Download - Game Programming (3D Pipeline Overview)
Game ProgrammingGame Programming(3D Pipeline Overview)(3D Pipeline Overview)
Game ProgrammingGame Programming(3D Pipeline Overview)(3D Pipeline Overview)
2014. Spring
3D Pipeline Overview
■ 3D Game Engine No matter how powerful target platform is, you will always
need more
■ Contents Fundamental Data Types
• Vertex, Color, Texture• Geometry Formats
Graphics Pipeline• Visibility determination
Clipping, Culling, Occlusion testing
• Resolution determination (LOD)• Transform, lighting• Rasterization
3D Rendering Pipeline (for direct illumination)
Transform into 3D world coordinate system
Illuminate according to lighting and reflection
Transform into 3D camera coordinate system
Transform into 2D camera coordinate system
Clip primitives outside camera’s view
Transform into image coordinate system
Draw pixels (includes texturing, hidden surface, …)
Coordinate Systems
X
Y
ZX
Y
Z
Left-handedCoordinate System
(typical computer graphicsReference system – Blitz3D, DarkBASIC)
Right-handed
Coordinate System
(conventional Cartesianreference system)
Transformations
■ Transformation occurs about the origin of the coordinate system’s axis
Translate Scale
Rotate
Order of Transformations Make a Difference
Translate along X 1;
Rotate about Z 45
Rotate about Z 45; Translate along X 1
Box centered atorigin
Order of Transformations
Hierarchy of Coordinate Systems
■ Also called: Scene graphs Called Skeletons in DarkBASIC
because it is usually used to represent people (arms, legs, body).
Local coordinate system
Transformations
Viewing Transformations
World X
Y
Z
The Camera
Parallel Projection
Perspective Projection
Lighting
■ Ambient basic, even illumination of all objects in a
scene ■ Directional
all light rays are in parallel in 1 direction - like the sun
■ Point all light rays emanate from a central point
in all directions – like a light bulb■ Spot
point light with a limited cone and a fall-off in intensity – like a flashlight
Cone anglePenumbra angle(light starts to drop off
to zero here)
Diffuse Reflection(Lambertian Lighting Model)
The greater the angle between the normal and the vector from the point to the light source, the less light is reflected. Most light is reflected when the angle is 0 degrees, none is reflected at 90 degrees.
Specular Reflection(Phong Lighting Model)
• Maximum specular reflectance occurs when the viewpoint is along the path of the perfectly reflected ray (when alpha is zero).
• Specular reflectance falls off quickly as alpha increases.
• Falloff approximated by cosn(alpha).• n varies from 1 to several
hundred, depending on the material being modelled.
• n=1 provides broad, gentle falloff• Higher values simulate sharp,
focused highlight.• For perfect reflector, n would be
infinite.
Fall off in Phong Shading
Small nLarge n
Approximating Curved Surfaces withFlat Polygons
Flat Shading – each polygon face has a normal that is used to perform lighting calculations.
Texture Maps Used in Tank Game
Fundamental Data Types
■ Vertices Store in XYZ coordinates in a sequential manner Most today’s graphics processing units (GPUs) will only use floats
as their internal format Simple approach
• Can be used for primitives with triangles that share vertices between their face
Ex) Triangle 0, 1st vertex Triangle 0, 2nd vertex Triangle 0, 3rd vertex Triangle 1, 1st vertex Triangle 1, 2nd vertex Triangle 1, 3rd vertex (…)Ex) a cube (8 vertices, 6 faces, 12 triangles) 12(triangles) x 3(vertices) x 3 (floats) x 4 (bytes) 432 bytes
DisadvantageRepeating the same vertices many times
Fundamental Data Types
Indexed Primitives• Two lists
Vertex list : Put the vertices in a list Indexed list
− Put the indices of the vertices for each face Ex) a cube: a cube (8 vertices, 12 triangles)vertex list: 8(verices) x 3(float) x 4(bytes) =96 bytesindexed list: 12(triangles) x 3(vertices index: unsigned integer) x 2
(bytes) = 72 bytes Total: 96 + 72 = 168 bytes (about 40%)
• Advantage Sending half the data is twice as fast All phases in the pipeline can work with this one
• Disadvantage Additional burden
− A vertex shared two faces that have different materials identifiers and texture coordinates
zz
zz
zzzz
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Fundamental Data Types
■ Quantization A lossy techniques
• Minimize the loss and achieve additional gain Downsampling
• Storing values in lower precision data types• Ex) coding floats into unsigned short or unsigned byte
Decompression (=reconstruction)• TC methods
Truncate and then centers Ex) truncate the floating point to an integer
to decode, decompressed by adding 0.5
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반올림 (Round)
Fundamental Data Types
■ Color RGB (or RGBA) color space
• Ex) floating-point black(0,0,0), white (1,1,1)• 24 bits modes
Color coded as bytes are internally supported by most APIs and GPUs
Some special case• Hue-Saturation-Brightness• Cyan-Magenta-Yellow• BGR colors (Targa texture format)• Luminance value
Fundamental Data Types
Alpha• Encode transparency (32 bit RGBA value)• The lower value, the less opacity
0: invisible (transparency) 255 (opaque)
• Disadv. Using alpha values embedded into texture maps
− Make the texture grow by one forth To save precious memory
− Using a regular RGB map and specifying alpha per vertex instead
Fundamental Data Types
■ Texture Mapping Increase the visual appeal of a scene by simulating the appearance
of different materials Two issues
• which texture map will use for each primitives
• How the texture will wrap around it which texture map will use
• Side effect A shared vertex
− A single vertex can have more than one texture
• Multitexturing (=multipass rendering) (chap. 17) Layer several textures in the same primitive to simulate a
combination of them
Fundamental Data Types
How textures are mapped onto triangles• (U, V) map that vertex to a virtual texture space• Usually stored as floats into the range(0, 1)• Special effects
Reflection map (on the fly)− Create the illusion of reflection by applying the texture
Geometry Formats■ Geometry Formats
How we will deliver the geometry stream to the graphics subsystem The geometry packing methods
• An optimal way Can achieve x2 performance
■ Geometry stream Five data types
• Vertices, normals, texture coordinates, colors• Indices to them to avoid repetition• Ex) 3 floats per vertex, 2 floats per texture, 3 floats per normal, 3
floats per color V3f T2f N3f C3f 132 bytes per triangle Ex) Pre-illuminated vertices (static lighting) V3f T2f N0f C3f 96 bytes per triangle Ex) bytes V3b T2b N0 C1b 18 bytes per triangle
A indexed mesh usually takes b/w 40 and 60 % of the space by the original data set
A Generic Graphics Pipeline
■ A Generic Graphics Pipeline Four stages
• Visibility determination Clipping, Culling, Occlusion testing
• Resolution determination LOD analysis
• Transform, lighting• Rasterization
Hardware Graphics Pipelines
CPUCPU GPUGPU
GPU Fundamentals:The Graphics Pipeline
■ A simplified graphics pipeline Note that pipe widths vary Many caches, FIFOs, and so on not shown
GPUCPU
ApplicationApplication TransformTransform RasterizerRasterizer ShadeShade VideoMemory
(Textures)
VideoMemory
(Textures)Xformed,Xformed,
LitLitVerticesVertices
(2D)(2D)
FragmentsFragments(pre-pixels)(pre-pixels)
FinalFinalpixelspixels
(Color, Depth)(Color, Depth)
Graphics StateGraphics State
Render-to-textureRender-to-texture
VerticesVertices(3D)(3D)
GPU Fundamentals:The Modern Graphics Pipeline
■ Programmable vertex processor!
■ Programmable pixel processor!
GPUCPU
ApplicationApplication VertexProcessor
VertexProcessor RasterizerRasterizer Pixel
ProcessorPixel
ProcessorVideo
Memory(Textures)
VideoMemory
(Textures)Xformed,Xformed,
LitLitVerticesVertices
(2D)(2D)
FragmentsFragments(pre-pixels)(pre-pixels)
FinalFinalpixelspixels
(Color, Depth)(Color, Depth)
Graphics StateGraphics State
Render-to-textureRender-to-texture
FragmentProcessorFragmentProcessor
VerticesVertices(3D)(3D)
VertexProcessor
VertexProcessor
GPU Pipeline: Transform
■ Vertex Processor (multiple operate in parallel) Transform from “world space” to “image space” Compute per-vertex lighting
GPU Pipeline: Rasterizer
■ Rasterizer Convert geometric rep. (vertex) to image rep. (fragment)
• Fragment = image fragment Pixel + associated data: color, depth, stencil, etc.
Interpolate per-vertex quantities across pixels
GPU Pipeline: Shade
■ Fragment Processors (multiple in parallel) Compute a color for each pixel Optionally read colors from textures (images)
Visibility culling(Clipping & Culling)
View frustum culling (=clipping)
Occlusion culling
Back-face culling
Clipping
■ Clipping The process of eliminating unseen geometry by testing it
against a clipping volume, such as screen• If the test fails, Discard that geometry
The clipping test must be faster than drawing the primitives The camera has horizontal aperture of 60 degrees (standard)
• 60/360: 17 % of geometry visible , 83% discard• Ex) FPS, driving simulators
Clipping methods• Triangle Clipping• Object Clipping
Bounding Sphere, Bounding Box
Clipping
■ Triangle Clipping Clipping each and every triangle prior to rasterizing it The triangle level test will not provide good results hardware clipping
• Great performance with no coding• But, not using the bus very efficiently
Sending whole triangles through the bus to the graphics card
− Clipping test in the graphics chip− Send lots of invisible triangles to the card
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels
Clipping
■ Object Clipping Work on an object level
• if a whole object is completely invisible discard• if a whole object is completely or partially within the
clipping volume H/W triangle clipping will be used Bounding Volume (Ex: spheres and boxes)
• Represent the boundary of the object • False positive
The BV will be visible but the object won’t
• Provide us with constant-cost clipping methods Ex) 1000-triangle object vs. 10000-trianle object
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels
Clipping
■ Bounding Sphere Defined by its center and radius
• Center (SCx, SCy, SCz), radius (SR) Given six clipping planes View volume
Ax + By + Cz + D = 0 (clipping plane)• A,B,C: plane normal• D: defined by A,B,C and a point in the
plane Clipping test
A * SCx + B * SCy + C * SCz + D < -SR• Return true if the sphere lies in the
hemispace opposite the plane normal
Clipping
Advantage• Inexpensive The test is the same as testing a point • Rotation invariance
Disadvantage• Tend not to fit the geometry well
Lots of false positive Ex) a pencil-shaped object
Clipping
■ Bounding Boxes Provide a tighter fit
• Less false positive will happen• More complex than with spheres• Don’t have rotational invariance
Boxes can either be axis aligned or generic• An axis-aligned bounding box (AABB)
Parallel to the X, Y, Z axes
• Defined by two points (from all the points) the minimum X, Y, Z value found in any of them the maximum X, Y, Z value found in any of them
Culling
■ Culling Eliminate geometry depending on its orientation Well-formed object
• The vertices of each triangle are defined in CCW order• Eliminate About one half of the processed triangles
House mesh Polygon soup
Culling
Boundary representation (B-rep)• A volume enclosed by the object• No openings or holes that allow us to look inside
Culling
Culling Test• 3D well-formed object
Back-facing normals Cull away− The faces whose normals are pointing away from the
viewpoint
• The simplest form of culling Culling take place inside the GPU
Object culling is by far less popular than object clipping• The benefit of culling: 50%, clipping: about 80%• If your geometry is prepacked in linear structures, you
don’t want to reorder it because of the culling
front
backeye
Culling
■ Object Culling Reject back-facing primitives
• Classify different triangles in an object according to their orientation (load time or preprocess)
• Create the cluster Partition of the normal space value
− Longitude and latitude− Use a cube as an intermediate
primitive
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels
Occlusion Testing
■ After Clipping and culling Some redundant triangles can still survive Camera-facing primitive overlap each other
• Painting them using Z-buffer to properly order them (overdraw)
• Occluder: the one closest to the viewer
■ Occlusion prevention policies Indoor rendering (chap. 13)
• Potentially Visible Set (PSV) culling• Portal rendering
Outdoor rendering (chap. 14)
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels
Occlusion Testing
■ Occlusion Testing Draw the geometry front to back
• Large occluders are painted first if its BV will alter the Z-buffer
• Paint the geometry if the BV will not affect the results
• Reject the object (fully behind other objects)
Resolution Determination
■ Examples: huge mountains and thousands of trees Clipping (aperture of 60) 1/6 of the total triangle Culling 1/12 (= 1/2 * 1/6) Occlusion about 1/15 Geometry
• Terrain 20km*20km square terrain patch with sampled every meter
400 million triangle map
• Trees Realistic tree 25,000 triangle One tree for every 20 meters 25 billion triangle per
frame ??
Level-of-detail rendering
■ use different levels of detail at different distances from the viewer
Level-of-detail rendering
■ not much visual difference, but a lot faster
use area of projection of BV to select appropriate LOD
Resolution Determination
■ Multiresolution Human visual system tends to focus on larger, closer object
(Especially if they are moving) Two components
• A resolution-selection (heuristic) The distance from the object to the viewer
− Far from perfect (The object is far away, but huge??) The area of the projected polygon
− Perceived size, not with distance
• Rendering algorithms that handles the desired resolution A discrete approach (memory intensive) (Noticeable popping)
− Simply select the best-suited model from a catalog of varying quality models
A continuous method (high CPU hit)− Derive a model with the right triangle count on the fly
Clipping, Culling, and occlusion tests determine what we are seeingResolution test determine how we see it
Transform and Lighting
■ Transform stage Perform geometric transformation to the incoming data
stream (rotation, scaling, translation, …) Handle the projection of the transformed vertices to screen-
coordinates (3D coord. 2D coord.)
■ Lighting stage Most current API and GPUs offer H/W lighting
• Only per-vertex Global illumination
• must be computed per pixel Light mapping (using multitexturing) (chap.17)
• Stores the light information in low-resolution textures Allow per-pixel quality at a reasonable cost
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels
Rasterization
■ Rasterization (perform fully in H/W) The process by which our geometry is converted to pixel
sequences on a monitor Immediate mode
• Sending primitives individually down the bus, one by one• Easiest to code• the worst performance (bus fragmentation)
glBegin (GL_TRIANGLES);glColor3f(1, 1, 1);glVertex3f(-1, 0, 0);glVertex3f(1, 0, 0);glVertex3f(0, 1, 0);glEnd();
Rasterization
Packed primitives• Vertex arrays(OpenGL), vertex buffers(DirectX)• Pack all the data in one or more arrays
Three separate array: vertex, color and texture information
• Use a single call to deliver the whole structure to the H/W Interleaved arrays
• Interleave the different types of informationEx) V[0].x V[0].y V[0].z C[0].r C[0].g C[0].b T[0].u T[0].v (…)
ApplicationStage
GeometryStage
RasterizationStage
3D Triangles 2D Triangles Pixels