3D Graphics 2 - Chalmers · 2008-11-19 · 08-11-19 Simulation Engines 2008, Markus Larsson 2 Camera management Three basic kinds of cameras in games Firstperson Camera attached to

08-11-19 Simulation Engines 2008, Markus Larsson 1

3D Graphics 2

Simulation Engines 2008Chalmers University of Technology

Markus [email protected]


Camera management Three basic kinds of cameras in games

Firstperson Camera attached to the player and inherits the exact

motion of the player Examples: Farcry, Doom, Quake, etc

Scripted Camera moves along predefined paths Examples: Alone in the dark, Resident Evil

Third person The camera is located outside the body of the player and

shows both the avatar and the environment Examples: Super Mario 64, Gears of War


Thirdperson camera: Constraints

The camera should never be closer to a wall than the near plane It should never go outside a level It should translate and rotate smoothly to always try to stay at a specific

point in relation to the player character It should smooth out discontinuities in the character's movement It should be tested for collision detection It should be able to enter the character when needed


Thirdperson camera: Algorithm Calculate the destination point for the camera from the character's position

The destination point is calculated by applying a displacement and rotation from the position of the player character. Different camera views can have different displacements, and it may be possible to switch between them.

Check the validity of the destination point (it could be on the wrong side of a wall)

Perform a ray intersect between character and destination point (there should be no intersection with the world geometry)

If the point is invalid, move the camera back towards the character so that it is positioned on the correct side of the wall

Calculate approximate translation and rotation motion and animate it over a series of frames (animation speed is a tweakable constant)

Check for collision using the bounding box of the camera during the motion


Shaders and shader languages

1990s Development of hardware for rendering of textured

3D primitives Hardware T&L introduced Dynamic lighting done using Gouraud shading

2000s Perpixel shading Realtime procedural textures Advanced texture mapping techniques


In pursuit of realism

Traditionally, research on realistic computer graphics has focused on global illumination methods such as ray tracing and radiosity Do not work in real time

Pixar introduced the concept of shaders in RenderMan and showed that GI is not strictly necessary for realistic images Instead, shaderbased methods based on local

reflections can be used


What is a shader?

Three different interpretations from Watt & Policarpo, 2003 Cstyle module in the RenderMan API used for high

level control of rendering components (surface, volume and light shaders)

A combination of render states and texture maps for a multipass or multitexture render of an object on fixedpipeline GPUs

New hardware functionality for controlling the rendering of primitives on a perpixel or pervertex level on programmablepipeline GPUs; these are called pixel shaders and vertex shaders, respectively

The last one is important one


Shader types Vertex shader

Called for every vertex in a 3D primitive Allows for effects such as hardware skinning, perturbation of water

surface, etc Pixel (fragment) shader

Called once for every fragment in a 3D primitive (not pixel, because a fragment in a 3D primitive could correspond to one or several pixels depending on filtering settings, etc)

Can be used for procedural texture, normal maps, etc Geometry shader

Can add to and remove vertices from a mesh and be used for adding geometry too costly to process on the CPU

Allows displacement mapping, etc Unified Shader Model in DirectX 10 (Shader Model 4.0)


Shader languages OpenGL Shading Language (GLSL)

Part of the OpenGL specification since OpenGL 1.4 Highlevel language similar to C/C++

Cg (C for Graphics) Nvidia's proprietary shader language Extremely similar to HLSL, but works on both

OpenGL and DirectX Microsoft HLSL

Works on DirectX 9 and 10 Nvidia and Microsoft collaborated on its

development


Reflective surfaces: Environment maps

Commonly used for reflections Precomputed textures

Standard environment maps Single texture representing the

scene in one texture as ifreflected from a steel ball

Cubic environment maps Cube map consisting of six textures unfolded on to a

cube The most used format on modern hardware

Can also be used for advanced lighting


Particle systems

One of the most useful tools available Smoke, water, fire, etc

Each individual particle has a small or zero geometrical extent, but together form cloudlike objects

Each particle system cancontain thousands or eventens of thousands of particles Be extremely cautious of

multiple render states


Particle systems Consists of one or several emitters and a number of particles Each emitter is responsible for spawning new particles every

time update according to some distribution Each particle contains information about its current position,

size, velocity, shape and lifetime Each update

Emitters generate new particles New particles are assigned initial attributes All new particles are injected into the particle system Any particles that have exceeded their lifetime are

extinguished (and usually recycled) The current particles are updated according to their scripts The current particles are rendered


Water

Ideally based on fluid simulations Way to costly for realtime use

Mostly represented by a simpleplane/quad

In more advanced scenariosrepresented by a displaced grid

Reflections are either entirely faked or by using a planar mirror The scene is inverted and the reflection is rendered to a

texture which is then rendered using projective texturing onto the water mesh


Explosions There is no universal method Usually a combination of effects are used in combination until it

“looks good” Often animated billboards of prerecorded explosions are

combined with debris that is either actual geometry or particles Nowadays, often actual models are used which is moved by the

physics engine


Lightmaps Precomputed lighting and shadows on static geometry in

the scene Advantages

Allows for baking advanced lighting such as radiosity Extremely fast

Disadvantages Only works on static

geometry Requires a lot of pre

computing for good quality May require a lot of

memory


Stencil shadows Used with dynamic shadows Uses the stencil buffer Advantages

Creates crisp shadows withoutaliasing artifacts

Stable and potentially very fast algorithm Disadvantages

Difficult to get soft shadows (but possible) Extruding the shadow volumes on hardware is cumbersome

and requires modifications to the meshes Requires multiple render passes Patent problems with Carmack's reverse


Stencil shadows Empty the stencil buffer Draw the whole scene with ambient lighting

The zbuffer is filled and the color buffer is filled with the color of surfaces in shadow

Turn off updates to the zbuffer and color buffer and draw the frontfacing polygons of the shadow volumes

Increments the stencil buffer; all pixels in or behind shadow volumes receive a positive value

Repeat for backfacing polygons Decrement the stencil buffer; the values in the stencil buffer

will decrease where we leave the shadow volumes Draw the diffuse and specular materials in the scene where the

value of the stencil buffer is zero


Shadow maps Texture based shadows Renders the scene from the light's point of view For each fragment, checks if it is the the closest

one to the light Advantages

Can be done almost entirely on the GPU Works very similar to drawing the scene

regularly and allows reuse of frustum checkcode, etc

Good candidate for soft shadow algorithms Disadvantages

Precision problems and aliasing artifacts Good for directional and spot lights but omni

lights may require up to six individual texturesper light


Soft shadows

There are lots of techniques, but usually shadow maps with multiple samples per texel are used

Often extremely expensive to compute “Free” in lightmaps


Textures

Texture coordinates (U,V) Unfolds the model

Paint directly on the model with software such as BodyPaint, Zbrush, etc

Texturespace/Tangentspace can be interesting for advanced effects such as Normal mapping


Displacement mapping Commonly used technique in

raytracers Not possible on DirectX 9

hardware Can not move vertexes that do

not exist Can be hacked for certain uses

by doing texture lookups in thevertex shader

Possible for real on DirectX 10hardware

Geometry shaders allowtessellation of objects withoutCPU intervention


Normal maps Highpoly surfaces are transferred to

models by storing normals in textures Normals are normally stored in

tangent space Can produce extremely good results

with fairly low computational costs Works well on most types of objects lit

with dynamic lights Often, tools like Zbrush are used to

create highpoly objects, but normal maps can also be created from heightmaps


Tangent space


Parallax mapping Also know as offset mapping as it offsets the texture

coordinates Very cheap and fairly convincing on flat surfaces High quality versions require an iterative process, which is

more expensive, but produce extremely good results (where applicable)


Cel shading

Non photorealistic technique Can be done either by drawing multiple passes

or in a shader by examining the scalar product of the normal and view direction

Check the sample in the Ogre SDK


High Dynamic Range rendering Often done by cheating Can be done for real on Shader Model 3.0 hardware

Render to a target with higher precision than can be drawn

Then scale the values in a second pass to simulate different levels of exposure


PRT

Precomputed radiance transfer can be used to simulate soft shadows, interreflections and subsurface scattering

Uses spherical harmonics to encode and approximate the light transfer function

Decoded in a programmable shader Only works on low frequency effects

with a reasonable number ofparameters

Lights can move, but objects can notmove relative to each other

Check the DirectX SDK


Post processing

Generally done by first rendering the scene to a texture and then applying that texture to a fullscreen quad which is drawn with a pixel shader applied to it

Multiple postprocessing effects can be combined either in sequence or by blending between them


Bloom Can be done best when combined with HDR rendering In essence: apply gaussian blur on areas with intense light A cheap substitute for actual blur is to downsample the

image a few times and then combining the original image with the downsampled versions


Depth of Field DOF is extremely important in real world photography and film The modern approach (Thorsten Sheuermann, 2004)

Downsample and preblurthe image

Use variable size filterkernel to approximatecircle of confusion

Blend between originaland preblurred imagefor better image quality

Take measures to prevent“leaking” sharp foregroundinto blurry background


Motion blur The lack of motion blur is mostly why

2530fps in a game seems stutteringbut works with film

The simplest form of motion blur isdone by using the accumulationbuffer and combining previousframes with the current

Does not work well without veryhigh framerates

When used on meshes directly, a vertex shader stretches some vertices along the direction of motion and fades their transparency

Can be done in post processing by first drawing the scene to a velocity buffer which determines how much to blur the scene in the post processing pass


Materials in Ogre

material Material/SOLID/TEX/sandstone.jpg { technique { pass { diffuse 0.8 0.8 0.8 specular 0.5 0.5 0.5 12.5 texture_unit { texture sandstone.jpg } } }}


Post processing in Ogre

There are plenty of predefined post processing effects

Compositor allows for easy access to the effects

Use compositor scripts to add effects or combine multiple effects

Check the Ogre SDK for demos


Shadows in Ogre

Extremely easy mSceneMgr->setShadowTechnique(“techniquename”);

Stencil shadows SHADOWTYPE_STENCIL_MODULATIVE SHADOWTYPE_STENCIL_ADDITIVE

Shadow maps SHADOWTYPE_TEXTURE_MODULATIVE SHADOWTYPE_TEXTURE_ADDITIVE

There are also more advanced methods Check the example in the SDK


Next lecture

Monday, 24 November

Artificial Intelligence

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3D Graphics 2 - Chalmers · 2008-11-19 · 08-11-19 Simulation Engines 2008, Markus Larsson 2 Camera management Three basic kinds of cameras in games Firstperson Camera attached to