Meshes

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Meshes

• Dr. Scott Schaefer

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3D Surfaces
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3D Surfaces
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3D Surfaces
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3D Surfaces
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3D Surfaces
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3D Surfaces
Vertex Table
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3D Surfaces
Polygon Table
Vertex Table
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3D Surfaces
Polygon Table
Vertex Table
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3D File Formats

• Way too many!
• OBJ
• STL
• MA
• PLY
• MAX, 3DS
• BLEND
• DAW
• LWO
• X
• .

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OBJ File Format (Simplified)

• Text Format Easy to read!
• Line based file consisting of
• Vertices (starts with v)
• Faces (starts with f)
• Comments (starts with )

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OBJ File Format (Simplified)

• Vertex Definition
• v x y z vertex with coordinates (x,y,z)
• vt u v texture coordinates (u,v)
• vn nx ny nz normal (nx,ny,nz)

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OBJ File Format (Simplified)

• Face Definition
• f g1/t1/n1 g2/t2/n2
• Faces may be variable length!
• Indexing starts at 1 not 0!
• First index is guarantee to be geometry index
• ti, ni are optional

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OBJ File Format Example

• This is a comment
• v 0 0 0
• v 0 1 0
• v 1 0 0
• v 0 0 1
• f 1 2 3
• f 1 3 4
• f 1 4 2
• f 2 3 4

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OBJ File Format Example

• This is a comment
• v 0 0 0
• v 0 1 0
• v 1 0 0
• v 0 0 1
• vn -1 -1 -1
• vn 1 1 1
• vn -1 1 1
• vn 1 -1 1
• f 1//2 2//1 3//3
• f 1//2 3//3 4//4
• f 1//2 4//4 2//1
• f 2//1 3//3 4//4

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OBJ File Format Complications

• Vertices may appear after faces
• Faces may not be triangles (must triangulate)
• Faces may use negative indexing

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3D File Formats

• Vertices may appear after faces
• Faces may not be triangles (must triangulate)
• Faces may use negative indexing
• Not all OBJ files conform to the OBJ standard

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Drawing 3D Objects with OpenGL

• Immediate Mode
• Display Lists
• Vertex Arrays
• Vertex Buffer Objects

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Immediate Mode

• glBegin ( GL_TRIANGLES )
• glNormal3f ( nx1, ny1, nz1 )
• glVertex3f ( x1, y1, z1 )
• glNormal3f ( nx2, ny2, nz2 )
• glVertex3f ( x2, y2, z2 )
• glEnd ( )

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Immediate Mode

• Advantages
• Easy to send a few polygons to OpenGL
• Disadvantages
• Lots of data transferred on the stack
• Lots of function calls
• Vertices duplicated many times
• Really, really slow

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Display Lists

• Initialize display list
• firstList glGenLists ( numLists )
• glNewList ( firstList, GL_COMPILE_AND_EXECUTE )
• // draw model in immediate mode
• glEndList ( )
• Draw the display list
• glCallList ( firstList )

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Display Lists

• Relies on graphics driver for optimization
• Advantages
• Faster than immediate mode
• Disadvantages
• Still not great performance

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Vertex Arrays

• glEnableClientState ( GL_(VERTEX/TEXTURE_COORD/NOR
  MAL)_ARRAY )
• glVertexPointer ( numComp, GL_FLOAT, bytes to
  next vertex, pointer to geometry)
• glTexCoordPointer ( numComp, GL_FLOAT, bytes to
  next texture coord, pointer to texture coords)
• glNormalPointer ( GL_FLOAT, bytes to next normal,
  pointer to normals )
• glDrawElements ( GL_TRIANGLES, number of indices
  in array, GL_UNSIGNED_INT, pointer to index
  array)
• glDisableClientState ( GL_(VERTEX/TEXTURE_COORD/NO
  RMAL)_ARRAY )

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Vertex Arrays

• Advantages
• Only transfers vertices once and not an expected
  value of 6 times
• Much faster than even display lists
• Format similar to most mesh formats
• Disadvantages
• Still transfers vertices to GPU every frame
• Bandwidth to GPU very problematic

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Vertex Buffer Objects

• New Functions
• glGenBuffersARB
• glBindBufferARB
• glBufferDataARB
• Use glVertexPointer with NULL pointer as
  argument
• Use glDrawElements with NULL pointer as argument

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Vertex Buffer Objects

• Advantages
• Can store data directly in video memory or AGP
• Greatly reduces bandwidth problem to GPU
• Disadvantages
• Not supported by MS OpenGL headers
• Must load function pointers out of DLL using
  WGLGetProcAddress
• Dynamic geometry does get as much benefit

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Performance
Nvidia 7300 GT Nvidia 8800 GTX
Immediate Mode 15.5 15.5
Display Lists 28.3 22.0
Vertex Arrays 33.5 35.5
Vertex Buffer Objects 50.0 200.0
Frames per second displaying a 413,236 triangle
model. CPU was an Intel Core 2 6700.