Texture Mapping Concepts - Introduction: Computer Graphics | CS 470, Study notes of Computer Graphics

Download Texture Mapping Concepts - Introduction: Computer Graphics | CS 470 and more Study notes Computer Graphics in PDF only on Docsity!

Computer Graphics

CS 470

Computer Science and Electrical Engineering Dept. West Virginia University

October 15, 2008

Outline

1 Texture Mapping Concepts

2 Texture Mapping in OpenGL

Texture Mapping Concepts

Texture - Definition

Texture

An image whose pattern is suggestive of the structure or composition of a

material.

Examples of textures.

Texture Mapping Concepts

Texture Mapping

Texture Mapping

Coloring the fragments of a primitive using pixels from a texture image.

The pixels in a texture image are often referred to as ’texels’.

Until now, the primitives we rendered have been either

Flat shaded : solid color, or

Smooth shaded : colors interpolated from vertices.

To increase the level of detail, we would need to increase the geometric

complexity (number of triangles).

Texture Mapping Concepts

How To Perform Texture Mapping

2D → 3D mapping

Define a texture mapping function

from 2D image coordinates to 3D

vertex coordinates

Associate a ”texture coordinate”

with each vertex.

I Texture coordinate is a vertex

attribute, like color or normal

vector.

I Specified like other attributes.

Linearly interpolate texture

coordinates per fragment.

Shade fragments using the color

of the image at the interpolated

coordinates.

Texture Mapping Concepts

Issues in Texture Mapping

Texture sampling.

I Note : Interpolated texture coordinates may not be integers.

How to incorporate lighting.

Rendering artifacts.

I Aliasing.

Speed / Efficiency.

I Texture images should stay on the server-side (GPU).

Handling the boundary of the texture image.

I Handling texture coordinates out of bounds.

I Tiling or repeating the texture image over a primitive.

Texture Mapping in OpenGL

Overview

1 Create a texture object and specify the texture image.

I Load data from file or generate procedurally

I glBindTexture*()

I glTexImage*()

2 Indicate how the texture mapping is to be performed.

I glTexParameter*()

I glTexEnv*()

3 Enable texture mapping.

I glEnable(GL_TEXTURE_2D);

4 Draw the scene.

I Specify texture coordinates : glTexCoord*()

I Specify vertices : glVertex*()

See also Chapter 9 of the OpenGL Programming Guide (red book online).

Texture Mapping in OpenGL Texture Objects

Texture Objects

Encapsulates the texture image and some parameters.

Textures are stored on the graphics server when possible.

Client refers to texture objects by an unsigned int identifier.

(Similar to the way display lists used IDs)

The relevant OpenGL functions are:

glGenTextures

I Generates a range of unused texture IDs (similar to glGenLists.)

glBindTexture

I The first call with a particular ID creates the texture object.

I The named texture object becomes ’bound’.

glTexImage*

I Upload the image data in the specified format.

Texture Mapping in OpenGL Texture Objects

Texture Creation Details

void glTexImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, const GLvoid *pixels)

Uploads the texture image to the currently bound object.

target : GL_TEXTURE_1D, GL_TEXTURE_2D, GL_TEXTURE_3D

level : level-of-detail (mipmap level) - 0 is the base image level.

internalformat : texel format within texture object (server-side).

width, height : dimensions of the texture.

border : border width - either 0 or 1.

format : format of the image data (client-side).

type : type of the image data (client-side).

pixels : void pointer to the image data (client-side).

I Load from file.

I Generate in program.

Texture Mapping in OpenGL Texture Objects

Texture Level-Of-Detail : Mipmaps

void glTexImage2D(GLenum target, GLint level , GLint internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, const GLvoid *pixels)

Mipmaps are different resolution versions of the same texture image.

Mipmap level 0 is the largest, most detailed image

Increasing level by 1 usually decreases width and height by half.

Texture Mapping in OpenGL Texture Objects

Internal format

You will always specify, at least, the color components

GL_LUMINANCE : 1 color component per texel - grey scale

GL_LUMINANCE_ALPHA : 2 color components per texel

- grey scale with transparency

GL_RGB : 3 color components per texel

GL_RGBA : 4 color components per texel

You can also specify sizes

GL_LUMINANCE8 : 8 bit grey scale

GL_LUMINANCE8_ALPHA8 : 16 bits per texel

GL_RGB5_A1 : 16 bits per texel

GL_RGB8 : 24 bits per texel

GL_RGBA : 32 bits per texel

Texture Mapping in OpenGL Texture Objects

Format

void glTexImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height, GLint border, GLenum format , GLenum type, const GLvoid *pixels)

The format of the data in client-side system memory.

GL_LUMINANCE

GL_LUMINANCE_ALPHA

GL_RGB

GL_RGBA

Only color components are specified.

Texture Mapping in OpenGL Texture Objects

Example texture object creation

#define checkImageWidth 64 #define checkImageHeight 64 GLubyte checkImage[checkImageHeight][checkImageWidth][4]; GLuint texName;

void init_texture(void) { makeCheckImage(); //fill the checkImage array glGenTextures(1, &texName); glBindTexture(GL_TEXTURE_2D, texName); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, checkImageWidth, checkImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, checkImage);

}

Texture Mapping in OpenGL Texture Object Parameters

Texture Object Parameters

Texture magnification.

I Function used when the pixel being textured maps to an area less than or

equal to one texel.

Texture minification.

I Function used when pixel being textured maps to an area greater than one

texel.

Texture wrapping mode.

I How to handle texture coordinates out side the range [0, 1].

I Can handle the s and t coordinates differently