Yesterday I was playing SF4 and was using C. Viper.
At one point, I started a super/ultra attack but was immediately grappled by Zangief. Shouldn't I have been invincible during the super/ultra attack?
This got me wondering - if two players attack each other at the same moment, how can I tell who will be hit?
I apologize if the terms I use for describing the various actions are inaccurate.
A lot of Ultras are completely invulnerable indeed, but only at the very start. For example, as a rough guess Ibuki Ultra 2 is invulnerable only for the 2-3 first frames but it becomes active only at frame 9. You got a whole 6-7 frames of vulnerability in there. Also, it means you can trade in the ultra (double hits). Let's assume the invulnerability only lasts 2 frames. Normal grabs start in 3 frames. So if you throw in the Ultra in grab range and Zangief reacts by doing a grab during the animation, he'll catch you at frame 3, after your invulnerability ended and before you landed the first hit, thus you lost.
To your question, I'd say that in a single frame, the game first checks if grabs connect, then if hits connect. If both players hit at the same time, well then, they do get hit at the same time which allows you to do draws and the like. But most of the time the question on who hits who is decided by who hits first at the frame level since double hits aren't that common.
Well Zangief's Grabs have a lot of priority and range so depending on the super/ultra and when it was activated it can be grabbed ... but it all depends on which super or ultra you were trying to do. All Supers and Ultras are not completely invincible they may just have some invincibility on the start up frames of the move.
It depends on how fast the attack is.Lets say I do a light punch and you do a medium punch.Light punch would win but then there's reach,reach that some Lp can't do.It's really complex.The winner of the exchange is which attack dominates the 2 things better than the other one.
Everything in Street Fighter is determined by hitboxes, hurtboxes etc. Every move has different characteristics which in the end boil down to these boxes but can also be expressed by frames. These hitboxes are invisible in the game, but recently a tool got released that allows you to enable them while playing, see here and here.
Only looking at frame data might misguide you. In the end it just boils down to "try it in training mode" and recently "and activate SF4 Box Viewer".
A few more additions to grabs: There are various states that can make a grab whiff: If the opponent is airborne (for example: jump or backdash), if the opponent is invincible or if the opponent is in hitstun (For example after he blocks one of your attacks, but you can move before he does).
