Half-Life Alyx liquid shaders: Why they look so good

Half-Life Alyx liquid shaders: Why they give the impression of being so good

We’ve all been taking on hobbies this 12 months. And whereas I’m positive my life has been enriched by marathoning The Crown and astral projecting out of my physique on weekends, Matthew Wilde had a lockdown venture that’s a little bit extra substantial: he made boozy bottle shaders for Half-Life: Alyx.

The shaders have been added to the sport in a Could patch, and so they instantly began getting love on the web. It’s straightforward to see why: they give the impression of being actually, actually good. The shader makes bottles seem clear, with liquid that reacts if you shake the bottle. Earlier than the patch, the bottles made a sloshing sound when shaken, however in any other case they appeared opaque.

I wished to learn the way the shader labored, and there was no higher particular person to ask than Wilde himself.

Wilde is a VFX developer, and he was working at Valve when Alyx was in improvement — however on Dota Underlords. He made a wine glass shader for the Dota character Jull, which caught the eye of Scott Dalton on the Half-Life: Alyx crew. There are many bottles scattered round in Alyx, however one specific stage takes place in a distillery and requires the participant to work together with bottles quite a bit.

Growing a shader that made the bottles look filled with liquid would make the extent that rather more spectacular. And Wilde wouldn’t be ranging from nothing, both: the sport was principally carried out, with detailed property and a strong lighting system in place.

“All the bottles existed as models, and they were all beautifully textured and they look great,” Wilde mentioned. “And so immediately, we could see that it was really promising.”

However with Alyx’s launch date looming, there simply wasn’t time to implement the brand new shader with out inflicting efficiency points.

“If there was just the occasional bottle around… that probably wouldn’t have been a problem,” Wilde instructed me. “But again, it was this distillery level that had thousands of props, and thousands of bottles.”

The shader acquired shelved for launch, and although Wilde was disenchanted, he took it in stride. Half-Life: Alyx got here out on March 23, 2020. That very same day, Jay Inslee, the governor of Washington state, issued a keep at residence order that may shut down Valve’s Bellevue places of work.

“And so we had time suddenly,” Wilde mentioned. “Lockdown gave us the opportunity to have a look back at things that we hadn’t managed to do previously.”

Making the shader

Shaders are packages that inform 3D objects find out how to look. This particular shader is a pixel shader, which tells each pixel what coloration to be, primarily based on how it’s lit. Every little thing that’s rendered in a recreation has a shader.

“It’s just that some shaders are a bit more involved than others,” Wilde instructed me.

Shaders find yourself accounting for lots of the small print that we see. Really rendering one thing in 3D — the entire grooves and knots in tree bark, for instance — can be time-consuming and performance-heavy. However a shader can create the phantasm of tree bark, or armor, or pores and skin, or beer foaming in a bottle, even when the item it’s utilized to is simply fabricated from easy, easy polygons.

Now ensconced at residence, Wilde returned to the shader. Alyx already had a lighting system, so what the shader wanted to do was determine what values to make use of when the lighting was calculated. The shader takes under consideration inputs like base colours, textures, and reflections for each the liquid and the bottle itself, after which combines all them collectively.

“And then what you output is another color,” Wilde mentioned. “You do it for every pixel, and you get a result at the end.”

A gif showing how the shader combines liquid and a bottle to create the effect of liquid inside a bottle.

Combining liquid and a beer bottle in Half-Life: Alyx
GIF: Matthew Wilde/Valve

The shader is, firstly, the results of obsessive statement. Wilde fairly actually sat in his basement, selecting up and enjoying with bottles to see how liquid behaved.

“You look at a shader and you see all the work that’s gone into it, and it’s pages and pages of stuff, but that has never just kind of flown out of someone’s mind, you know, straight into the file,” Wilde instructed me. “It’s always been a case of building it up slowly… and then getting ideas as you go along, kind of sketching it out.”

Wilde would make observations of the real-life bottle — the froth that develops on beer, or the best way liquid catches the sunshine — after which tweak these inputs within the shader in order that his creation mirrored actuality. It was like making a pointillist portray, pixel by pixel. Notably, there’s no liquid simulation occurring right here.

“It’s really made to behave how I have determined that it should behave, not anything more realistic than that,” Wilde mentioned. “There is no simulation, it’s just observed.”

That’s why the wording in Alyx’s patch notes could be very intentional: “Bottles now appear to have liquid inside them that sloshes around as you shake them.”

Reflection and refraction

Dice mapping has lengthy been how video games deal with reflections, and even with ray-tracing looming in our future, that gained’t change in a single day. A dice map consists of six views baked right into a dice. Relying on the place the participant’s digital camera is pointing, the sport will pull an approximate view from the dice to create a mirrored image. It’s why mirrors don’t actually work in video games — dice maps don’t create excellent reflections, and so they actually don’t have to.

There’s an important instance of that in Marvel’s Spider-Man on PS4, the place reflections on buildings may present one thing from close by, however not essentially essentially the most correct factor. We’re often fairly able to suspending disbelief, as a result of the reflection seems ok.

This booze shader makes use of a dice map to create reflections, but in addition to create the phantasm of refraction — which is mild passing via an object, reasonably than bouncing off it.

The bottles are usually not clear. However they look clear, as a result of Alyx is pulling the dice map in your approximate coordinates, and projecting that pre-baked picture onto the aspect of the bottle you’re . It reveals you a distorted view of the house, however it’s a wild phantasm that completely works.

“Using them for a bottle where things are already a bit distorted, where everything’s a bit dirty… it’s actually a really good way of doing it,” mentioned Wilde. “I think it also helps that when you’re looking at glass and liquid, no one knows how it should look, just as an instinctive thing. You just accept when you start looking through glass and things go a bit weird.”

Looking through a wine bottle in Half-Life: Alyx to demonstrate that it is opaque and empty.

For those who stick a wine bottle in your face, you’ll see that it’s, in actual fact, empty.
Gif: Polygon by way of Valve

The dice map picture is doubly distorted the place the shader says there’s liquid. Wilde combines reflections on high of reflections to make it seem to be there’s two surfaces: the glass bottle, and the liquid “inside” it.

And that’s the way you get the phantasm of liquid inside a clear glass bottle — when in actual fact, the bottle is empty and opaque.

Pretend physics

What I like most about this shader is that it’s detailed sufficient to move our psychological threshold of realism — though, once more, there’s no simulation concerned.

“Everything is faked, everything is cheated. Everything is just kind of observed, and tweaked, and values modified until it just about feels right,” Wilde mentioned. “When I shake a bottle, [the liquid] kind of wobbles around a bit. So we make it wobble around a bit, inasmuch as the wobble looks about right.”

It’s this wobble that originally delayed the shader, as there wasn’t an environment friendly method to get the knowledge into the sport. Ultimately, the efficiency value was negligible, as a result of Wilde’s colleagues at Valve realized they might retailer knowledge within the shader’s vertex coloration.

The vertex coloration is “a relic from an earlier time before realistic lighting and textures,” Wilde instructed me in an e mail. “In this instance, it was just a color that was applied across the whole thing. And we didn’t use that color for anything.”

A vodka bottle held up to the light and then shaken.

A vodka bottle in Half-Life: Alyx
Gif: Polygon by way of Valve

And so the agitation (or, professionally, sloshiness) of the liquid, the course of gravity, and foam or bubbles, are all saved in these unused RGB channels. Once I wave a bottle round wildly, the speed of the bottle is used to drive agitation, making the liquid seem to slosh round. Bubbles or foam are additionally tied to velocity, and progressively decay when motion stops. However all essentially the most intensive calculations — lighting, reflections, shadows — are already being carried out for each object within the recreation, no matter whether or not the participant goes nuts with bottles or not.

Why it succeeds

Though engaged on a lockdown venture involving copious quantities of alcohol may sound isolating, when it got here to implementing the shader and cracking the efficiency problem, Wilde wasn’t alone — Alyx’s visible results are a crew effort.

“The success of it comes from, first of all, the fact that the assets themselves are just so good to start with,” Wilde mentioned. “When you’re writing a shader, certainly this kind of shader… you’re piggybacking onto an existing pipeline and existing lighting setups. So it’s just about trying to exploit what’s there as best you can.”

Shaking a bottle is likely to be easy to do in actual life, however it’s not so easy to make a digital bottle reply in variety. With time and albeit spectacular powers of statement, Half-Life: Alyx’s boozy bottles got here to really feel so proper that we don’t have to droop disbelief after we see them. It’s the type of intelligent, inventive considering that recreation improvement will all the time profit from, regardless of how superior expertise will get.

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