Guide to understanding Hit Boxes in Street Fighter
What is a Hit Box
There are two types of Hit Boxes in Street Fighter games, a (red) Attacking Box and a (blue) Hittable Box. When you do Ryu's Shoryuken in Super Street Fighter 2 Turbo HD Remix with the hit box display turned on, a red square shows up around his arm, this is the Attacking Box.
On the flip side, at almost all other times Ryu's body is highlighted with blue Hittable Boxes, which shows you the areas he can currently be hit at.
Jumping, crouching or pretty much doing anything alters the size, shape and location of the Hittable Boxes, as shown by the images below.
When Hit Boxes collide
For a move to connect in Street Fighter a red Attacking Box must overlap with your opponent's blue Hittable Box. Zangief is demonstrating this on Blanka here.
Although the hit boxes pretty much match up with the graphics in the Street Fighter series, there are definitely some exceptions. As you can see below on the left, although Guile's leg is clearly touching Zangief, the hit is still not registering because the hit boxes are just slightly too far away.
If Guile moves a bit closer though, his red box overlaps with Zangief's blue box and the kick connects, as shown on the right.
While there are only two hit boxes, both Attacking and Hittable Boxes can overlap with each other.
Some moves have a huge red Attacking Box and shrink your blue Hittable Boxes quite a bit, and vice versa. Often time it's these properties that define how a good a move is — or isn't.
Having a move with a HUGE Attacking Box that shrinks your Hittable Box considerably is probably going to be one of your best moves, especially if it has a fast start up and recovery time.
E. Honda's Headbutt in HD Remix is one of those attacks, while his feet/leg area is vulnerable, touching any other part of his body is going to result in being struck, and his head and the very top part of his back cannot be hit at all — but it can hit you.
Invincible Frames
Some moves have invincible frames or invulnerability, which means they have no Hittable Boxes for a short period of time.
Ken's Medium Punch Shoryuken is completely invulnerable in HD Remix during its start up period and the entire time he's traveling upwards. You can only hit him as he's coming down, when the blue hit boxes appear.
Just for comparison, let's check out Sagat's Hard Punch Tiger Uppercut. It offers a MUCH smaller window of invulnerability, and soon after the move is executed Sagat is vulnerable on the top of his head and from his waist on down.
To avoid being knocked out of his Tiger Uppercut, most Sagat players try to time this move so it connects right before the blue Hittable Boxes appear, letting him punch his opponent without worrying about being hit in the process.
How does the game decide which moves beat each other?
Some people assume that the moves in Street Fighter have a mysterious 'priority', meaning there's some way the game decides which moves win out if they're both done at nearly the same time, but in reality everything is based off the location of your hit boxes.
If you stick out an attack that connects with your opponent's Hittable Box near the same time they stick out an attack that also connects with your blue boxes, your attacks will 'trade hits'.
Conclusion
Learning the hit box properties your moves have can make you a better player. If you understand exactly where and how your moves can and cannot be countered, you can use them that much more effectively.
While most of Capcom's Street Fighter games do not have hit box displays, you can still learn where they are through a trial and error process, which most people have been doing for years.
Coupled with frame data, understanding how hit boxes work help you dissect the games and really understand what's going on while you're playing.
