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#31 Defending The Pass


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Hello Coaches!


Welcome to another Dev Diary for NewSim on SimFBA!


This Dev Diary continues to explain the transition to the new game engine. Last time, we talked about defending against the run. This time, we will talk about more defense. Specifically about how to stop the pass! Since I have to go into pretty much everything that a pass play can be, this will likely be a long one, so buckle up!


Like I mentioned last time, defenses are divided up into three "levels" (DL, LBs, DBs). Each of these levels has a job when it comes to the pass. The first and second can rush the passer (and eventually sometimes the third), and the second and third provide the coverage.


As such, there are three separate things that can happen on a pass play: An attempted pass, a pass rush and/or a run (QB scramble). Every play will have at least one of these things, but sometimes a pass rush can still result in an attempted pass or a run. Let's look at each one individually.


Attempted Pass:

The most common occurrence on a pass play is when the QB attempts to complete a pass to an intended receiver. Each play has an initial intended receiver, but sometimes the play is "redirected" to another receiver. This is to simulate the QB going through reads and progressions. While the distribution of the redirected passes (who it gets redirected to) is hard-coded, the chance that a redirection occurs is dynamic depending on the play, and can be changed through input files on the backend.


The attempted pass process takes all of the information we talked about in Dev Diary #26: Throwing and Dev Diary #28: Catching and combines that with the coverage information we will talk about in a moment to produce the final chance of 4 possible events:

  • Completed Pass
  • Incomplete Pass
  • Interception (based on defender)
  • Interception (based on overall defense)

The last two are probably confusing, but just think of it like this: the first interception category is when a pass is thrown poorly enough that the defender has a chance to intercept it. If they catch well enough, they will. If not, it falls incomplete. The second interception category is for when the QB makes a poor read/decision and misses a defender "lurking". This defender isn't the one covering the receiver, but rather is another player that cut underneath the route, or perhaps was a safety coming in over the top.


If the pass is completed, a number is rolled to see how far the receiver got with the ball, and then the play is complete. If the pass was incomplete, the play is over. If the pass was intercepted, the defense will return it, and that mostly depends on the speed of the intercepting player.


Pass Rush:

Sometimes, a play calls for a pass rush check. This is more likely on longer passes and plays with more pass rushers. There are two types of pass rush checks: Matchups and Overall Pass Rush.


Matchups: The sim selects a pass blocker and their corresponding pass rusher and has them face off. Whatever the difference between the blocker and the rusher's skills are will be added to the chance of a sack. So if a pass rusher is better than a pass blocker, there will be a high chance of a sack, and vice versa. Obviously, if there is a sack, this pass rusher will get the credit for the sack.


Overall Pass Rush: The sim adds up the pass blocking of all pass blockers, and compares it to the sum of the pass rush skills of all of the pass rushers. Similar to the matchup, the difference between these two numbers is compared and applied to the sack range. However, differences here are DOUBLED before they are added to the sack range. So a difference of 2 becomes a difference of 4, making a bigger difference.


There are four things that can happen after a pass rush:

  • Sack
  • Run
  • Pass
  • Interception

The Sack is pretty obvious. We will talk about running in the next section. The pass and interception basically follows the same as a regular pass attempt, with the following difference: some QB types will be better at throwing on the run and under pressure than others! Specifically, in order of best to worst:


  1. Balanced
  2. Scrambling
  3. Field General
  4. Pocket

NOTE: Scrambling is lower than Balanced because of the likelihood that they are going to run, which we will get to next.


So the chances that one of the four things that can happen on a pass rush happening are also determined by the QB type:


Scrambling QBs are quite slippery and will likely be sacked the least. They also will choose to run with the ball over passing it, and will throw it the least under pressure.


Balanced QBs are decent at avoiding pass rush, while not as good as Scrambling QBs. They are more likely to throw than Scramblers, but will still decide to tuck it and run a fair amount.


Field General QBs are a bit vulnerable to pass rushers and will go down more often than our previous types, but make up for this with a decreased likelihood of throwing INTs as well as using their arm to get out of a bad situation rather than their legs.


Pocket QBs are the perfect target for pass rushers. They very rarely can avoid the rush, but when they do, they usually make the defense pay. They are the least likely to throw and interception, and the most likely to complete a pass if they avoid the rush. What they will pretty much never do is run with the ball. Which leads us to our next section:


QB Scramble:

Finally, a pass rush can induce a QB to take off with the football themselves. This is most dangerous and likely facing against Scrambling QBs, but is pretty much not a threat against Pocket QBs. This event is treated just like a regular run (just without any blocking or defending modifiers). The QB uses their agility (like an outside run) to determine how far they run with the football, and can have a breakaway run just like any other run play.


Whew. That's a lot! But it is all to try to make the sim more realistic! Also, imagine having to code all this! :thisisfine:


So what skills make a pass defender? Well, that depends on whether you want a defender to rush the passer or to drop into coverage. If you want the player to rush the passer, the following skills help:

  • Pass Rush
  • Speed
  • Strength
  • Football IQ

Ok, so how do each of these things affect how well a run defender will play?


Pass Rush: This skill is pretty straightforward. The higher this value is, the better a player is at getting past blockers and sacking/putting pressure on the QB.


Speed: High speed players will get a small boost to their ability to rush the passer.


Strength: Players with high strength also have a slight bonus.


Football IQ: Intelligence can provide a slight benefit or malus.


What about a player in coverage?

  • Speed
  • Man Coverage
  • Zone Coverage

Speed: This skill specifically applies to covering deep routes. You don't want slow players covering speed WRs running streaks!


Man Coverage: This skill will directly influence the completion chance against a covered receiver while in a man coverage assignment against them.


Zone Coverage: This skill will directly influence the completion chance against a covered receiver while in a zone coverage assignment against them.


Now, I bet you noticed a little tidbit in there about man and zone assignments. Well, now we have the ability to determine whether corners, linebackers and safeties are playing man or zone for a given play! This will allow you to play to your defense's strengths!


Ok, so that was a TON of words, but if you made it this far, I have a treat for you! All of what I talked about is currently coded up and functional! Here is a little taste:








That is it for this dev diary. I apologize for the length, but I hope it was worth it. Comment with any questions you have and I'll be sure to answer them as best as I can!


Until next time,



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  • 1 year later...

Can we get some clarification on how Zone Defense works? Does CB1 still match up against WR1 automatically but use their Zone Defense score instead? Or is there more variability in the matchup?


In a similar vein - how are coverage assignments calculated for safeties? Are they guarding TEs, slot WRs, etc?

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For now the matchups are still pretty much the same, but the zone attribute is applied to determine the outcome of the matchup rather than the man attribute. For safeties, it is a matter of how many deep safeties you have. The more deep safeties, the more deep routes have penalties applied to them.


This is all for now, and might change in the future as we improve. But for now that's how it works.

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Based on things like whether or not you are blitzing, and if you played the pass or run on that play. Playing the pass leaves 2 safeties, playing the run leaves one, and blitzing more than 1 player leaves zero.
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