Dr. Ayman Eldeib · 2020. 3. 21. · Dr. Ayman Eldeib Systems & Biomedical Engineering Department...

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SBE 306B - Computer Systems III

Texture Mapping

Dr. Ayman Eldeib

Systems & Biomedical Engineering Department

Spring 2020

Computer Graphics Texture Mapping

Texture Mapping

geometry

image

x

y

z screen

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Why Texture Mapping?

Scenes created with diffuse lighting look convincingly three-dimensional, but are flat, chalky, and “cartoonish”

Phong lighting lets us simulate materials like plastic and (to a lesser extent) metal, but scenes still seem very cartoonish and unreal

Big problem: polygons are too coarse-grained to usefully model fine surface detail

Solution: texture mapping


Adding surface detail helps keep CG images from looking so simple

Explicitly modeling this detail in geometry can be very expensive

Zebra stripes, wood grain, writing on a whiteboard

Texture mapping pastes images onto the surfaces in the scene, adding realistic fine detail without exploding the geometry

Cont.Why Texture Mapping?

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Texture Mapping and OpenGL Pipeline

Images and geometry flow through separate pipelines that join at the rasterizer

“complex” textures do not affect geometric complexity

vertices geometry pipeline

image pixel pipeline

Rasterizer(screen)

vertices

image


Texture Mapping

Map variations in 2D image to 3D object

3D object 2D image

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Adds Visual Detail

Texture MappingCont.


Spring 2011 Computer GraphicsTexture Mapping

Bump Mapping

Texture = change in surface normal!

Sphere w/ diffuse textureSwirly bump map

Sphere w/ diffuse texture

and swirly bump map

Texture MappingCont.

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Displacement Mapping

Cont.

Texture Mapping


In short: it is impractical to explicitly model fine surface detail with geometry

Solution: use images to capture the “texture” of surfaces

Texture maps can modulate many factors that affect the rendering of a surface such as:

Color or reflectance (diffuse, ambient, specular)

Transparency (smoke effects)

Cont.

Texture Mapping

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Texture Mapping Polygonal Models

To each vertex (x,y,z in object coordinates), must associate 2D texture coordinates (u,v)

So texture fits “nicely” over object


geometry

+ =

image texture map

Texture Mapping

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Texture Mapping Option:

Varieties of Projections


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

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Planar Mapping

Drop z coord (u,v) = (x,y)


Cylindrical Mapping

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Spherical Mapping


Cube Mapping

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Cube Mapping Cont.


When scan converting a polygon (triangle) must map

each pixel to texture image

In reality, each pixel in raster image does not map to a

pixel in texture image

Actually, each pixel maps to a region in texture - Scale

|PIX| < |TEX|

oEasy: interpolate (bilinear) between texel values

|PIX| > |TEX|

oHard: average the contribution from multiple texels

|PIX| ~ |TEX|

oStill need interpolation!

Interpolation of Texture Coordinates

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Interpolation

Geometrically correct interpolation

Nearest neighbor

Bilinear

Bicubic

Scale

interpolate

Interpolation of Texture Coordinates Cont.


Texture Mapping

Interpolation methods

Geometrically correct interpolation

o Must interpolate in world space not screen space

Nearest neighbor

o Take value of nearest pixel

Bilinear

o Linearly average four nearest pixels based on distance

Bicubic

o Weighted average of surrounding pixels

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Texture Mapping Fundamentals

A texture is typically a 2-D image

Image elements are called texels

A texture is typically a 2-D array of texels

Value stored at a texel affects surface appearance in

some way

The mapping of the texture to the surface determines

the correspondence, i.e., how the texture lies on the

surface

o Mapping a texture to a triangle is easy.

o Mapping a texture to an arbitrary 3-D shape is more

complicated.


Texture Mapping Rendering

Rendering uses the mapping:

Find the visible surface at a pixel

Find the point on that surface corresponding to that

pixel

Find the point in the texture corresponding to that

point on the surface

Use the parameters associated with that point on the

texture to shade the pixel

Using triangulated meshes reduces the problem to

mapping a portion of the image to each triangle

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We typically parameterize the texture as a function in (u, v)

For simplicity, normalize u & v

Associate each triangle with a texture

Give each vertex of the triangle a texture coordinate (u, v)

For other points on the triangle, interpolate texture

coordinate from the vertices

Much like interpolating color or depth

Cont.Texture Mapping Rendering


MIP-maps

For a texture of 2n x 2n pixels, compute n-1 textures,

each at ½ the resolution of previous

This multiresolution texture is called a MIP-map

Original Texture Lower Resolution Versions

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Keep textures pre-filtered at multiple resolutions

For each pixel, linearly interpolate between

two closest levels

Fast, easy for hardware

MIP-maps


User Generated Mappings

For complex 3-D objects, mapping textures is still something of an art…so we often let the user do it

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Applying Texture using OpenGL

specify texture

read or generate image

assign to texture

assign texture coordinates to vertices

specify texture parameters

wrapping, filtering

Three Steps


Specify Texture Image

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

dimensions of image must be powers of 2

glTexImage2D( target, level, components, w, h, border, format, type, *texels );

glTexCoord*() specified at each vertex

Texel colors are processed by pixel pipeline

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Texture Application Methods Filter Modes

minification or magnification

special mipmap minification filters

Wrap Modes

clamping or repeating

e.g. glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )

Texture Functions

how to mix primitive’s color with texture’s color

blend, modulate or replace texelsglTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param )GL_TEXTURE_ENV_MODE modes: GL_MODULATE, GL_BLEND, GL_REPLACE

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Dr. Ayman Eldeib · 2020. 3. 21. · Dr. Ayman Eldeib Systems & Biomedical Engineering Department...

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