Translated by Optical Flow

In the middle of last year, Nintendo's "The Legend of Zelda: Tears of the Kingdom" sold more than 10 million copies in three days after its release, and the total global sales of this game have now exceeded 20 million copies, continuing the momentum of its predecessor "Breath of the Wild".

What is still fresh in people's memories is that "The Legend of Zelda: Tears of the Kingdom" has taken a step forward in the open world design, and others are still stuck in the Chicken and Lower Quest modes, this game has allowed players to "go to heaven and earth" in a seamless large world.

Recently, the development team of The Legend of Zelda: Tears of the Kingdom (hereinafter referred to as Tears of the Kingdom) shared the game's technical experience in creating a seamless 3D open world (that is, an open world that makes full use of the sky, ground, and cave scenes) at CEDEC.

Note: The PPT in the article can be scanned and translated with one click on WeChat.

01

A two-dimensional, breath of the wild

As the predecessor of Tears of the Kingdom, The Legend of Zelda: Breath of the Wild (hereinafter referred to as Breath of the Wild) laid the technical framework for both games. For example, in order to achieve a seamless large world, the production team adopted a time-lapse rendering technology with more polygon performance advantages; In order to be compatible with the gameplay with a high degree of freedom, the production team also used a physics engine that is difficult to make mistakes in physics calculations in various environments, so as to present highly flexible movements.

Going backwards from what the production team wanted to achieve, the biggest problem they faced in the process of delineating the technical scope was "wide world" and "seamless movement".

For the former, in the early stages of the development of Breath of the Wild, after brainstorming, the production team finally decided on the direction of "making a vast, flat world", in other words, the production team regarded the world of Breath of the Wild as a "two-dimensional" world.

This mindset also helps to simplify the design of the player's movement seamlessly. As a result, Breath of the Wild's map can draw LODs (Level of Detail) effects that are "rough in the distance, delicate in the near distance" and "important objects can be displayed at a distance, and objects that are too detailed will only be displayed when they are close", so that the player can smoothly read the data as they move in the 2D plane.

Starting from the goal you want to achieve, these technologies are sometimes created to create other functions that are "unachievable". As for the mechanism of "loading according to the 2D data on the map", what can't be achieved? That's the "three-dimensional structure".

In other words, while Breath of the Wild is a wall-climbing and gliding 3D action game, it is still a game that is confined to a two-dimensional open world in terms of game design and experience.

In contrast, Tears of the Kingdom has been thinking about how to expand vertically to the ground, sky, and underground since the brainstorming stage of the early development team. This kind of thinking was also driven by the production team's goal of challenging "what Breath of the Wild can't achieve."

In addition, Breath of the Wild must take into account the hardware performance compatible with the Wii U, while Tears of the Kingdom is a Switch-specific game so there are few hardware limitations. So in general, the technical difficulty in the development of the Tears of the Kingdom open world is "how to expand the two-dimensional elements in Breath of the Wild into a three-dimensional design".

02

Realization of a three-dimensional "cave system"

As the open world of Tears of the Kingdom shifted to a 3D design, Sky Island, caves, and underground areas were involved in the development of the scene, so it was necessary to rethink data loading and LOD from a 3D perspective from a technical perspective.

The first thing the team focused on was the cave, which contained the floor, walls, and ceiling. To this end, the production team has developed a "cave system" that can not only read and LOD 3D cave data, but also seamlessly map the cave terrain.

Using this cave system, the crew was able to easily construct and present a huge space with seamless loading. So they created a huge underground cavern, which, when paired with the surface scene, creates a seamless surface + underground area.

In the process of experimenting with what gameplay could be designed in this vast space, the production team slowly came up with ideas such as exploring the darkness, fighting powerful underground enemies, and fighting between riders.

After repeated tests, the production team came to the conclusion that even if there was a huge field underground, it could bring more gameplay design ideas to the production team. Further, if the underground scene was linked to the surface, the production team could also design more "discovery-play" loops, so later the game was born to find corresponding hints for each other underground and on the surface.

When creating an underground scene, the production team will design based on the data above ground, such as a depression in the ground where there is a bulge in the ground, and a checkpoint underground where there is a shrine in the ground.

Similar to the underground caverns, the creation of the Sky Island also uses the "cave system".

03

Redesigned loading system

While expanding the Tears of the Kingdom's open world to three-dimensional, the loading system has also been redesigned to make the game more flexible to load than Breath of the Wild. For example, one of them is a mechanism that allows you to flexibly replace objects according to the game logo when the terrain of the village becomes larger according to the progress of the game.

In Breath of the Wild, a loading screen appears when entering a large dungeon, but in Tears of the Kingdom, the production team built a universal data prefetch separate area for seamless loading.

On the other hand, since the algorithm for loading the system comes from Breath of the Wild, how to read the data from the underground and the sky becomes a problem. For example, if the Underground scene is in an underground space away from the surface, the player will not have the opportunity to see both the Underground and the Surface at the same time.

Therefore, the production team believes that loading the ground when it is underground, and loading the ground when it is on the ground, so that the vast world can be presented at a high frequency, so only when the player reaches a certain depth, will the ground scene be unloaded and the underground scene will be loaded.

However, during the first implementation, the unloading and loading could not be completed in time, causing the player to freeze in the process of falling underground. Theoretically, the underground and the ground are connected, and when the player goes directly from the ground to the underground, the loading speed is enough, but it is still flawed as a game product. Therefore, it is also necessary for the production team to keep up with loading and unloading when the player is falling at high speed.

As a result, the production team used four methods to make improvements. The first is to divide bottlenecks and time-consuming processes into parts that are needed immediately and other parts by "using analyzers for visualization and process reordering", and schedule the former to idle time processing when loading is not performed, and then delay processing of the latter.

In order to solve the bottleneck caused by this sorting, the second solution is to "reduce the number of files that need to be loaded".

In a two-dimensional world-based loading system, loading is performed based on the display distance set for each object, but even things that are not actually visible, such as the bottom of a cliff, are loaded. Therefore, when moving to 3D, the production team set up camera points at multiple locations that the player could reach, and reduced the number of files that needed to be loaded based on the data of "what objects are not visible from these points".

The production team also used a third method, "reducing the size of the files that had to be loaded". One example is to determine if the resolution of the texture used in the model is appropriate, and if lowering the resolution is not a problem, then automatically reduce the resolution, thus reducing the load file size.

However, even if the above three methods are fully utilized, there are still cases where the loading is not completed in time, so the fourth method for the production team is "the start time of loading in advance".

Specifically, firstly, according to all the openings used to enter the underground scene, make a list of the documents required to enter the underground; Then, as the player approaches the hole, have the game load as many associated files as memory allows. Through the above four methods, Tears of the Kingdom enables the player to move seamlessly from the ground to the underground.

So, how does the sky world load?

In the early days, Sky Island was still in the trial-and-error stage of game planning, so it was clear that they had adopted an "aggressive Sky Island placement strategy". However, if the loading technology is implemented before the sky island design is finalized, incompatible sky island ideas and designs cannot be adopted.

So, the team took advantage of Breath of the Wild's loading system to create an environment where game planners had no limit to experiment with the Sky Island design and create what they really needed. As a result, there are many items and props designed on the Sky Island, as well as a large amount of content related to the main story, as well as small content related to air movement and traversing terrain, as well as details such as how to get on the Sky Island.

Based on the above functions and positioning, the production team only loads one large empty island and multiple small empty islands according to the player's actions and stores them in memory, while the unloaded empty island will present pre-generated LOD low-poly based on the configuration information. By defining "units" in this way and determining the allocation in advance, the process of unloading and replacing new islands can be handled at high speed in the game.

From the production team's point of view, creating a seamless world and establishing a smooth production process are somehow a joint thing. In addition to the technology to achieve a seamless world, it is also necessary to have a flexible approach and a good workflow to get the most out of each process.

The production team finally talked about how when playing Tears of the Kingdom, some scenes are where you are looking at the world, and some scenes are where you only focus on the objects in front of you, and by repeatedly switching between these scenes, the whole world can be seamlessly connected in the player's mind.

In game development, the production team needs to consider not only a full production cycle, but also the details of game optimization within 1/1000th of a second, or the iteration of the entire team. From a programming point of view, the most important thing to realize the seamless world of Tears of the Kingdom is to be able to "switch back and forth between the micro and macro".

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