1 L Feb-18-04 SMD159, Buffers and Texture Mapping Overview - - PDF document

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Feb-18-04 SMD159, Buffers and Texture Mapping 1 L

INSTITUTIONEN FÖR SYSTEMTEKNIK

Buffers and Texture Mapping

David Carr Fundamentals of Computer Graphics Spring 2004

Based on Slides by E. Angel

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Overview

• Buffers
• Additional OpenGL buffers
• Reading and writing buffers
• Blending
• Texture mapping
• Mapping Methods

+ Texture mapping + Environmental Mapping + Bump Mapping

• Basic strategies

+ Forward versus backward mapping + Point sampling versus area averaging

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Buffers

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Buffer

Define a buffer by its spatial resolution (n x m) and its depth k, the number of bits/pixel pixel

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OpenGL Buffers

• Color buffers
• Front
• Back
• Auxiliary

+ Undisplayed + System dependent

• Overlay

+ Chroma key animation

• Depth
• Accumulation
• High resolution buffer
• Stencil
• Holds masks

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Writing in Buffers

• Conceptually, we can consider all of memory as a large two-

dimensional array of pixels

• We read and write rectangular block of pixels
• Bit block transfer (bitblt) operations
• The frame buffer is part of this memory

frame buffer (destination) writing into frame buffer source memory

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Writing Model

• Read destination pixel before writing source

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Writing Modes

• Source and destination bits are combined bitwise
• 16 possible functions (one per column in table)

replace OR XOR

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XOR mode

• Recall from Chapter 3 that we can use XOR by enabling logic
• perations and selecting the XOR write mode
• XOR is especially useful for swapping blocks of memory such as

menus that are stored off screen If S represents screen and M represents a menu the sequence S ¨ S ⊕ M M ¨ S ⊕ M ([S ⊕ M] ⊕ M) S ¨ S ⊕ M ([S ⊕ M] ⊕ S) swaps the S and M

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The Pixel Pipeline

• OpenGL has a separate pipeline for pixels
• Writing pixels involves

+ Moving pixels from processor memory to the frame buffer + Format conversions + Mapping, Lookups, Tests

• Reading pixels

+ Format conversion

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Raster Position

• OpenGL maintains a raster position as part of the state
• Set by gl.glRasterPos*()
• 2D or 3D position in various forms
• The raster position is a geometric entity
• Passes through geometric pipeline
• Eventually yields a 2D position in screen coordinates
• This position in the frame buffer is where the next raster

primitive is drawn

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Buffer Selection

• OpenGL can draw into or read from any of the color buffers (front,

back, auxiliary)

• Default to the back buffer
• Change with gl.glDrawBuffer and gl.glReadBuffer
• Note that format of the pixels in the frame buffer is different from that
• f processor memory and these two types of memory reside in

different places

• Need packing and unpacking
• Drawing and reading can be slow

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Bitmaps

• OpenGL treats 1-bit pixels (bitmaps) differently than

multi-bit pixels (pixelmaps)

• Bitmaps are masks which determine if the

corresponding pixel in the frame buffer is drawn with the present raster color

• 0 fi color unchanged
• 1 fi color changed based on writing mode
• Bitmaps are useful for raster text fonts

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Raster Color

• Same as drawing color set by gl.glColor*()
• Fixed by last call to gl.glRasterPos*()
• Geometry drawn in blue
• Ones in bitmap use a drawing color of red

gl.glColor3f(1.0, 0.0, 0.0); gl.glRasterPos3f(x, y, z); gl.glColor3f(0.0, 0.0, 1.0); gl.glBitmap(……. gl.glBegin(GL_LINES); gl.glVertex3f(…..)

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Drawing Bitmaps

gl.glBitmap(width, height, x0, y0, xi, yi, bitmap)

first raster position second raster position

• ffset from raster

position increments in raster position after bitmap drawn

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Pixel Maps

• OpenGL works with rectangular arrays of pixels called

pixel maps or images

• Pixels are in one byte (8 bit) chunks
• Luminance (gray scale) images 1 byte/pixel
• RGB 3 bytes/pixel
• Three functions
• Draw pixels: processor memory to frame buffer
• Read pixels: frame buffer to processor memory
• Copy pixels: frame buffer to frame buffer

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OpenGL Pixel Functions

gl.glReadPixels(x,y,width,height,format,type,myimage)

start pixel in frame buffer size type of image type of pixels pointer to processor memory

byte myimage[512][512][3]; gl.glReadPixels(0,0, 512, 512, GL.GL_RGB, GL.GL_UNSIGNED_BYTE, myimage); gl.glDrawPixels(width,height,format,type,myimage)

starts at raster position

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Image Formats

• We often work with images in a standard format (JPEG,

TIFF, GIF)

• How do we read/write such images with OpenGL?
• No support in OpenGL
• OpenGL knows nothing of image formats
• Some code available on Web
• Basic plan of attack
• Find (write) a class that reads/writes into a standard data

structure

• Write a translation to a pixel map
• Transfer pixel map to OpenGL

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Texture Mapping

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Application

Image

• Want to:
• Map realistic variation onto an
• bject
• Limit model complexity
• Solution directly map a digital

pattern to an object

• Example, mapping

reflections to Geri’s glasses Box texture map Centered on the lens

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The Limits of Geometric Modeling

• Graphics cards can render over 10 million polygons per

second

• But, that is insufficient for many phenomena
• Clouds
• Grass
• Terrain
• Skin

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Modeling an Orange

• Consider the problem of modeling an orange (the fruit)
• Start with an orange-colored sphere
• Too simple
• Replace sphere with a more complex shape
• Does not capture surface characteristics (small dimples)
• Takes too many polygons to model all the dimples

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Modeling an Orange (Alternate)

• Take a picture of a real orange, scan it, and “paste”
• nto simple geometric model
• This process is texture mapping
• Still might not be sufficient because resulting surface

will be smooth

• Need to change local shape
• Bump mapping

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Three Types of Mapping

• Texture Mapping
• Uses images to fill inside of polygons
• Environmental (reflection mapping)
• Uses a picture of the environment for texture maps
• Allows simulation of highly specular surfaces
• Bump mapping
• Emulates altering normal vectors during the rendering

process

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Texture Mapping

Geometric Model Texture Mapped

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Environment Mapping

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Bump Mapping

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Where Does Mapping Take Place?

• Mapping techniques are implemented at the end of the

rendering pipeline

• Very efficient because few polygons pass down the

geometric pipeline

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2D image 3D surface

Is It Simple?

• Although the idea is simple
• Map an image to a surface
• There are 3 or 4 coordinate systems involved

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Coordinate Systems

• Parametric coordinates
• May be used to model curved surfaces
• Texture coordinates
• Used to identify points in the image to be mapped
• World Coordinates
• Conceptually, where the mapping takes place
• Screen Coordinates
• Where the final image is really produced

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Texture Mapping

parametric coordinates texture coordinates world coordinates screen coordinates

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Mapping Functions

• Basic problem is how to find the maps
• Consider mapping from texture coordinates to a point a

surface

• Appear to need three functions

x = x(s,t) y = y(s,t) z = z(s,t)

• But we really want to go

the other way

s t (x,y,z)

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Backward Mapping

• We really want to go backwards
• Given a pixel, we want to know to which point on an object it

corresponds

• Given a point on an object, we want to know to which point in

the texture it corresponds + Need a map of the form

s = s(x,y,z) t = t(x,y,z)

• Such functions are difficult to find in general

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Two-Part Mapping

• One solution to the mapping problem is to first map the

texture to a simple intermediate surface

• Example: map to cylinder

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Cylindrical Mapping

• Parametric cylinder
• Maps rectangle in u,v space to cylinder of:
• Radius r and height h in world coordinates
• Maps from texture space

x = r cos 2p u y = r sin 2pu z = v/h s = u t = v

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Spherical Map

• We can similarly use a parametric sphere
• But, must decide where to put the distortion
• Spheres are use in environmental maps

x = r cos 2pu y = r sin 2pu cos 2pv z = r sin 2pu sin 2pv

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Box Mapping

• Easy to use with simple orthographic projection
• Also used in environmental maps

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intermediate actual

Second Mapping

• Map from intermediate object to actual object
• Normal vectors from intermediate to actual
• Normal vectors from actual to intermediate
• Vectors from center of intermediate

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point samples in u,v (or x,y,z) space point samples in texture space miss blue stripes

Aliasing

• Point sampling of the texture can lead to aliasing errors

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• A better but slower option is to use area averaging
• Note that pre image of pixel is curved

pixel pre image

Area Averaging

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Questions?