2

Texture Mapping Until now, colors and shades determined the

vertices color Texture is the characteristic physical structure

given to an object by the size, shape, arrangement of its parts

By texture mapping we apply image data to a geometric primitive

3

Basic strategyThree steps to apply a texture1. Specify the texture

read or generate image assign to texture enable texturing

2. Specify texture parameters wrapping, filtering

3. Assign texture coordinates to vertices Proper mapping function is left to application

4

Texture Mapping

5

OpenGL pipline Images and geometry flow through separate

pipelines that join at the rasterizer- “complex” textures do not affect geometric

complexity

6

Specifying a Texture Image Define a texture image from an array of texels

(texture elements) in CPU memory Glubyte my_texels[512][512][3];

Enable texture mapping glEnable(GL_TEXTURE_2D) OpenGL supports 1-3 dimensional texture

maps

7

Create Texture Objects Define a handle to different available texture

states (image and params)

glGenTextures(1, &texture[texture_num])

First argument tells OpenGL how many texture objects we would create

Second argument is a pointer to the place where OpenGL will store the names (unsigned integers) of the texture objects Texture[ ] is of type GLuint

8

Binding to a texture object To specify the text object that we are

going to define its specifics

glBindTexture(GL_TEXTURE_2D, texture[texture_num])

9

Define image as textureglTexImage2D( target, level,

internalFormat, w, h, border, format, type, texels )

target: type of texture, e.g. GL_TEXTURE_2Dlevel: used for mipmapping (discussed later)internalFormat: elements per texelw, h: width and height of texture imageborder: used for smoothing (discussed later)format and type: describe texelstexels: pointer to texel array

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE,

my_texels);

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Converting a texture image

OpenGL requires texture dimensions to be powers of 2

If dimensions of image are not powers of 2

gluScaleImage(format, w_in, h_in,type_in, *data_in, w_out, h_out,type_out, *data_out); data_in is source image data_out is for destination image

Image interpolated and filtered during scaling

11

Mapping a texture Once a texture is uploaded to the graphics card,

its resolution has no importance Texture is mapped to “texture coordinates”

between 0.0 to 1.0 (s,t) glTexCoord*() specified at each vertex

s

t1 ,1

0 ,1

0 ,0 1 ,0

(s, t( = )0.2, 0.8)

(0.4 ,0.2)

(0.8 ,0.4)

A

B C

a

bc

Texture Space Object Space

12

Typical code

13

Texture parameters OpenGL a variety of parameter that

determine how texture is applied Wrapping parameters determine what happens

if s and t are outside the (0,1) range Filter modes allow us to use area averaging

instead of point samples Mipmapping allows us to use textures at

multiple resolutions Environment parameters determine how

texture mapping interacts with shading

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Wrapping modeglTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP )

glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )

15

Filter ModesModes determined by

glTexParameteri( target, type, mode )

glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST);

glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR);

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Magnification and MinificationA group of texels can cover a pixel (minification) orOne texel can cover a group of pixels (magnification)

Can use point sampling (nearest texel) or linear filtering( 2 x 2 filter) to obtain texture values

Texture PolygonMagnification Minification

PolygonTexture

17

NN interpolation

18

Bilinear interpolation

19

Mipmapped textures Texturing far/close objects Mipmapping allows for pre-filtered texture maps

of decreasing resolutions Lessens interpolation errors for smaller textured

objects Declare mipmap level during texture definitionglTexImage2D( GL_TEXTURE_*D, level, … ) GLU mipmap builder routines will build all the

textures from a given imagegluBuild*DMipmaps) … (

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Mipmapping

N*N

N/4*N/4

N/2*N/2

ΝΝΝΝ 23112

1612

412 ΝΝΝΝ 2

3112

1612

412

What about

memory ?

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Texture functions Controls how texture is applied

glTexEnv{fi}( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, param)

GL_TEXTURE_ENV_MODE modes GL_MODULATE: modulates with computed shade GL_BLEND: blends with an environmental color GL_REPLACE: use only texture color GL_DECAL: alpha value is considered

22

Typical Codevoid init(void){

glClearColor (0.0, 0.0, 0.0, 0.0);glShadeModel(GL_FLAT);

glEnable(GL_DEPTH_TEST);

makeCheckImages;()

glGenTextures(2, texName);glBindTexture(GL_TEXTURE_2D, texName[0]);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,GL_NEAREST);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,GL_NEAREST);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, checkImage);

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Typical Code – (cont’d)

glBindTexture(GL_TEXTURE_2D, texName[1]);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,GL_NEAREST);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,GL_NEAREST);

glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, otherImage);

glEnable(GL_TEXTURE_2D);

}

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Typical Code – (cont’d)void display(void){glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);glBindTexture(GL_TEXTURE_2D, texName[0]);glBegin(GL_QUADS);glTexCoord2f(0.0, 0.0); glVertex3f(-2.0, -1.0, 0.0);glTexCoord2f(0.0, 1.0); glVertex3f(-2.0, 1.0, 0.0);glTexCoord2f(1.0, 1.0); glVertex3f(0.0, 1.0, 0.0);glTexCoord2f(1.0, 0.0); glVertex3f(0.0, -1.0, 0.0);

glEnd;()glBindTexture(GL_TEXTURE_2D, texName[1]);glBegin(GL_QUADS);glTexCoord2f(0.0, 0.0); glVertex3f(1.0, -1.0, 0.0);glTexCoord2f(0.0, 1.0); glVertex3f(1.0, 1.0, 0.0);glTexCoord2f(1.0, 1.0); glVertex3f(2.41421, 1.0, -1.41421);glTexCoord2f(1.0, 0.0); glVertex3f(2.41421, -1.0, -1.41421);

glEnd;()glFlush;()

}

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OFF files

OFF files are standard format to represent 3D model Starts with number of points, then number of

polygons ( usually triangles ) List of points List of polygons – <# points of polygon>< index of

points> Easy to write OFF file reader and draw these

models.

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Assignment2