Chapter II: Brief Introduction to Large Outboard Scene Retrierators
Section 1: Indoor vs. Outdoor
Section 2: VOXEL vs.lod
Section III: Dynamic Terrain VS. Static Terrain
Section IV: Other
First part
Speech
Real-time rendering techniques for outdoor scenes are hot technology in the game programming world. At the same time it also has the same important role in other fields. Such as GIS systems, flight simulation systems, VR systems, and digital global technology are in real time rendering techniques that do not open outdoor scenes. A excellent outdoor scenery rendering technology can create a very realistic and persuasive virtual natural environment outside the guaranteed time. For example, Nova Logic's famous 3D shooting game Delta Force series, in addition to simulating various trees, weeds, and various weather effects in addition to various terrain such as snow, grass, deserts.
Real-time rendering of outdoor scenes has many technical difficulties, in the following sections, we will make a detailed introduction and solutions for some major issues.
The main contents of this article are divided into two parts: one, rendering of large-scale terrain. Second, how to improve the authenticity of the scene. Part 2 and the third part.
3D rendering of the scene is inseparable from the 3D API. At present, there are two popular 3D APIs, and SGI's OpenGL and Microsoft Direct3D. The two APIs have their own advantages that they can use the hardware acceleration function. But OpenGL is an open standard with better graft performance, which works under Linux and FreeBSD. Even hardware acceleration (NVIDIA has launched a driver specifically for Linux / FreeBSD). So in this article, I used OpenGL. But if you are familiar with the principles, it is also small, and D3D to 8.0 is very like OpenGL.
Chapter II: Brief Introduction to Large Outboard Scene Retrierators
One. Outdoor vs.
Below we make a contrast between the indoor scenes and outdoor scenarios, let's take a look at the main difficulties of real-time rendering of outdoor scenes.
At present, the most successful business interior game engine has a quake / doom series, the Unreal series engine. They are all based on BSP technology. Through BSP technology, plus PVS, Portal can reduce the complexity of the scene, and can even connect an indoor scenario and an outdoor scene through Portal technology, and further exceeding the scope of this article.
We know that when we stand in a room, we can admire the scenes just the arrangement of this room and the scenes that can be seen through the door and windows of this room. You can only walk within the constraints of the wall. In other words, we are in a limited space. We have enough reason to prevent people from passing through the world outside the wall, and there is enough reason to constrain the field of view within a high wall.
Picture 1-1)
A typical indoor scenario, using the Id Software Quake Iii map file, rendering with www.gametotures.net
However, these constraints are impossible in an outdoor scene. In a flying simulator, you can drive a plane to fly far from any one. Because the fact is true, if you like, you can drive a plane to fly around the earth without worrying about the wall to prevent you from moving forward. In other words, an ideal size of an outdoor scene is unlimited! In addition to the size of the scene, the viewing is also infinite. If you stand at the height, you can overlook any places than your bottom, that is, you have almost unlimited vision.
Figure (1-2)
Typical outdoor scenario, picture from North Navigation Dang Zhengsheng Demo: Nature Wing2.0 Screenshot
We know that unlimited large scenes require unlimited scene data. But this is impossible, we can only hope that the bigger the scene is, the main part of the outdoor scene is the rendering of the terrain, and the terrain data has determined the size of the scene. So how to save these terrain data into a primary problem. (However, today, the memory cost is rapidly declining, this problem has become not very prominent.) Secondly, the unlimited field of view, the unlimited field of view represents the unlimited element (simple geometry for 3DAPI support) Graphics, see OpenGL / Direct3D SDK, see OpenGL / Direct3D SDK, which is also impossible. The quantity of the primitive is increased at the speed of the size of the scene. Light considers terrain data, a 2048x2048 terrain, if it is not considered to reduce the degree of detail and cropping, it will render 8M triangle, such a triangular shape is still far from the PC and cannot achieve interactive frame rate. Therefore, how to reduce the number of primitives to rendering the terrain is a key issue for rendering of outdoor scenes. Other situations are as well as a field surface derivative: trees, weeds, landforms. At the same time, the weather is effective, such as rain, snow, wind and lightning, etc. These things are basically unnecessary in an indoor environment. Moreover, these effects require a high price, and some don't even simulate it.
two. Voxel VS LOD
In summary, we know the rendering of key terrains in outdoor scenes. We need a technology to reduce terrain rendering overhead.
At present, terrain rendering technologies have two Voxel and Lod, let me be a simple introduction.
Voxel is also Volumetric Pixel. That is, the so-called "voxel", it is relative to pixels, if the pixel is a two-dimensional rectangle, then Voxel is a three-dimensional cube. Its principle is relatively simple. James Sharman claimed to have come up with this approach in 1995. The previous mentioned Delta Force game is Voxel technology. About the detail technology of VOXEL is not the focus of this article, I am not ready to do an in-depth introduction. Voxel has a natural advantage that it doesn't matter when it is rendered, and there is no relationship with the scene, and it will never render more things (self-cropping capabilities). Its complexity is only related to the field of view we need, and resolution. Moreover, it is possible to achieve a relatively ideal speed without using hardware acceleration (Delta Force I does not use hardware acceleration), and the generated image is also more delicate. Its disadvantages are not enough flexible.
LOD is also the abbreviation of Level of Detail, different from Voxel technology, which is a polygon, real 3D rendering technology. It simplifies the details of the object based on certain rules, and we can choose the difference in objects of different details as needed. As the viewer is close to the observer, the degree of detail is selected, and vice versa. Used in terrain rendering, sometimes we also call it a multi-resolution Terrain rendering technology.
Figure (1-3)
Based on VOXEL's rendering scene, picture from China Game Developer Network, Chen Peng "Handself Edible Voxel 3D Engine"
Figure (1-4)
Based on LOD rendering results, the image is from Demo: Sim-Nature, this article: SIM-Nature.
The LOD algorithm processes more complicated, but it allows us to freely control our scene rendering, more convenient to use the graphics hardware acceleration function. And other objects can be combined in the scene. Such as trees, sun, and particle systems, etc., the sky can be convenient to let the observer observe the scene at any angle, and we can just let the camera rotate a certain angle. But this is more difficult in Voxel because Voxel is very troublesome when dealing with non-horizontal sight. LOD technology is terrain rendering technology that will be used and implemented herein.
Figure (1-5)
Topical grids are processed by different details. The picture comes from Demo: SIM-Nature in this article.
three. Dynamic terrain VS static terrain
Terrain rendering is usually divided into dynamics and static.
The detail of the static terrain can be uniform or uneven. But the details are usually calculated in advance, there are many advantages in the static terrain of uneven detail: such as the plain's landforms can use the bottom details, and the high-level detail level is used. A more intuitive example is that the car has a certain restriction in the place where you can reach the place, we can build a relatively high detail level in the nearby place, and use less detail in far away. In this way, an irregular terrain can be established. For example, it can build a terrain model along the direction of the track, which saves a lot of space.
Dynamic terrain is related to viewpoint. It is a method of this article will adopt and implemented. With the movement of the viewpoint, the terrain grid will be updated. This is a more advanced algorithm relative to static terrain. The scene established in this way is more in line with human visual characteristics, that is, the details they see are changing. The establishment of the dynamic terrain grid and the update takes an additional time, but this overhead is worth it. The establishment of a dynamic terrain grid is more complicated. It needs to pay attention to many things: how to decide the details, how to avoid cracks is two major problems. At the same time, it should also remove the invisible terrain part, geometric change (with detail, the breathing of the terrain surface) should also be considered. These issues are discussed in detail in the sections.
Four: Others
We have said that in a rendering of an outdoor scene. Some elements other than terrain are also important, these elements can improve the authenticity of the scene. This article will implement some of the elements, including: trees, ground details, sun, sky have moved blurred effects. They will be discussed in the third part of this article.
