如果使用一般的紋理過濾，當觀察方向跟模型表面不是相互垂直的的情況下，會出現(xiàn)紋理信息的丟失，表現(xiàn)為圖像看上去比較模糊，如下圖所示，遠處場景的細節(jié)信息很差：

針對這種情況，可以采用同向異性過濾的方式處理紋理，在過濾紋理的時候，考慮到觀察角度不同，使紋理本身沿著模型表面傾斜的方向進行延伸。

使用如下語句查詢當前系統(tǒng)支持的最大同向異性過濾的數(shù)值，數(shù)值越大，表示沿著最大變化方向所采樣的紋理單元越多，顯示效果就越好：

GLfloat max_TexAni;    //查詢允許的各向異性數(shù)量
	glGetFloatv(GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &max_TexAni);

以“ GL_TEXTURE_MAX_ANISOTROPY_EXT ”為參數(shù)調(diào)用“glTexParameterf ”函數(shù)，并設置同向異性過濾的數(shù)值，就可以使設置生效。以下是經(jīng)過同向異性過濾設置的顯示效果：

遠處紋理的細節(jié)增強了很多。以下是工程代碼：

#define WindowWidth  400    
#define WindowHeight 400
#define WindowTitle  "OpenGL Mip紋理貼圖&&同向異性過濾"

#include <Windows.h>
#include "GL/glew.h"
#include <GL/freeglut.h>
#include <stdio.h>
#include <stdlib.h>

//定義紋理對象編號    
GLuint texGround;
GLuint texWall;
GLuint texSky;

#define BMP_Header_Length 54  //圖像數(shù)據(jù)在內(nèi)存塊中的偏移量    
static GLfloat angle = 0.0f;   //旋轉角度    
static GLfloat zPosition = 10;

// 函數(shù)power_of_two用于判斷一個整數(shù)是不是2的整數(shù)次冪    
int power_of_two(int n)
{
	if (n <= 0)
		return 0;
	return (n & (n - 1)) == 0;
}

/* 函數(shù)load_texture
   28.* 讀取一個BMP文件作為紋理
   29.* 如果失敗，返回0，如果成功，返回紋理編號
   30.*/
GLuint load_texture(const char* file_name)
{
	GLint width, height, total_bytes;
	GLubyte* pixels = 0;
	GLuint last_texture_ID = 0, texture_ID = 0;

	// 打開文件，如果失敗，返回    
	FILE* pFile = fopen(file_name, "rb");
	if (pFile == 0)
		return 0;

	// 讀取文件中圖象的寬度和高度    
	fseek(pFile, 0x0012, SEEK_SET);
	fread(&width, 4, 1, pFile);
	fread(&height, 4, 1, pFile);
	fseek(pFile, BMP_Header_Length, SEEK_SET);

	// 計算每行像素所占字節(jié)數(shù)，并根據(jù)此數(shù)據(jù)計算總像素字節(jié)數(shù)    
	{
		GLint line_bytes = width * 3;
		while (line_bytes % 4 != 0)
			++line_bytes;
		total_bytes = line_bytes * height;
	}

	// 根據(jù)總像素字節(jié)數(shù)分配內(nèi)存    
	pixels = (GLubyte*)malloc(total_bytes);
	if (pixels == 0)
	{
		fclose(pFile);
		return 0;
	}

	// 讀取像素數(shù)據(jù)    
	if (fread(pixels, total_bytes, 1, pFile) <= 0)
	{
		free(pixels);
		fclose(pFile);
		return 0;
	}

	// 對就舊版本的兼容，如果圖象的寬度和高度不是的整數(shù)次方，則需要進行縮放    
	// 若圖像寬高超過了OpenGL規(guī)定的最大值，也縮放    
	{
		GLint max;
		glGetIntegerv(GL_MAX_TEXTURE_SIZE, &max);
		if (!power_of_two(width)
			|| !power_of_two(height)
			|| width > max
			|| height > max)
		{
			const GLint new_width = 256;
			const GLint new_height = 256; // 規(guī)定縮放后新的大小為邊長的正方形    
			GLint new_line_bytes, new_total_bytes;
			GLubyte* new_pixels = 0;

			// 計算每行需要的字節(jié)數(shù)和總字節(jié)數(shù)    
			new_line_bytes = new_width * 3;
			while (new_line_bytes % 4 != 0)
				++new_line_bytes;
			new_total_bytes = new_line_bytes * new_height;

			// 分配內(nèi)存    
			new_pixels = (GLubyte*)malloc(new_total_bytes);
			if (new_pixels == 0)
			{
				free(pixels);
				fclose(pFile);
				return 0;
			}

			// 進行像素縮放    
			gluScaleImage(GL_RGB,
				width, height, GL_UNSIGNED_BYTE, pixels,
				new_width, new_height, GL_UNSIGNED_BYTE, new_pixels);

			// 釋放原來的像素數(shù)據(jù)，把pixels指向新的像素數(shù)據(jù)，并重新設置width和height    
			free(pixels);
			pixels = new_pixels;
			width = new_width;
			height = new_height;
		}
	}

	// 分配一個新的紋理編號    
	glGenTextures(1, &texture_ID);
	if (texture_ID == 0)
	{
		free(pixels);
		fclose(pFile);
		return 0;
	}

	// 綁定新的紋理，載入紋理并設置紋理參數(shù)    
	// 在綁定前，先獲得原來綁定的紋理編號，以便在最后進行恢復    
	GLint lastTextureID = last_texture_ID;
	glGetIntegerv(GL_TEXTURE_BINDING_2D, &lastTextureID);
	glBindTexture(GL_TEXTURE_2D, texture_ID);
	/*glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	130.    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); */

	GLfloat max_TexAni;    //查詢允許的各向異性數(shù)量
	glGetFloatv(GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &max_TexAni);

	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAX_ANISOTROPY_EXT, max_TexAni);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR_MIPMAP_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
	glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

	/* glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0,
	 140.    GL_BGR_EXT, GL_UNSIGNED_BYTE, pixels); */

	gluBuild2DMipmaps(GL_TEXTURE_2D, 3, width, height, GL_BGR_EXT, GL_UNSIGNED_BYTE, pixels);

	glBindTexture(GL_TEXTURE_2D, lastTextureID);  //恢復之前的紋理綁定    
	free(pixels);
	return texture_ID;
}


void Display(void)
{
	// 清除屏幕    
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	// 設置視角    
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(65, 1, 1, 100);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	gluLookAt(0, 0, zPosition, 0, 0, 0, 0, 1, 0);
	glRotatef(angle, 0.0f, 1.0f, 0.0f); //旋轉    

	 // 繪制左側墻壁以及紋理    
	glBindTexture(GL_TEXTURE_2D, texWall);
	glBegin(GL_QUADS);
	glTexCoord2f(0.0f, 0.0f); glVertex3f(-5.0f, -5.0f, 100.0f);
	glTexCoord2f(30.0f, 0.0f); glVertex3f(-5.0f, -5.0f, -100.0f);
	glTexCoord2f(30.0f, 2.0f); glVertex3f(-5.0f, 5.0f, -100.0f);
	glTexCoord2f(0.0f, 2.0f); glVertex3f(-5.0f, 5.0f, 100.0f);
	glEnd();

	//繪制右側墻  
	glBegin(GL_QUADS);
	glTexCoord2f(0.0f, 0.0f); glVertex3f(5.0f, -5.0f, 100.0f);
	glTexCoord2f(30.0f, 0.0f); glVertex3f(5.0f, -5.0f, -100.0f);
	glTexCoord2f(30.0f, 2.0f); glVertex3f(5.0f, 5.0f, -100.0f);
	glTexCoord2f(0.0f, 2.0f); glVertex3f(5.0f, 5.0f, 100.0f);
	glEnd();

	//繪制地板  
	glBindTexture(GL_TEXTURE_2D, texGround);
	glBegin(GL_QUADS);
	glTexCoord2f(0.0f, 0.0f); glVertex3f(-5.0f, -5.0f, 100.0f);
	glTexCoord2f(0.0f, 1.0f); glVertex3f(5.0f, -5.0f, 100.0f);
	glTexCoord2f(25.0f, 1.0f); glVertex3f(5.0f, -5.0f, -100.0f);
	glTexCoord2f(25.0f, 0.0f); glVertex3f(-5.0f, -5.0f, -100.0f);
	glEnd();

	//繪制頂層  
	glBindTexture(GL_TEXTURE_2D, texSky);
	glBegin(GL_QUADS);
	glTexCoord2f(0.0f, 0.0f); glVertex3f(-5.0f, 5.0f, 100.0f);
	glTexCoord2f(0.0f, 3.0f); glVertex3f(5.0f, 5.0f, 100.0f);
	glTexCoord2f(35.0f, 3.0f); glVertex3f(5.0f, 5.0f, -100.0f);
	glTexCoord2f(35.0f, 0.0f); glVertex3f(-5.0f, 5.0f, -100.0f);
	glEnd();
	glutSwapBuffers();
}

void SpecialKey(GLint key, GLint x, GLint y)
{
	if (key == GLUT_KEY_UP)
	{
		zPosition += 1.0f;
	}
	if (key == GLUT_KEY_DOWN)
	{
		zPosition -= 1.0f;
	}
	if (key == GLUT_KEY_LEFT)
	{
		angle += 0.5f;
	}
	if (key == GLUT_KEY_RIGHT)
	{
		angle -= 0.5f;
	}
	glutPostRedisplay();
}

int main(int argc, char* argv[])
{
	// GLUT初始化    
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
	glutInitWindowPosition(100, 100);
	glutInitWindowSize(WindowWidth, WindowHeight);
	glutCreateWindow(WindowTitle);
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_TEXTURE_2D);    // 啟用紋理    
	texGround = load_texture("ground.bmp");  //加載紋理    
	texWall = load_texture("wall.bmp");
	texSky = load_texture("sky.bmp");
	glutDisplayFunc(&Display);   //回調(diào)函數(shù)    
	glutSpecialFunc(&SpecialKey);
	glutMainLoop(); //循環(huán)調(diào)用    
	return 0;
}

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