Saturday 24 October 2020

Bending Blightbound's world to lower the horizon

In 3D games perspective is often treated as a given; a law of nature. But it doesn’t have to be that way: with some shader trickery or clever modelling, perspective can be manipulated to achieve certain compositions that may not be realistic, but look more interesting and are still convincing to the player. One such example is how we kept the horizon on screen in our new game Blightbound by subtly bending the world.

At Ronimo we come from a world of 2D games. In 2D, composition can be whatever you like. That’s why our art director Gijs Hermans may sometimes want to ignore standard perspective rules and instead looks at what he wants to achieve visually. Thus early in development Gijs came to me and said he wanted the camera to look down quite a bit, but still have the horizon in view. In fact, he wanted the horizon to be quite a bit below the top of the screen. His reasoning was that visuals look much better when you don’t see just the floor most of the time. Het position of the horizon is an important tool for shaping a composition.

The origin of this request is a clash that often happens in game development: pretty visuals versus gameplay clarity. Our artists spend a lot of time on achieving both goals simultaneously. A very successful example of this is the way gameplay objects and backgrounds are drawn in a different style in our previous game Swords & Soldiers 2, as I described in this blogpost.

In a game where depth matters, like Blightbound, a low camera is problematic because it makes it difficult to see whether you are standing in front of an enemy or behind them. A high camera solves this, but a high camera removes the horizon from view, making the image a lot more boring.

When Gijs came to me with this request, I thought of two possible solutions: either give the camera a wider field-of-view, or bend the world to move the horizon down. We tried the easiest solution first: wide field-of-view. However, it turned out this needed to be set so wide that the entire perspective looked skewed. Extreme field-of-view often isn’t very pretty and it definitely wasn't in Blightbound.

The alternative I came up with is bending the world down the further it is from the camera. This is an effect that’s used in a bunch of games to create a sense that the world is very small, making the world feel cutesy and funny. However, Blightbound is intended to be a dark fantasy game, definitely not something cute and funny, so we didn’t want anything that extreme. I figured that with some tweaking it might be possible to achieve a more subtle version of this that still keeps the horizon in view but doesn’t have the funny vibe.

My implementation of this effect is quite simple. In the vertex shader I bend down the world depending on the Z-position of the vertex in the world. The nice thing of implementing it this way is that our gameplay code and level design tools can assume a flat world, making them a lot simpler. The bending only exists during rendering, so gameplay logic doesn't need to take it into account.

A minor challenge in implementing this bend is how to handle lighting and shadows. When the camera moves forward and the world bends, we don’t want the lighting on objects to change, since that would make the bending very obvious and would make the player focus on the backgrounds instead of on the gameplay. Also, objects in the background shouldn't be more bright because they are rotated towards the light by the bend. My solution was to calculate all lighting, shadows and fog as if there is no world bend.

Also, a little technical note: since the bend happens on the vertices, objects need to have enough vertices. A big square plane for the ground with no vertices in between can’t be bent. Occasionally this caused bugs where a small object would float above a big object because the big object didn’t have enough vertices to be bent correctly.

The bend effect is quite fun to see in action when set to an extreme value. However, any kind of geometric deformation is quite noticeable when the camera moves, so we chose to fix the bend in the world instead of letting it move with the camera.

A few different settings for the bend, including the final one used in Blightbound.

As you can see in the video, the bend effect is kept quite subtle in Blightbound. We didn’t want that cutesy/funny effect at all, since this is intended to be a dark fantasy game. Our level artist Ralph Rademakers tweaked the effect and the camera a lot until he got it to a point where it felt like there was no bend at all, just a natural camera. However, if you compare with and without bend, you can see that the bend makes a huge difference in what you actually see. And that’s exactly how it was intended: achieve the desired composition but don’t make it look like anything weird is going on.

And then came the fog! The bend effect was implemented when we hadn’t figured out the lore of the world yet. We didn’t know then that we would want to have so much fog. In fact, the working title of the game used to be AwesomeKnights instead of Blightbound! Once we finally decided on the lore we knew that the world of Blightbound is covered in “blight”, a corrupting fog. To match that, Ralph added a lot of fog to all the levels. This creates a great atmosphere, but… hides the horizon!

Does that make the world bend useless? No, definitely not. It's still used in quite a few levels to change the perspective and have a more horizontal view on the background, even if we can’t see as far as before. It’s a more subtle tool than originally intended, but still a very useful tool.

I think the bend effect we used here is a wonderful example of the kind of graphics programming I enjoy most: looking at what’s needed from an artistic standpoint, and then making tech that achieves that. I’m personally not very interested in realistic rendering: 3D is just a tool to make cool art, whatever the shape or type. The bend technique used here makes no sense whatsoever from a physical standpoint, but it adds to making Blightbound a prettier, more compelling game.

Sunday 18 October 2020

Screen Space Reflections in Blightbound

An important focus during development of our new game Blightbound (currently in Early Access on Steam) is that we want to combine 2D character animation with high quality 3D rendering. Things like lighting, normal maps, depth of field blur, soft particles, fog and real-time shadows are used to make it all gel together and feel different from standard 2D graphics. One such effect I implemented into our engine is SSR: Screen Space Reflections. Today I’d like to explain what SSR is and what fun details I encountered while implementing it into our engine.

A compilation of places with reflections in rain puddles in Blightbound.

Reflections in games can be implemented in many ways. The most obvious way to implement reflections is through raytracing, but until recently GPUs couldn’t do this in any reasonable way, and even now that GPU raytracing exists, too few people have a computer that supports it to make it a feasible technique. This will change in the coming years, especially with the launch of Xbox Series X and Playstation 5, but for Blightbound that’s too late since we want it to look good on currently common GPUs.

So we need something else. The most commonly used techniques for implementing reflections in games are cubemaps, planar reflections and Screen Space Reflections (SSR). We mostly wanted to use reflections for puddles and such, so it’s important to us that characters standing in those puddles are actually reflected in real-time. That means that static cubemaps aren’t an option. A pity, since static cubemaps are by far the cheapest way of doing reflections. The alternative is dynamic reflections through cubemaps or planar reflections, using render-textures. These techniques are great, but require rendering large portions of the scene again to render-textures. I guessed that the additional rendercalls and fillrate would cost too much performance in our case. I decided to go for Screen Space Reflections (SSR) instead.

The basic idea behind SSR is that since you’re already rendering the scene normally, you might as well try to find what’s being reflected by looking it up in the image you already have.

SSR has one huge drawback though: it can only reflect things that are on-screen. So you can’t use a mirror to look around a corner. Nor can you reflect the sky while looking down at the ground. When you don't focus on the reflections this is rarely a problem, but once you look for it you can see some really weird artefacts in reflections due to this. For example, have a look at the reflections of the trees in this video of Far Cry 5.

So we’re looking up our reflections in the image of the scene we already have, but how do we even do that? The idea is that we cast a ray into the world, and then look up points along the ray in the texture. For this we need not just the image, but also the depth buffer. By checking the depth buffer, we can look whether the point on our ray is in front of the scene, or behind it. If one point on the ray is in front of whatever is in the image and the next is behind it, then apparently this is where the ray crossed the scene, so we use the colour at that spot. It’s a bit difficult to explain this in words, so have a look at this scheme instead:

Since SSR can cast rays in any direction, it’s very well suited for reflections on curved surfaces and accurately handles normal maps. We basically get those features for free without any extra effort.

A demo in the Blightbound engine of SSR, with a scrolling normal map for waves.

At its core SSR is a pretty simple technique. However, it’s also full of weird limitations. The challange of implementing SSR comes from working around those. Also, there are some pretty cool ways to extend the possibilities of SSR, so let’s have a look at all the added SSR trickery I implemented for Blightbound.

First off there’s the issue of transparent objects. Since the world of Blightbound is covered in a corrupting mist (the “blight”), our artists put lots of foggy layers and particles into the world to suggest local fog. For the reflections to have the right colours it’s pretty important that these fog layers are included in the reflections. But there are also fog layers in front of the puddles, so we can't render the reflections last either.

The solution I chose is simple: reflections use the previous frame instead of the current frame. This way we always look up what's being reflected in a complete image, including all transparent objects. The downside of this is of course that our reflection isn't completely correct anymore: the camera might have moved since the previous frame, and characters might be in a different pose. However, in practice the difference is so small that this isn't actually noticeable while playing the game.

Transparent objects pose another problem for SSR: they're not in the depth buffer so we can't locate them correctly. Since Blightbound has a lot of 2D animation, lots of objects are partially transparent. However, many objects can use alpha test. For example, the pixels of a character's texture are either fully transparent of not transparent at all. By rendering such objects with alpha test, they can write to the depth buffer without problems.

This doesn't solve the problem for objects with true transparency, like smoke, fog and many other special effects. This is something that I don't think can be reasonably solved with SSR, and indeed we haven't in Blightbound. If you look closely, you can see that some objects aren't reflected at all because of this. However, in most cases this isn't noticeable because if there's an opaque object closely behind it, then we'll see the special effects through that. While quite nonsensical, in practice this works so well that it seems as if most explosions are actually reflected correctly.

Transparent objects like this fire aren't in the depth buffer and thus can't be found for reflections. However, if another object is close behind, like the character on the left, then the transparent object is reflected through that. The result is that it seems as if many special effects are properly reflected.

Having perfectly sharp reflections looks artificial and fake on many surfaces. Reflections in the real world are often blurry, even more so the further the reflected object is from the surface. To get reflections that accurately blur with distance I've applied a simple trick: the rays get a slight random offset applied to their direction. This way objects close remain sharp and objects get blurrier with distance. Artists can tweak the amount of blur per object.

However, this approach produces noise, since we only cast one ray per pixel. We could do more, but that would be really heavy on performance. Instead, to reduce the noise a bit, when far enough away I also sample from a low-resolution blurry version of the scene render-texture. This is a bit redundant but helps reduce the noise. Finally, by default in Blightbound we do supersampling anti-aliasing (SSAA) on the screen as a whole. This results in more than one ray being shot per screen-pixel. Only on older GPUs that can't handle SSAA is this turned off.

Another issue is precision. For really precise reflections, we would need to take a lot of samples along the ray. For performance reasons that's not doable though, so instead we make small jumps. This however produces a weird type of jaggy artefacts. This can be improved upon in many different ways. For example, if we would render the reflections at a lower resolution, we would be able to take a lot more samples per ray at the same performance cost. However, with how I implemented SSR into our rendering pipeline that would have been quite cumbersome, so I went for a different approach which works well for our specific situation:
  • More samples close to the ray's origin, so that close reflections are more precise.
  • Once the intersection has been found, I take a few more samples around it through binary search, to find the exact reflection point more precisely. (The image below is without binary search.)
  • The reflection fades into the fog with distance. This way the ray never needs to go further than a few meters. This fits the world of Blightbound, which is basically always foggy.
The result of these combined is that we get pretty precise reflections. This costs us 37 samples along a distance of at most 3.2 meters (so we never reflect anything that's further away than that from the reflecting surface).

Any remaining imperfects become entirely unnoticeable since blur and normal maps are often used in Blightbound, masking any artefacts even further.

A challenge when implementing SSR is what to do with a ray that passes behind an object. Since our basic hit-check is simply whether the previous sample was in front of an object and the next one is behind an object, a ray that should pass behind an object is instead detected as hitting that object. That's unintended and the result is that objects are smeared out beyond their edges, producing pretty ugly and weird artefacts. To solve this we ignore a hit if the ray dives too far behind an object in one step. This reduces the smearing considerably, but the ray might not hit anything else after, resulting in a ray that doesn't find an object to reflect. In Blightbound we can solve this quite elegantly by simply using the level's fog colour for such failed rays.

By default, SSR produces these kinds of vertical smears. Assuming objects have a limited thickness reduces this problem greatly, but adds other artefacts, like holes in reflections of tilted objects.

This brings us to an important issue in any SSR implementation: what to do with rays that don't hit anything? A ray might fly off the screen without having found an object, or it might even fly towards the camera. Since we can only reflect what we see, SSR can't reflect the backs of objects and definitely can't reflect anything behind the camera. A common solution is to have a static reflection cubemap as a fallback. This cubemap needs to be generated for each area so that the reflection makes sense somewhat, even if it isn't very precise. However, since the world of Blightbound is so foggy I didn't need to implement anything like that and can just fall back to the fog colour of the area, as set by an artist.

The final topic I would like to discuss regarding SSR is graphics quality settings. On PC users expect to be able to tweak graphics quality and since SSR eats quite a lot of performance, it makes sense to offer an option to turn it off. However, what to do with the reflecting objects when there's no SSR? I think the most elegant solution is to switch to static cubemaps when SSR is turned off. Static cubemaps cost very little performance and you at least get some kind of reflection, even if not an accurate and dynamic one.

However, due to a lack of time that's not what I did in Blightbound. It turned out that just leaving out the reflections altogether looks quite okay in all the spots where we use reflections. The puddles simply become dark and that's it.

For reference, here's the shader code of my SSR implementation. This code can't be copied into an engine directly, since it's dependent on the right render-textures and matrices and such from the engine. However, for reference when implementing your own version of SSR I expect this might be useful.

SSR is a fun technique to implement. The basics are pretty easy, and real-time reflections are a very cool effect to see. As I've shown in this post, the real trick in SSR is how to work around all the weird limitations inherent in this technique. I'm really happy with how slick the reflections ended up looking in Blightbound and I really enjoyed implementing SSR into our engine.