GPU Pro 7: 高级渲染技术探索 - CSDN文库

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I

Geometry

Manipulation

This section of GPU Pro contains two chapters that describe rendering techniques

used in recent games to enrich the environment and increase the visual quality.

The section begins with a chapter by Anton Kai Michels and Peter Sikachev,

who describe the procedural snow deformation rendering in Rise of the Tomb

Raider. Their deferred deformation is used to render trails with depression at the

center and elevation on the edges, allowing gradual reﬁlling of the snow tracks,

but it can also easily be extended to handle other procedural interactions with

the environment. The technique is scalable and memory friendly and provides

centimeter-accurate deformations. It decouples the deformation logic from the

geometry that is actually aﬀected and thus can handle dozens of NPCs and works

on any type of terrain.

The second chapter in this section deals with Catmull-Clark subdivision sur-

faces widely used in ﬁlm production and more recently also in video games because

of their intuitive authoring and surfaces with nice properties. They are deﬁned

by bicubic B-spline patches obtained from a recursively subdivided control mesh

of arbitrary topology. Wade Brainerd describes a real-time method for rendering

such subdivision surfaces, which has been used for the key assets in Call of Duty

on the Playstation 4 and runs at FullHD at 60 frames per second.

—Carsten Dachsbacher

4 I Geometry Manipulation

the core of this technology is a novel technique called deferred deformation,which

decouples the deformation logic from the geometry it aﬀects. This approach can

scale with dozens of NPCs, has a low memory footprint of 4 MB, and can be easily

altered to handle a vast variety of procedural interactions with the environment.

This chapter aims to provide the reader with suﬃcient theoretical and practical

knowledge to implement deferred deformation in a real-time 3D application.

Procedural terrain deformation has remained an open problem in real-time

rendering applications, with past solutions failing to provide a convincing level

of detail or doing so with a very rigid set of constraints. Deferred deformation

delivers a scalable, low-memory, centimeter-accurate solution that works on any

type of terrain and with any number of deformable meshes. It renders not only

the depression in the trail center but also elevation on the trail edges and allows

for gradual reﬁlling of snow tracks to emulate blizzard-like conditions.

Some terminology used in this publication with regards to snow trails and

deformation will be outlined in Section 1.2. Section 1.3 then takes a look at

past approaches, where they succeeded and why they were ultimately not suit-

able for Rise of the Tomb Raider. Section 1.4 outlines a simple, straightforward

algorithm for rendering snow deformation that will serve as a prelude to de-

ferred deformation in Section 1.5, which elaborates on the core ideas behind the

technique and the use of compute shaders to achieve it. Section 1.6 details the

deformation heightmap used in our algorithm and how it behaves like a sliding

window around the player. Section 1.7 explains how the snow tracks ﬁll over time

to emulate blizzard-like conditions. Section 1.8 covers the use of adaptive hard-

ware tessellation and the performance beneﬁts gained from it. Finally, Section 1.9

discusses alternate applications of this technique and its future potential.

1.2 Terminology

Here is a collection of terms used throughout the article (see Figure 1.2):

• Snow height: The vertical coordinate (in our case

vertex.z)ofthesnow

prior to any deformation.

• Deformation points: 3D points estimating Lara’s feet and other objects that

cause deformation.

• Foot height: The vertical height of a deformation point (

point.z).

• Trail depression: The part of the trail that is stomped down and is lower

than the original snow height.

• Trail elevation: The small bump along the edges of the trail caused from

pushing snow out of the way.

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