OpenGL（二十三） 各向异性纹理过滤

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;
}