Simulation Engines TDA571|DIT030 3D Graphics - Part 2 Tommaso - - PowerPoint PPT Presentation

Simulation Engines TDA571|DIT030 3D Graphics - Part 2

Tommaso Piazza

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IDC | Interaction Design Collegium

Administrative stuff

• Today 3D Graphics presentation
• Meet me in front of Grace Hopper
• Have you started coding?
• Where are your repositories?

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IDC | Interaction Design Collegium

Camera management

• Three basic kinds of cameras in games
• First-person
• Camera attached to the player and inherits the exact

motion of the player

• Examples: Farcry, Doom, Quake, etc
• Scripted
• Camera moves along pre-defined 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

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Third-person 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

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Third-person 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

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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
• Per-pixel shading
• Real-time procedural textures
• Advanced texture mapping techniques

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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, shader-based methods based on local

reflections can be used

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What is a shader?

• Three different interpretations from Watt & Policarpo,

2003

• C-style 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 multi-

pass or multi-texture render of an object on fixed-pipeline GPUs

• New hardware functionality for controlling the rendering of

primitives on a per-pixel or per-vertex level on programmable-pipeline GPUs; these are called pixel shaders and vertex shaders, respectively

• The last one is the important one

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Shader types

• Vertex shader
• Called for every vertex in a 3D primitive (modifies color, lightning, position...)
• Allows for effects such as hardware skinning, perturbation of water surface, etc
• http://www.youtube.com/watch?v=QHXjhfxAns0
• 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
• http://www.youtube.com/watch?v=91gjSmfSIgw
• 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
• http://www.youtube.com/watch?v=IRzAxtBWtV8
• Unified Shader Model in DirectX 10 (Shader Model 4.0)

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Shader languages

• OpenGL Shading Language (GLSL)
• Part of the OpenGL specification since OpenGL 1.4
• High-level 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

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Reflective surfaces: Environment maps

• Commonly used for reflections
• Precomputed textures
• Standard environment maps
• Single texture representing the

scene in one texture as if reflected from a steel ball

• Can also be used for advanced lighting

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Reflective surfaces: Cube maps

• Cubic environment maps
• Cube map consisting of six

textures unfolded on to a cube

• The most used format on

modern hardware

• The texture coordinate is a

vector that specifies which way to look from the center

• f the cube mapped cube to

get the desired texel

http://en.wikipedia.org/wiki/File:Panorama_cube_map.png 12

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Particle systems

• One of the most useful tools available
• Smoke, water, fire, etc
• http://www.youtube.com/watch?v=nmd6hIjgexs
• Each individual particle has a small or zero

geometrical extent, but together form cloud- like objects

• Each particle system can

contain thousands or even tens of thousands of particles

• Be extremely cautious of

multiple render states

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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

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Water

• Ideally based on fluid simulations
• Way to costly for real-time use
• Mostly represented by a simple

plane/quad

• In more advanced scenarios

represented 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

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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

• r particles
• Nowadays, often actual models are

used which is moved by the physics engine

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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

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Stencil shadows (shadows as volumes)

• Used with dynamic shadows
• Uses the stencil buffer
• Advantages
• Creates crisp shadows without

aliasing 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

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Stencil shadows

• Empty the stencil buffer
• Draw the whole scene with ambient lighting
• The z-buffer is filled and the color buffer is filled with the color of

surfaces in shadow

• Turn off updates to the z-buffer and color buffer and

draw the front-facing polygons of the shadow volumes

• Increments the stencil buffer; all pixels in or behind shadow volumes

receive a positive value

• Repeat for back-facing 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

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Shadow maps

• Texture based shadows
• Renders the scene from the light's point
• f 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 check code, 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 textures per light

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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

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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

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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 the vertex shader

• Possible for real on DirectX 10

hardware

• Geometry shaders allow tessellation of
• bjects without CPU intervention

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Normal maps

• High-poly 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
• f objects lit with dynamic

lights

• Often, tools like Zbrush

are used to create high-poly objects, but normal maps can also be created from heightmaps

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Parallax mapping

• Also know as offset mapping

as it offsets the texture coordinates

• Very cheap and fairly

convincing on flat surfaces

• High quality versions requires

iterative process, which is more expensive, but produce extremely good results (where applicable)

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Cel shading

• Non photo-realistic technique
• Can be done either by drawing multiple passes
• r in a shader by examining the scalar product
• f the normal and view direction
• Check the sample in the Ogre SDK

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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

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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 of parameters

• Lights can move, but objects can not

move relative to each other

• Check the DirectX SDK

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Post processing

• Generally done by first rendering the scene

to a texture and then applying that texture to a full-screen quad which is drawn with a pixel shader applied to it

• Multiple post-processing effects can be

combined either in sequence or by blending between them

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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

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Depth of Field

• DOF is extremely important in real world photography

and film

• The modern approach (Thorsten Sheuermann, 2004)
• Downsample and pre-blur

the image

• Use variable size filter

kernel to approximate circle of confusion

• Blend between original

and pre-blurred image for better image quality

• Take measures to prevent

“leaking” sharp foreground into blurry background

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Motion blur

• The lack of motion blur is mostly why

25-30fps in a game seems stuttering but works with film

• The simplest form of motion blur is

done by using the accumulation buffer and combining previous frames with the current

• Does not work well without very

high 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

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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 } } } }

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IDC | Interaction Design Collegium

Post processing in Ogre

• There are plenty of pre-defined 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

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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

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All together

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