ECSE-4750 Computer Graphics Fall 2004

Prof. Michael Woznywozny@cat.rpi.edu

TA. Abhishek Gattanigattani@rpi.edu

TA. Stephenmeists@rpi.edu

We have discussed

Polygons – square, cube, etc. Quadric surfaces – disk, cylinder, sphere

etc. Bezier Curves, Bezier Surfaces, B-splines,

Nurbs surfaces etc.

Based on these basic elements, we can build objects with very complex shape -> render.

Why we need textures?

Thousands of small polygons to give the appearance of a real object.

Expensive to compute, render and manage Object too smooth and regular to look real Texture mapping allows real images to be

applied to objects and is less expensive computationally

Texture Mapping to add realism

Steps in Texture Mapping

Create and specify a texture object. Specify texture application method Enable texture mapping mode Draw (map texture co-ordinates to object

points)!

Texture Dimensions

2D (common) 1D = 2D with Height =1

– e.g. texture vary only in one direction

3D = Layers of 2D textures– e.g. MRI/CT data

Ref: Rosalee Wolfe’s book

Texture Data

RGBA Depth Luminance Intensity

Texel: one texture element

Texture Application Modes (texture functions)

Replace Modulate/Scale Blend

Loading texture data

jpeg, png, tiff, gif ?? Idea: convert to BMP and load.Note: Handle all errors when you write I/O code.

#include <GL/glaux.h> //for reading bitmap image

Function prototype– AUX_RGBImageRec *LoadBMP(char

*Filename) // Loads A Bitmap Image– Read about AUX_RGBImageRec in more detail.

Loading texture data (contd..)

Create a file handle– FILE *File=NULL; // File Handle

Try to open the file– File=fopen(Filename,"r"); // Check To See If The File Exists

Close the file and load bitmapif (File) // Does The File Exist?

{ fclose(File); // Close The Handle

// Load The Bitmap And Return A Pointer

return auxDIBImageLoad(Filename);}

Using the texture data

Gluint texture[1]; //Storage for one texture Create Storage Space For The Texture

– AUX_RGBImageRec *TextureImage[1];

Load the texture– TextureImage[0]=LoadBMP("Data/NeHe.bmp")

Naming a texture object– glGenTextures(1, &texture[0]);– Non-zero unsigned integer may be used as a texture names

Using the texture data (contd..)

Bind the named texture to texture target– glBindTexture(GL_TEXTURE_2D, texture[0]);

Defined a 2-D Texture– glTexImage2D(GL_TEXTURE_2D, 0, 3,

TextureImage[0]->sizeX, TextureImage[0]->sizeY, 0, GL_RGB, GL_UNSIGNED_BYTE, TextureImage[0]->data);

glTexImage2D

void glTexImage2D( GLenum target, //The target texture. Must be GL_TEXTURE_2D.

GLint level,//The level-of-detail number.

GLint internalformat,//The number of color components in the texture

GLsizei width,//Must be 2n + 2(border) for some integer n. GLsizei height,//Must be 2m + 2(border) for some integer m. GLint border,//The width of the border. Must be either 0 or 1. GLenum format,//The format of the pixel data.

GLenum type, //GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP

const GLvoid *pixels //pointer to the image data in memory

);

Texture Parameters

void glTexParameterf(GLenum target, GLenum pname, GLfloat param );

– glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); // Linear Filtering

– glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); // Linear Filtering

– For GL_TEXTURE_WRAP_S, GL_TEXTURE_WRAP_T you can specify GL_CLAMP or GL_REPEAT.

Free Up Space

Image Dataif (TextureImage[0])// If Texture Exists

{if (TextureImage[0]->data)// If Texture Image Exists{

// Free The Texture Image Memoryfree(TextureImage[0]->data);

}free(TextureImage[0]);// Free The Image Structure

} Texture Data

– void glDeleteTextures( GLsizei n, const GLuint *textures);– Deletes named textures

Changes to Init

int InitGL(GLvoid) {

if (!LoadGLTextures())// Jump To Texture Loading Routine ( NEW ){

return FALSE;// If Texture Didn't Load Return FALSE ( NEW )}

glEnable(GL_TEXTURE_2D);// Enable Texture Mapping ( NEW )glShadeModel(GL_SMOOTH);// Enable Smooth ShadingglClearColor(0.0f, 0.0f, 0.0f, 0.5f);// Black BackgroundglClearDepth(1.0f);// Depth Buffer SetupglEnable(GL_DEPTH_TEST);// Enables Depth TestingglDepthFunc(GL_LEQUAL);// The Type Of Depth Testing To DoglHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);// Really Nice Perspective Calculationsreturn TRUE;// Initialization Went OK

} Mode prescedence: Gl_TEXTURE_ID < GL_TEXTURE_2D <

GL_TEXTURE_3D < GL_TEXTURE_CUBE_MAP.

Texture Compression

Compress While Loading– GL_COMPRESSED_* in internalformat of

glTexImage*D

Load a Compressed Texture Image– glCompressedTextImage*D– Compression format should be supported by

graphics card.

Texture Borders

Texture repetition has problems at borders esp. with linear filtering.

Each texel has eight neighbors except the borders. So where do edge pixels get their neighbors? From the texture border!

Simple approach: Copy the values of the adjacent texture into the border.

Texture rendering

Texture mapping is image resampling Some common problems

Mipmaps

Why do we need them? – Shimmering, flashing, scintillations when texture

is scaled to fit smaller objects in dynamic scenes.

Mipmaps: Pre-filtered texture maps at different resolutions

OpenGL figures out which map to use when No Free Lunch! Increased storage and

computation

Why we need Mipmaps?

Without With

Mipmaps

Texture averaging/interpolation

Nearest (Linear)– texel nearest to the pixel under consideration

Linear (Bilinear)– weighted average of the 4 nearest texels to the

pixel under consideration

Trade off between image quality and speed

Mipmaps (Contd..)

Specifying maps– Manually: glTextImage2D called each time for

each map with different parameters– Automatically: build using gluBuild*DMipmaps()

gluBuild2DMipmaps(GL_TEXTURE_2D,3,p->sizeX,p->sizeY,GL_RGB,GL_UNSIGNED_BYTE, p->data)