Portal Stories: Mel

### Portal Stories: Mel

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XYZ Aug 1, 2015 @ 8:54am
What exactly does the Propulsion Gel do?
In the chapter 3, there is a chamber with 3 laser beams where you have to coat a cube with the propulsion gel and let it slide down on a hard light bridge.

Well, this makes me curious about what exactly does the propulsion gel do. Initially I thought that it just increases the acceleration and the maximum speed of the player. But now it seems that it also reduces the friction. This is interesting because that, from a physical perspective, the fact that propulsion gel reduces the friction simply contradicts the fact that it increase the acceleration, as the acceleration must come from the friction.

Could anyone explain to me what exactly does the propulsion gel do in the official settings? And exactly how is it implemented in the physics engine of Portal 2?
Last edited by XYZ; Aug 1, 2015 @ 8:55am
_galaxy Aug 1, 2015 @ 9:00am
any screenshots? D:
XYZ Aug 1, 2015 @ 9:11am
Originally posted by ℜenato ℒopes ⌬:
any screenshots? D:

Can't post a screenshot right now because I am posting this on another computer that doesn't have Mel installed, but I think this chamber is quite memorable, if you have played through chapter 3 you can't forget it.
Smidge204 Aug 1, 2015 @ 11:56am
The propulsion gel works exactly the same in the official game, if that's at least part of what you're asking. The mechanic of having a cube slide down a ramp is actually in the official game.

As for implementation at the programming level, the easiest way I can imagine is to give it a negative coefficient of friction. It's entirely possible there are special rules for the player and other objects, though, so it speeds up the player only but reduces friction for physics objects.

Asbestos is pretty mysterious!
Last edited by Smidge204; Aug 1, 2015 @ 11:58am
_galaxy Aug 1, 2015 @ 1:33pm
oh I know what puzzle you are talking about
(btw I didn't put the cube sliding, what was your solution? O.o)
Smidge204 Aug 1, 2015 @ 1:52pm

Skip to 10 minutes - that's the chamber in question, with developer commentary. Spoilerific of course...
Last edited by Smidge204; Aug 1, 2015 @ 1:52pm
_galaxy Aug 1, 2015 @ 1:58pm
oh I tough you were saying to slide the cube to get to the other side xD you were talking about the laser thing. oh ok.
XYZ Aug 2, 2015 @ 5:16pm
Originally posted by Smidge204:
The propulsion gel works exactly the same in the official game, if that's at least part of what you're asking. The mechanic of having a cube slide down a ramp is actually in the official game.

As for implementation at the programming level, the easiest way I can imagine is to give it a negative coefficient of friction. It's entirely possible there are special rules for the player and other objects, though, so it speeds up the player only but reduces friction for physics objects.

Asbestos is pretty mysterious!

Wow, the idea for negative coefficient of friction is awesome : D.

Still I have a question: if that is how propulsion gel is implemented, a cube coated with propulsion gel would actually accelerate to a highspeed on the ground if we give it an initial speed, which is not observed in the game.
Tmast98 Aug 2, 2015 @ 9:23pm
Hi! We did not change the way gels are programmed or implemented. You can observe the properties of speed gel with a cube in Portal 2's cooperative campeign. If you are curious regarding how gels are implemented, here is a small summary:

Originally posted by Jeep Barnnet:
We [Valve] brought in Ilya Rosenberg to help implement his remarkably fast algorithm for real-time particle isosurface extraction and rendering way back.

That tech was floating around in the Source engine for years, looking for a home, and Portal 2’s gels seemed like a great fit. In simpler terms it takes a set of points (could also be called particles or metaballs) and each frame generates a custom mesh of polygons (or isosurface) that wraps smoothly around them. This is generated in chunks and in a specific order based on the player’s position and camera angles to achieve maximum performance while preserving transparency rendering orders.

Performance was extremely important in Portal 2’s Co-op splitscreen where the game can be potentially rendering the gels from 20 to 30 angles in the same scene! We also did some special work to make them draw seamlessly across portals. This required allowing the metaballs to be duplicated across both sides of the portal with the lighting information blended across the polygons near the edge.

Christopher Onorati
Level Designer
Last edited by Tmast98; Aug 2, 2015 @ 9:24pm