# ‘Penumbra: Black Plague’ Developer Diary #3 – Screens

One of the main features in Penumbra: Black Plague is the physics. This is a feature that adds a lot of immersion and gives new opportunities for gameplay. However, it has not been without problems to implement it and that is what we will focus on in this diary entry. We will go through some of the different problems we encountered and explain how we solved them.

## Implementing physics

At the core of the physics in Penumbra: Black Plague there is an engine called Newton Game Dynamics. This engine makes sure objects collide and react realistically in the game world. But having the physics running is far from enough if you want to have an interactive game, there’s a lot of work left to be done.

Our first challenge was to get the physics engine working with the game and the main thing in Penumbra is to be able to interact with objects. We wanted the interaction to be natural, simple and allow the player to intuitively understand the controls. However to get it all working we could not just slap on a generic system, instead we had to specialize every object in the world so that it reacts as you expect it to. First of all we have three different types of interaction that we use for each object: grab, push and move. Grabbing is used for objects that you simply pick up, push is used on larger objects that cannot be lifted and move is used on things like drawers, doors and levers. Once the type of object has been determined several properties needed to be tweaked. How much should the object weigh, is it slippery, should it be rotated along with the player and so on. All of these things needed to be tweaked carefully and the wanted behavior often changed depending on the situation.

Another challenge was getting the actual interaction algorithms to work well. Once you had a finely tuned an object, you need to have some controls in the game that uses the tuned properties and makes sure that the object behaves as planned. For example, one issue that arose was that the forces used to correct the orientation of an object were way too large at times. Meaning that when you picked up a small box it got so much energy that it was able to “punch“ large metal crates, making them fly several meters. The first fix was to set a max limit, however the object was not corrected quickly enough when doing so. This lead us instead to lowering the mass of the object, but in turn this lead to objects not being able to be lifted because of other items inside them (for example a trashcan with trash). The final solution was to have all of these controls set on an object basis, letting some object get their weight lowered, some got a max force limit, etc. As if things where not complicated enough, once we started to add AI to the world, it got even more problematic. The problem with normal AI routines is that they assume the world is static, which is certainly not the case in Penumbra: Black Plague where almost every object can be moved. This meant that a route the AI had picked out could be blocked at any time and become invalid. To solve this we made the AI constantly check if the path was clear and if it was not, have them update their paths to go around the obstacle. Also, the AI needed to have routines to see if a blocked path could perhaps be cleared (by pushing or kicking and breaking) instead of going around, which took a lot of tweaking to get right. In the end we had to tweak all world objects (again) and make sure that the AI would handle them correctly. Countless of hours have been put into checking so that the enemies behave intelligently in the many situations that can arise.

## Using the physics to design puzzles

As mentioned above, a lot of work went into to setting the correct properties to various objects so that they reacted correctly in the world. This was especially true when designing and creating the puzzles in the game levels. We wanted to have as few inventory based puzzles as possible and instead have the player physically interacting as much as possible. Having this goal proved to require a lot of work to make sure that the puzzles where fun and worked as planned.

The best and worse thing with physics puzzles is that you can never plan for every event. There are endless of ways in which the physics can be used and this means that you can never be sure that you have tested every single way in which the player can approach a puzzle. The good thing with this is that the player can solve puzzles in ways, we as designers, never planned. The bad thing is that the puzzle might be broken in unexpected ways too. For example if you need to cross a rift using boards, it is hard to prevent the player from simply throwing down all the boards into the rift, making the puzzle unsolvable. During the development of the game we tried hard to minimize these sorts of things, however we also didn’t want to be overprotective either. Because of this we made the decision to leave a way to destroy the puzzle, if it made the way to solve it the more open. As long as it was very obvious when the player destroyed a puzzle we figured this approach would be a lot better than holding a player’s hand every step of the way, making the game much more linear.

Another grim feature of a physics system is its chaotic nature. By chaotic we mean that the slightest change in starting conditions might lead to vast changes in the result. For example, at one point in the game we wanted a corpse to fall down, but the physics would never behave the way we wanted it to. Every time we thought we had it in the right starting pose, it would still fall completely wrong on a later occasion. This forced us to add several correctional forces to make sure the corpses landed where it should and a task we at first thought would be simple, turned out to consume over a day’s worth of job.

What we have just described is just a little part of all the obstacles we had with the physics, but hopefully it will give you a glimpse of the work we have put into getting them really good. Although the physics have been hard work, we believe that it has added a lot to the game and given it a unique feel and atmosphere. Hopefully all of you who play the game will think likewise!