TERMINOLOGY Rendering: the entire process of drawing an image
to the screen Vertex: a single 3D vector (x, y, z) Edge: a line
between two vertices Face: Most often 3 vertices and their edges
Triangle: why a triangle? Simple Can linearly interpolate on them
(later) Can construct other objects with them
Slide 3
THE GRAPHICS PIPELINE OVERVIEW Objects are made of primitives,
and primitives are made of vertices (i.e. geometry) Vertex
Processing (no triangles yet) Coordinate transformations into clip
space [-1, 1] Compute color for each vertex (not pixel) Clipping
and Primitive Assembly Clipping volume culls out geometry outside
the frustum, and clips geo that straddles Assemble sets of vertices
into lines and polygons Rasterization Determine which pixels are
inside each polygon (primitive) The output is a set of fragments
for each primitive Fragment Processing Fills in the pixels in the
frame buffer (what youre seeing right now!)
Slide 4
COORDINATE SYSTEMS We have a bunch of them Object space (or
local space) the space when the object is modeled World space the
game space where everything happens Camera space the view of the
world from the camera Screen space the actual screen We transform
the object from one space to the next Cameras have frustum defined
by Near plane objects closer than this cannot be seen Far plane
objects further cant be seen Aspect ratio Clip space is the area
inside of the frustum
Slide 5
BASIC PROBLEM We need to convert our 3D models and display them
on a 2D screen To do this, we use projections by defining a viewing
volume These flatten the 3D world There are two kinds: Orthographic
(aka parallel) All objects that have the same dimension are the
same size, regardless of distance Viewing volume is rectangular
Perspective Objects shrink with distance Viewing volume is shaped
like a pyramid
Slide 6
EXAMPLE (UPPER-RIGHT IS A PERSPECTIVE VIEW)
Slide 7
ORTHOGRAPHIC VIEW VOLUME (YOU CAN SEE THE PARALLEL NOW) Near
clipping plane Far clipping plane
Slide 8
PERSPECTIVE VIEW VOLUME Near clipping plane Far clipping
plane
Slide 9
IT ALL STARTS WITH BUFFERS Frame buffer The memory where the
visible screen is stored Theres only one Drawing directly to this
memory can be detected by the user Back buffer A secondary buffer
that you draw to Once everything is drawn, swap the buffer
(pointer) or copy it!
Slide 10
IT ALL STARTS WITH BUFFERS Depth buffer (aka z-buffer ) Objects
in the scene have a depth, and sorting is slow (aka The Painters
Algorithm) When drawing each object, only update the pixel if the
object is closer Stencil buffer A buffer that can reject on a
per-pixel basis Irregular shapes on the screen
TERMINOLOGY Wireframe rendering only the edges of the model
(old games)
Slide 14
TERMINOLOGY Hidden Surface Removal (HSR) occluded objects cant
be seen Backface culling - drawing only the triangles that are
facing the camera
Slide 15
BACKFACE CULLING
Slide 16
TERMINOLOGY Solid shading (this isnt a definition)
Slide 17
TERMINOLOGY Flat Shading simulate lighting
Slide 18
TERMINOLOGY A texture is an image (e.g.a jpg or procedurally
generated) Texture mapping using an image during the rasterization
process
Slide 19
TERMINOLOGY THE BLOCK PROGRAM Blending mixing colors by
rendering more than one thing in one spot The floor is rendered
semi-transparent (yes, there are two cubes)
Slide 20
SHADERS AND MATERIALS No more fixed-function pipeline When you
render a triangle, you run a vertex and pixel shader Shaders are
small programs used to process: A vertex translate, rotate, scale,
etc A pixel (or fragment) determines the final color of a pixel
using textures, light colors, normal of the plane, etc Common
languages: GLSL (OpenGL), Cg, HLSL (Microsoft) A material is the
combination of shaders, textures, normals, colors, reflectivity
that will determine the final color
Slide 21
GRAPHIC ORGANIZATION Rendering should be separate from game
logic Hardware differs on each machine A render object is something
that is renderable (duh) There is only one description of it
(saving geometry) A flock of birds is still just one bird Render
object instances Use a render object Save a state for each instance
on the screen (e.g. location, animation state) We typically cull
objects (i.e. prevent them from drawing) if: They are behind the
camera Outside the frustum
Slide 22
CHARACTERS A mesh is a collection of triangles Can be one mesh
Can be hierarchical (i.e. one object can have multiple meshes) A
skeleton describes how a mesh will be deformed for animation
Slide 23
LIMITING WHAT WE RENDER View frustum Near and far planes and
clipping outside the frustum Cull when possible (render volume
partitioning) Portals (how many portals are visible? Think of
windows and doors) Binary Space Partitions (BSPs) a tree structure
representing all of the game space Each node does not intersect
with any other node If a node isnt visible, neither is its child!
Quad/Oct Trees extensions of BSPs Potentially Visible Sets (PVSs)
each node has a list of other nodes
Slide 24
BSPS Wikipedia
Slide 25
QUADTREE PARTITIONING Collision detection Outdoor visibility
checking Algorithm (2D): X/Y : consider bits Left shift at each
level in tree When at leaf, stop
Slide 26
QUADTREE VISUALIZED
Slide 27
GRAPHIC PRIMITIVES Note: there are also point sprites for
particle systems
Slide 28
MESH AS TRIANGLE STRIP
Slide 29
VERTEX AND INDEX BUFFERS How many vertices in a cube? How many
faces in a cube? How many triangles in a cube? Usually twice as
many triangles as vertices Typically, vertices are stored in a
buffer and are numbered (0n-1) There is a separate list for faces
This list still has a type (triangle list, strip, etc) E.g. - a
triangle could be made from vertices 0, 4, 5 Called indices in an
index buffer
Slide 30
INDEXED STRIPS First strip is 5, 0, 6, 1, 7, 2, 8, 3, 9, 4.
Second is 10, 5, 11, 6, 12, 7, 13, 8, 14, 9 Also, once a vertex has
been processed, it may be cached (speedup)
Slide 31
TEXTURE MAPPING Applying an image to geometry 2D images
(rectangular) 3D images (volumetric such as a CAT scan) Cube
mapping Adds realism to the scene Vertices have additional
information: A 3D position Normals (for lighting) UV coordinates
(for textures)
UV COORDINATES (AKA TEXTURE COORDINATES) (0, 0)(1, 0) (1, 1)(0,
1) (1, 1)(0, 1) (0, 0)(1, 0) Note: there is a linear interpolation
of all the in-between UV coordinates!
WHAT HAPPENS NOW? (0, 0)(1, 0) (1, 1)(0, 1) (2, 2)(0, 2) (0,
0)(2, 0) Polygon to be textured
Slide 37
WHAT HAPPENS NOW? (0, 0)(1, 0) (1, 1)(0, 1) (2, 2)(0, 2) (0,
0)(2, 0) Polygon to be textured
Slide 38
WHAT ABOUT NOW? (0, 0)(1, 0) (1, 1)(0, 1)
Slide 39
APPLYING TEXTURES
Slide 40
ADDITIONAL TEXTURING Point sampling Bilinear filtering (when
magnifying) Averages 4 pixels based on distance Mipmapping Halving
the size of the texture Can cause pops when switching Trilinear
filtering Uses bilinear filtering Interpolates between mipmaps
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