如果使用一般的纹理过滤，当观察方向跟模型表面不是相互垂直的的情况下，会出现纹理信息的丢失，表现为图像看上去比较模糊，如下图所示，远处场景的细节信息很差：
针对这种情况，可以采用同向异性过滤的方式处理纹理，在过滤纹理的时候，考虑到观察角度不同，使纹理本身沿着模型表面倾斜的方向进行延伸。
使用如下语句查询当前系统支持的最大同向异性过滤的数值，数值越大，表示沿着最大变化方向所采样的纹理单元越多，显示效果就越好：
GLfloat max_TexAni; //查询允许的各向异性数量
glGetFloatv(GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &max_TexAni);以“ GL_TEXTURE_MAX_ANISOTROPY_EXT ”为参数调用“ glTexParameterf ”函数，并设置同向异性过滤的数值，就可以使设置生效。以下是经过同向异性过滤设置的显示效果：
远处纹理的细节增强了很多。以下是工程代码：
#define WindowWidth 400
#define WindowHeight 400
#define WindowTitle "OpenGL Mip纹理贴图&&同向异性过滤"
#include <Windows.h>
#include "GL/glew.h"
#include <GL/freeglut.h>
#include <stdio.h>
#include <stdlib.h>
//定义纹理对象编号
GLuint texGround;
GLuint texWall;
GLuint texSky;
#define BMP_Header_Length 54 //图像数据在内存块中的偏移量
static GLfloat angle = 0.0f; //旋转角度
static GLfloat zPosition = 10;
// 函数power_of_two用于判断一个整数是不是2的整数次幂
int power_of_two(int n)
{
if (n <= 0)
return 0;
return (n & (n - 1)) == 0;
}
/* 函数load_texture
28.* 读取一个BMP文件作为纹理
29.* 如果失败，返回0，如果成功，返回纹理编号
30.*/
GLuint load_texture(const char* file_name)
{
GLint width, height, total_bytes;
GLubyte* pixels = 0;
GLuint last_texture_ID = 0, texture_ID = 0;
// 打开文件，如果失败，返回
FILE* pFile = fopen(file_name, "rb");
if (pFile == 0)
return 0;
// 读取文件中图象的宽度和高度
fseek(pFile, 0x0012, SEEK_SET);
fread(&width, 4, 1, pFile);
fread(&height, 4, 1, pFile);
fseek(pFile, BMP_Header_Length, SEEK_SET);
// 计算每行像素所占字节数，并根据此数据计算总像素字节数
{
GLint line_bytes = width * 3;
while (line_bytes % 4 != 0)
++line_bytes;
total_bytes = line_bytes * height;
}
// 根据总像素字节数分配内存
pixels = (GLubyte*)malloc(total_bytes);
if (pixels == 0)
{
fclose(pFile);
return 0;
}
// 读取像素数据
if (fread(pixels, total_bytes, 1, pFile) <= 0)
{
free(pixels);
fclose(pFile);
return 0;
}
// 对就旧版本的兼容，如果图象的宽度和高度不是的整数次方，则需要进行缩放
// 若图像宽高超过了OpenGL规定的最大值，也缩放
{
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; // 规定缩放后新的大小为边长的正方形
GLint new_line_bytes, new_total_bytes;
GLubyte* new_pixels = 0;
// 计算每行需要的字节数和总字节数
new_line_bytes = new_width * 3;
while (new_line_bytes % 4 != 0)
++new_line_bytes;
new_total_bytes = new_line_bytes * new_height;
// 分配内存
new_pixels = (GLubyte*)malloc(new_total_bytes);
if (new_pixels == 0)
{
free(pixels);
fclose(pFile);
return 0;
}
// 进行像素缩放
gluScaleImage(GL_RGB,
width, height, GL_UNSIGNED_BYTE, pixels,
new_width, new_height, GL_UNSIGNED_BYTE, new_pixels);
// 释放原来的像素数据，把pixels指向新的像素数据，并重新设置width和height
free(pixels);
pixels = new_pixels;
width = new_width;
height = new_height;
}
}
// 分配一个新的纹理编号
glGenTextures(1, &texture_ID);
if (texture_ID == 0)
{
free(pixels);
fclose(pFile);
return 0;
}
// 绑定新的纹理，载入纹理并设置纹理参数
// 在绑定前，先获得原来绑定的纹理编号，以便在最后进行恢复
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; //查询允许的各向异性数量
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); //恢复之前的纹理绑定
free(pixels);
return texture_ID;
}
void Display(void)
{
// 清除屏幕
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// 设置视角
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); //旋转
// 绘制左侧墙壁以及纹理
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();
//绘制右侧墙
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();
//绘制地板
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();
//绘制顶层
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); // 启用纹理
texGround = load_texture("ground.bmp"); //加载纹理
texWall = load_texture("wall.bmp");
texSky = load_texture("sky.bmp");
glutDisplayFunc(&Display); //回调函数
glutSpecialFunc(&SpecialKey);
glutMainLoop(); //循环调用
return 0;
}
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