if a move has both launch and knockdown properties, define priority (launch wins, or knockdown after juggle limit).
• Simultaneous knockdown — trade into double knockdown needs type resolution per fighter independently.
• OTG during blockstring — decide if OTG can hit a defender in blockstun knockdown (usually no; knockdown clears block).
• Rollback determinism — lie-down frame counters must tick from fixed knockdown start, not animation blend length.

Harbor Brawl refactor: three types, one table

Pre-patch problems:

1. 47 moves had hand-authored canOTG flags unrelated to knockdown animation — balance patches broke silently.
2. Hard knockdown duration varied 22–51 frames with no design rule.
3. Training mode showed knockdown but not type or OTG window.

The refactor standardized:

• Hard: 32-frame lie-down, OTG immune, blue floor VFX.
• Soft: 16-frame lie-down, OTG frames 6–14, yellow floor VFX.
• Forced stand: 8-frame pop-up animation, white flash, no OTG.
• Every move's knockdown type set in the move spreadsheet column kd_type; engine derives OTG from type, not per-move override.
• One OTG per combo globally; second OTG attempt whiffs with distinct SFX.
• Training overlay: shaded bar for OTG window, icon for knockdown type on hit.

Sweeps and throw enders moved to hard; launcher second hits moved to soft. Average combo length held steady (5.1 hits) while oki-phase damage rose because attackers could finally plan routes around type instead of memorizing exceptions.

Technique decision table

Common pitfalls

• Invisible knockdown types. Same lie-down animation for hard and soft with different OTG rules feels like a bug. Use distinct VFX or training labels.
• Soft knockdown on every combo ender. OTG routes dominate; hard knockdown enders become unused.
• OTG window shorter than move startup. OTG sweeps with 14-frame startup cannot hit a 12-frame soft window — math the windows before ship.
• Hard knockdown with no oki payoff. 50-frame lie-down where attacker cannot reach in time wastes design — tune duration to character movement speed.
• Forced stand into immediate mixup with no escape. Pop-up into overhead with no frame gap is oppressive; leave 2–4 frame gap or pushback.
• OTG damage ignoring combo scaling. OTG hits should respect proration or they become the highest-damage route in every string.

Production checklist

• Define hard, soft, and forced-stand types with frame durations in the combat spec.
• Derive OTG eligibility from knockdown type, not per-move ad hoc flags.
• Set OTG window start/end frames and verify against OTG move startup data.
• Cap OTG uses per combo if infinite sweep loops are possible.
• Assign hard knockdown to high-payoff neutral enders; soft to mid-combo drops.
• Match lie-down duration to attacker movement so oki is physically reachable.
• Apply combo damage scaling to OTG hits.
• Add distinct VFX per knockdown type and OTG success/fail feedback.
• Expose knockdown type and OTG window in training mode and replays.
• Test trade knockdowns, OTG during tech, and juggle-limit forced stand.
• Validate knockdown frame counters under rollback netcode.
• Publish knockdown type per move in public frame data.

Key takeaways

• Hard knockdown ends combos and funds okizeme; soft knockdown opens OTG extensions; forced stand skips floor time entirely.
• OTG eligibility should derive from knockdown type with documented frame windows, not scattered per-move booleans.
• Harbor Brawl's unified taxonomy cut “random OTG” complaints by making knockdown payoff readable and plan-able.
• Balance requires hard enders for neutral resets and soft drops for lab depth — not one type for everything.
• Knockdown typing connects directly to wake-up, okizeme, and juggle systems; document all three together.

Related reading

• Game wake-up and getup systems explained — rise options after knockdown
• Game okizeme and meaty frame traps explained — pressure on wake
• Game juggle and launch combo systems explained — air routes before ground OTG
• Game grab and throw systems explained — throw knockdown and tech interplay