Double Buffered

A Programmer’s View of Game Design, Development, and Culture

GDC 2010: Reading the Player’s Mind Through His Thumbs: Inferring Player Intent Through Controller Input

Posted by Ben Zeigler on March 21, 2010

Okay, time for my last set of session notes. Here’s what happened at Reading the Player’s Mind Through His Thumbs: Inferring Player Intent Through Controller Input presented by Chris Zimmerman from Sucker Punch. The main topic of the session was the controls of Infamous (sorry, I refuse to use the stupid capitalization), which I quite enjoyed as a game. It presented a few pretty innovative ideas for interpreting player input, which I’m currently trying to figure out how to apply to various projects I’m working on. Stuff I heard:

Controls Immersion

  • For Infamous one of the goals was to push immersion as far as possible. The objective was to make you feel like you WERE Cole, instead of just controlling Cole. They should see and hear what Cole sees and hears, and do what Cole does.
  • However, players expect a certain amount of abstraction in their control interfaces. No player wants an accurate simulation of what it’s like to climb a rope,  which is actually kind of hard. Direct control just won’t work, both because of the complication of the world and the fact that players are somewhat bad at using controllers. Players want to express their wishes, and feel challenged but also successful
  • Chris performed an experiment asking players to point a ps3 stick at objects in a 3D scene. Players were fairly inaccurate, and formed a bell curve. Around 70% are within 15 degrees of the target, but not accurate beyond that. A second experiment asked them to push a button when a ball bounces. Players were within 50 ms most of the time, but not any more accurate then that (plus any A/V lag which is omnipresent).
  • Challenges for Infamous were large. Active and small enemies, inexact controllers. Featured jumping and climbing, and they had the design goal of everything that looked climbable had to be climbable. Turns out the city has a lot of things that look climbable.
  • They considered going with a solution where you would lean on a joystick to climb, like in Assassin’s Creed 2. But, they decided that didn’t feel skillful, and would be a problem given the density of the city. So, the goal was to require the player to execute correctly, but to liberally provide aid in the background.

Aiming

  • For aiming, the goal was to make it so the game ALWAYS shot exactly where you were aiming the reticle. So, the solution was to help in moving the reticle, not leave the reticle manual but help after firing. So, the game adjusts the “input” based on both the player’s real input and what the game thinks the player is trying to do. The movements have to be physically possible on the controller, it just helps the player out.
  • It takes the controller input and looks for targets that are near the direction the reticle is moving. It will adjust the reticle motion to be directly towards that object if it’s a good enough fit. It also slows down the reticle movement as you are close to the target, to give more time to fire. It also keeps track of where it was and where it’s going as well as button presses, so if the press happens within a certain time frame of passing over the target it counts as on target.
  • If there are no inferred targets the reticle is 100% player controlled. It excludes targets that are too far away, and all targets are treated as a line (because they have height) instead of a point. This means that if you’re heading towards a characters feet you’ll hit their feet with your attack, instead of magically jumping to their midsection.
  • For evaluating targets, lexicographic scoring is used. It rates the targets on several dimensions in priority order, and only uses lower dimensions if the top ones are equal. For shooting, this means that each target has two parts of the “egg” that move out from them. All valid centers of any egg rate higher than any outer sections of the egg, but it will use the closest section out of the two groups.

Jumping and Climbing

  • For jumping the goal was the same as targeting. The game allows air steering while in midair, so the game will only adjust your controller input to match a physically possible controller input. But, jumping is way more complicated because of the larger number of possible targets, the commonality of linear targets such as ledges, and various animation affordance issues. Failing to correctly predict while jumping is worse than when aiming.
  • The basic algorithm looked ahead about 0.75 seconds. There were a set of “illegal” filters it would check for all possible locations, and one preference score. It would first apply exclusion filters, in performance order where it would do the cheap tests first. After passing all filters it would heuristically score the remaining targets.
  • To score the targets it uses a golden section 1 dimensional optimizer, which was picked for execution speed. Scoring function was based on trajectory relative to final position. To modify the player input it computes the player’s time to land, and the relationship of the desired target and the current trajectory. It adjusts the player’s stick input to match the desired input. (Note: I missed a bit of the specific math, but there are a variety of ways to rate targets heuristically)
  • Ground landings have to be compared against features. You can’t just trace the feet because they will hit a wall before the ground in many situations (such as jumping up). It instead traces a set of polygons between the head and the feet, and clips them against geo. It stops the search if the polygons hit a segment the player can stand on.
  • The original design didn’t call for wall jumps, but they added them when players ended up hitting falls. When a player hits a wall they can “move” a certain amount or jump off, which ended up feeling more natural. To score wall collisions it traces a parabola at the knees.
  • They ended up doing some modification to running as well, to walk around small objects in the world. Super heroes don’t run into parking meters, why should the player?

Conclusion

  • Why does this sound so complicated? At first they tried stealing from Uncharted, where it computes the jump at take off. But, there were too many objects. Then they tried stealing from their own Sly Cooper games, but having a “grab” button that when activates automatically air steers the player towards the nearest surface.
  • But, the realistic graphics of Infamous brought with them increased expectations. Players were not willing to accept the floatiness of the Sly Cooper solution, which worked just fine in a cartoony world. So, they had to come up with something that matched the world better.
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