Texture mapping (đồ họa máy TÍNH SLIDE)

OpenGL Texture Mapping• Apply a 1D, 2D, or 3D image to geometric... Idea: Use Map Shape• Map shapes correspond to various projections – Planar, Cylindrical, Spherical • First, map square

Texture Mapping

Texture Mapping

• Important topic: nearly all objects textured

– Wood grain, faces, bricks and so on

– Adds visual detail to scenes

• Meant as a fun and practically useful lecture

Polygonal model With surface texture

Adding Visual Detail

• Basic idea: use images instead of more

polygons to represent fine scale color variation

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Option: Varieties of projections

[Paul Bourke] 5

Option: unfold the surface

[Piponi2000]

Option: make an atlas

[Sander2001]

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Option: it’s the artist’s problem

OPENGL TEXTURE MAPPING

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OpenGL Texture Mapping

• Apply a 1D, 2D, or 3D image to geometric

How to map object to texture?

To each vertex (x, y, z in object coordinates),

must associate 2D texture coordinates (s, t)

So texture fits “nicely” over object

s t

Idea: Use Map Shape

• Map shapes correspond to various projections

– Planar, Cylindrical, Spherical

• First, map (square) texture to basic map shape

• Then, map basic map shape to object

– Or vice versa: Object to map shape, map shape to

square

• Usually, this is straightforward

– Maps from square to cylinder, plane, sphere well

defined

– Maps from object to these are simply spherical,

cylindrical, cartesian coordinate systems

Planar mapping

• Like projections, drop z coord (s, t) = (x, y)

• Problems: what happens near z = 0?

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Cylindrical Mapping

• Cylinder: r, θ, z with (s, t) = (θ/(2π), z)

– Note seams when wrapping around (θ = 0 or 2π)

Spherical Mapping

• Convert to spherical coordinates: use latitude/long

– Singularities at north and south poles

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Cube Mapping

Cube Mapping

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Texture Mapping and the OpenGL Pipeline

• Images and geometry flow through separate

pipelines that join at the rasterizer

– "complex" textures do not affect geometric complexity

geometry pipeline vertices

pixel pipeline image

rasterizer

• Assign texture coordinates to vertices

• Specify texture parameters

– Wrapping, filtering

Applying Textures II

• Specify textures in texture objects

• Set texture filter

• Set texture function

• Set texture wrap mode

• Set optional perspective correction hint

• Bind texture object

• Enable texturing

• Supply texture coordinates for vertex

– Coordinates can also be generated

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Texture Objects

• Like display lists for texture images

– One image per texture object

– May be shared by several graphics contexts

• Generate texture names

glGenTextures(n, texIds[]);

– Returns n currently unused names for texture objects

in the array texIds

• Delete texture objects

glDeleteTextures(n, texIds[])

Texture Objects (cont.)

• Create texture objects with texture data and state

– When binding to a texId value of zero, OpenGL stops

using texture objects and returns to the default texture.

• Bind textures before using

glBindTexture(target, id);

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Specify Texture Image

• Define a texture image from an array of texels in CPU memory

glTexImage2D( target, level, components,

w, h, border, format, type,

texels[] );

– dimensions of image must be powers of 2

• Texel colors are processed by pixel pipeline

– pixel scales, biases and lookups can be done

Converting A Texture Image

• If dimensions of image are not power of 2

gluScaleImage(format, w_in, h_in,

type_in, data_in[], w_out, h_out,

type_out, data_out[]);

– *_in is for source image

– *_out is for destination image

• Image interpolated and filtered during scaling

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Specifying a Texture: Other Methods

• Use frame buffer as source of texture image

– uses current buffer as source image

Mapping a Texture

• Based on parametric texture coordinates

• glTexCoord*() specified at each vertex

Generating Texture Coordinates

• Automatically generate texture coords

Tutorial: Texture

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Texture Application Methods

– How to mix primitive’s color with texture’s color

• blend, modulate or replace texels

Perspective Correction Hint

• Texture coordinate and color interpolation

– either linearly in screen space

– or using depth/perspective values (slower)

• Noticeable for polygons “on edge”

Is There Room for a Texture?

• Query largest dimension of texture image

– typically largest square texture

– doesn’t consider internal format size

• glGetIntegerv(GL_MAX_TEXTURE_SIZE,

&size)

• Texture proxy

– will memory accommodate requested texture size?

– no image specified; placeholder

– if texture won’t fit, texture state variables set to 0

• doesn’t know about other textures

• only considers whether this one texture will fit all of memory

Texture Residency

• Working set of textures

– high-performance, usually hardware accelerated

– textures must be in texture objects

– a texture in the working set is resident

– for residency of current texture, check

GL_TEXTURE_RESIDENT state

• If too many textures, not all are resident

– can set priority to have some kicked out first

– establish 0.0 to 1.0 priorities for texture objects

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TEXTURE MAPPING APPLICATIONS

Texture Mapping Applications

• Modulation, light maps

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) (

( (

,

(s t I K I K N L K V R n S I K I K I T

Bump Mapping

• Texture = change in surface normal!

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Displacement Mapping

Illumination Maps

• Quake introduced illumination maps or light maps to

capture lighting effects in video games

Environment Maps

Images from Illumination and Reflection Maps:

Simulated Objects in Simulated and Real Environments

Gene Miller and C Robert Hoffman

• Compute on the fly,

e.g Perlin noise 

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Procedural Texture Gallery