HSR algorithm-occlusion culling

The current HSR (Hidden Surface Removal) algorithms can be divided into four types: backface culling, frustum culling, portal culling, and occlusion culling ). Their functions are as follows: backface culling is used to remove the triangle on the back of the mesh, frustum culling is used to remove objects outside the screen, and portal culling is used to remove objects that cannot be seen through the portal, the occlusion culling is used to remove the objects that are blocked by the objects in the scene. The first two algorithms are the most commonly used algorithms. For backface culling, they are already implemented on GPU hardware. However, to reduce the transmission bandwidth, you can also use the CPU in a proper place. It mainly refers to a large-scale mesh like terrain. How to Use it depends on how you organize terrain data, when you use triangle strip to render a terrain, we try to use block-based terrain for culling. When you use Triangle List to render the terrain, you can try to use a triangle to perform culling on it. You need to experiment with the method you actually use. Frustum culling is not explained here because it is too common, and portal culling is another article, simply put, if the camera cannot see the house through a door, all the things in the house are culling without rendering. Let's take a simple example to see why it cannot be completely eliminated:

The above can be seen as a house, and the portal is the door of the room. If we do portal culling computing, the NPC is visible, but because there is an object wall in front, it should be invisible, however, portal culling does not remove it, so the NPC will still be transferred to the GPU for rendering.

At this time, we need to use the occlusion culling technology to make up for this defect. Therefore, the OC technology must be used in a portal engine. Here is another outdoor example, there is a building outside. We can use the building doors and windows as the portal to perform culling on the building, and use the building as an occluder to perform OC operations on the entire scenario. Based on the information provided by the relevant literature, when the OC technology is used in the scenario, the FPS is increased by 30% to 70% than when the OC technology is not used, therefore, if possible, we should try to add the OC technology to your engine, and the OC technology is not only applicable to the portal engine, it can be used in any technology-built engine.

There are many OC algorithms, such as hzb and Hom, which are relatively powerful. However, if they are used in CPU computing, the efficiency is still very low and the real-time requirements are not met, however, the current GPU does not support its hardware implementation, so it is not the focus of this article. The OC algorithm introduced here is called the Z buffer algorithm of the range scanning line, this is an OC algorithm that can be completely eliminated and is very efficient. Before introducing it, we must clarify several concepts. In oC, blocks are called occluder, and blocks are called occludee. The OC algorithm checks whether occludee is blocked by occluder.

The large objects in the scenario can be used as occluder, such as houses and castles. Before OC computing, we need to obtain all occluder in the scenario. This is a very important step, this step is usually performed during scenario preprocessing. An occluder list is generated based on the scenario and saved to the file.

Next we will discuss how to obtain occluder. In this scenario, occluder usually needs to be manually specified because developing an algorithm to automatically obtain occluder is very complicated, at the same time, the number of occluder in the same frame must be limited due to the OC operation efficiency problem. Now, the terrain-based engine is popular in MMO. The objects that can be used as occluder are actually independent models. Therefore, the occluder is actually the convex hull that obtains the model, this type of algorithm is also quite similar to the qhull algorithm. However, if you use a program to obtain the hull, there are some limitations on the shape of the model, an alternative method is to specify a hull for the object during the artist's modeling and save it to the file, so that the shape of the model is not limited, therefore, the second case is to specify an appropriate object as the occluder when editing the scenario, and then save the convex hull of the object to the occluder list.

When we obtain occluder in the scenario, we can perform OC operations. First, let's take a look at the principle of the Z-buffer algorithm of the range scanning line.

The Polygon on the same plane on occluder is called the surface. The surface is connected by the edge at the beginning and end to form a closed space.

When N is occluder, A, B, and C are occludee, we assume that they are projected onto the screen as a rectangle. Here we call the top and bottom two edges A and B of N as scanning lines, it can be seen that if object A is behind N, then a must be blocked by N, which can be compared by comparing the Z value of the surface where N and a are located, this is the source of the Z buffer in this algorithm. Now we assume that all objects A, B, and C are behind n. We can see that A is completely blocked, and B is partially blocked, C is not blocked. How can I judge it?

First, let's look at the situation of A. There are four scanning lines of N and A in the ascending order of Y: B, D, C,, in the range composed of scanning line B and D, because there is no edge of a, skip and check the range composed of scanning line D and C. We call the edge between the two scanning lines as the active edge, they are saved in the active edge list. Each active edge stores two surface pointers, left surface and right surface. For example, for the left edge of N, its left surface does not point to the background of the screen, that is, the surface with the Z value as the maximum, and the right surface is the surface with the N value. Others are similar.

Now we check the activation edge list between scanning line D and line C. First, we obtain the left edge with the smallest activation edge of X value of A as, then obtain the activation edge of the left edge of X greater than a in the activation edge list of N, which is the right edge of N and compares the left surface of the two edges, the Z of the Left surface of the left edge of A is greater than the Z of the right edge of N, so the left edge of a is blocked by N, then we can use the same method to check whether the right edge of a is blocked, so that we can determine that A is completely blocked by N. The same method is used to check B. in the range between scanning line f and B, because n does not exist in the activation edge list, B can be determined that B is not completely blocked by N. Check C again. In the activation edge list between scanning line H and G, all edge X in the edge list is smaller than the left edge of C, and it can be determined that C is not blocked by N. For other complex situations, such as multiple occluder cases, you can verify by yourself that this algorithm can accurately determine whether an object is completely blocked under any circumstances.

I believe that you have fully understood the basic principle of the Z-buffer algorithm of the range scanning line through the above introduction. The following describes how to implement this algorithm. The steps are as follows:

1. Obtain all occluder in the scenario and remove all surface pairs of camera on the occluder.

2. Use near clip plane to perform clip operations on all surfaces, removing the surface located outside of near clip plane. If the surface and near clip plane intersect, in this way, the edges outside the nearclip plane are removed.

3. transform all vertices to the projection space. Pay attention to the nature of the projection space. The projection transformation is actually to convert frustum into a box, which is a cube for OpenGL, the minimum inflection point is (-1,-1,-1), and the maximum inflection point is (, 1). For dx, the minimum inflection point is (-1, -), the maximum inflection point is (, 1 ). The shape of all objects in the projection space on the screen does not change, which is also the reason for the transformation to the projection space. Calculate the projected area of each surface on the screen, remove the surface with a small area, and remove the surface outside of frustum, this is to prevent errors due to computational accuracy. The plane of the surface is also transformed to the projection space to facilitate the calculation of the Z value of the specified vertex.

4. Save all vertices converted to the projection space to the edge list. Note that the edge is saved in the method where the Y value of the edge starting point is smaller than the Y value of the end point, in addition, the Y value at the starting point is the Y value of edge. In the edge list, sort all edges in the ascending order of edge y. If Y is the same as X, if X is the same as the slope of edge. Because edge list is fully sorted by Y, we can also call it the y bucket. Note that if edge is parallel to the X axis, you do not need to add it to the edge list.

5. Search for scanning lines and calculate the list of active edges between scanning lines. From the above introduction, we can also get a general idea of how to obtain the scanning line. The scanning line must be on the edge endpoint. If the edge exists intersection, the intersection must also be used as the scanning line, then we need to traverse the y bucket to find all scanning lines between the scanning line y =-1 and y = 1, after obtaining all scanning lines, you need to calculate the list of active edges in the scanning line range. Note that the edge in the list must use the intersection between the edge and the scanning line as the endpoint, the list of activated edges is sorted by X, which is also called the X bucket.

6. For scenes, you need to check whether the blocked object has passed in its AABB as occludee. First, check whether the projection area of occludee on the screen is large enough. If it is too small, you do not need to perform OC on it, then, the AABB is converted into a box, and the surface and edge of the box are calculated. Then, the surface and edge of the box are converted to the projection space and scanned lines are searched based on the above method to obtain the list of activated edges. Starting from the smallest Scan Line of Y in occluder and occludee, the activation edge list is compared by interval to check whether occludee is completely blocked.

In this algorithm, the list of activated edges needs to be updated almost every frame. Therefore, the most computational part is here, however, after the edge list is activated, occlusion Calculation for occludee is very convenient and quick. It is suitable for OC operations on a large number of dynamic objects, especially for role models with high rendering costs.