One of the least discussed technical topics in fighting games is the existence of impact freeze, also known as hitstop. Someone asked about it earlier this week and i couldn’t find any articles addressing it, so i thought i’d talk about it today.
Frame data usually gives you startup, active period, and recovery for each move in the game. When an attack connects, the remainder of its active period is converted to recovery (assuming it’s not a multi-hit attack).
Impact freeze definitely exists in most fighting games and the easiest way to spot it is by mashing jabs. It’s impossible to chain a jab into another jab before it reaches an active frame. You can stand next to someone and double-tap jabs all day and you’ll never get one to whiff chain into another before it connects.
Now that we’ve established that mashed whiff jabs reach at least their first active frame, you can compare whiff jabs against connecting jabs. It’s easy to tell that whiffed jabs come out three or four times faster than connected ones. That’s because when they whiff, you’re looking at 3 frames of startup followed by 1 or 2 active frames and then canceled back to startup. When they connect, it’s like 3 frames startup, followed by around 10 frames of impact freeze and then back to startup. Therefore whiffed repetitions take around 4 frames, whereas connected reps last around 14.
Take a look at the Rufus/Viper Combo Challenge video for a few examples. First of all, Rufus’ j.DF+MK divekick connects almost on the last frame, right before he lands. Yet you can still see the impact of the attack. If there was no impact freeze then he’d basically shoot right through Viper’s leg and continue the combo with no pause whatsoever. The same thing is true of his s.LK -> s.HK chain, which was executed with only a 5 frame delay between the buttons, yet you can clearly see the s.LK connect.
As it turns out, Ultimate Mortal Kombat 3 has no impact freeze. On the ground, most punches are cancelable and most kicks aren’t. Shang Tsung chains s.LP -> s.HP then cancels into the Kung Lao transformation. Yet you only see s.HP animate for one frame before it’s canceled. Furthermore, Kung Lao cancels vertical j.HK into divekick and then vertical j.HP into divekick. In both cases the divekick actually whiffs, so it’s almost impossible to see the move that actually connects. You barely see the startup of j.HK and j.HP before they disappear on impact. That’s what it looks like when a game has no impact freeze.
Going back to the SF4 example, check out Viper’s c.HP xx HP Seismo Hammer. You can clearly see her c.HP animate before she cancels it. That command was completed quickly as well, so if there was no impact freeze then it would look like some phantom attack connected then immediately canceled into ground pound.
Impact freeze is important is because it stabilizes cancel timing. For example, Viper’s EX Seismo Hammer is a very slow move which always combos when she cancels c.HP into it. You have that entire 12-frame impact freeze window to complete the input. That’s what makes it easy. As long as you do it before impact freeze ends, she automatically cancels into the special move as soon as c.HP hitstop ends (but never sooner). If there was no impact freeze, then the timing on it would vary dramatically. For slow specials like that, you’d have to complete the command very quickly in order to get them to combo. Match gameplay would be a mess.
As for its attributes, each move causes a specific amount of impact freeze regardless of how it connects (antiair, air to air, air to ground, etc). In most games, light attacks freeze around 9 frames, medium attacks around 11, hard attacks around 13. In the oldschool SF2 series they were all around 14 frames regardless of button strength. Some specials have very long impact freeze while others have relatively short impact freeze.
Projectiles operate by slightly different rules. As expected, they cause impact freeze to the victim but not to the attacker. When a fireball connects, the victim freezes in place for around 12 frames, then starts getting pushed back. Since pushback has no effect in the corner, there’s no practical difference between one-sided impact freeze and pure hit stun if the victim is cornered. Lastly, impact freeze caused by other attacks has no effect on projectiles. For example, if Dhalsim’s DF+LP hits Ryu by reaching under his LP Hadoken, Ryu’s fireball will continue traveling forward without pause.
Knowing all this information isn’t particularly important for understanding gameplay or dealing with frame data, since most of it works fine behind the scenes. However, impact freeze is crucial for advanced charge character combos because it provides additional recharge time.
In a more practical setting, it’s also important for avoiding projectiles in matches using slides and jump attacks. For example, ST Dee Jay’s c.HK slide can go under certain projectiles only when it connects, because it freezes both characters without affecting the projectile. This works for invincibility as well. There’s a famous clip of Fei Long uppercutting through a Dee Jay fireball which wouldn’t have worked if the uppercut hadn’t connected and frozen during an invincible frame.