• 理論基礎
  多級紋理(Mipmap)：就是解析度遞減的同一紋理，根據距離觀察點的距離選著最適合的解析度紋理。
  它主要解決兩方面問題：一，閃爍，當螢幕上被渲染物體的表面與它所應用的紋理影象相比顯得非常小時，就會出現閃爍。尤其當相機和物體在移動的時候，這種負面效果更容易被看到。二，效能問題，載入了大量的紋理資料之後，還要對其進行過濾處理（縮小），在螢幕上顯示的只是一小部分。紋理越大，所造成的效能影響就越大。但同時它也會增加額外的記憶體，大約比原先多出1/3記憶體空間。

注：多級紋理使用步驟：1，使用glGenerateMipmap生成一組mipmap 2，設定好相應的過濾方式。

• 程式碼示例

#include <GLTools.h>
#include <GLShaderManager.h>
#include <GLFrustum.h>
#include <GLBatch.h>
#include <GLFrame.h>
#include <GLMatrixStack.h>
#include <GLGeometryTransform.h>

#ifdef __APPLE__
#include <glut/glut.h>
#else
#define FREEGLUT_STATIC
#include <GL/glut.h>
 

#endif

/*註釋：如果gltReadTGABits讀取失敗，可以自己定義這個介面，具體的定義前面章節有*/

GLShaderManager     shaderManager;          // Shader Manager
GLMatrixStack       modelViewMatrix;        // Modelview Matrix
GLMatrixStack       projectionMatrix;       // Projection Matrix
GLFrustum           viewFrustum;            // View Frustum
 

GLGeometryTransform transformPipeline;      // Geometry Transform Pipeline

GLBatch             floorBatch;
GLBatch             ceilingBatch;
GLBatch             leftWallBatch;
GLBatch             rightWallBatch;

GLfloat             viewZ = -65.0f;

// Texture objects
#define TEXTURE_BRICK   0
#define TEXTURE_FLOOR   1
#define TEXTURE_CEILING 2
#define TEXTURE_COUNT   3
GLuint  textures[TEXTURE_COUNT];
const char *szTextureFiles[TEXTURE_COUNT] = {
    "/Users/app05/Desktop/opengl/brick.tga",
    "/Users/app05/Desktop/opengl/floor.tga",
    "/Users/app05/Desktop/opengl/ceiling.tga" };

// 設定紋理過濾方式
void ProcessMenu(int value)
{
    GLfloat fLargest;
    GLint iLoop;

    for(iLoop = 0; iLoop < TEXTURE_COUNT; iLoop++)
    {
        glBindTexture(GL_TEXTURE_2D, textures[iLoop]);

        switch(value)
        {
            //單紋理，遠近調節時有明顯閃爍不自然情況
            case 0:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
                break;

            case 1:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
                break;
             //設定多級紋理的過濾方式(遠模糊近清晰，有效避免閃爍情況)
            case 2:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_NEAREST);
                break;

            case 3:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_LINEAR);
                break;

            case 4:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_NEAREST);
                break;

            case 5:
                glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
                break;
             //開啟各向異性過濾(過濾時還考慮了觀察的角度，效果最好方式，但也是最耗效能的)
            case 6:
                glGetFloatv(GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &fLargest);
                glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAX_ANISOTROPY_EXT, fLargest);
                break;
            //恢復為各向同性過濾(過濾時不考慮觀察角度，預設垂直方向取樣，如果是傾斜觀察，這種模式下會丟失一些紋理資訊)
            case 7:
                glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAX_ANISOTROPY_EXT, 1.0f);
                break;
        }
    }

    glutPostRedisplay();
}


void init()
{
    GLbyte *pBytes;
    GLint iWidth, iHeight, iComponents;
    GLenum eFormat;
    GLint iLoop;

    glClearColor(0.0f, 0.0f, 0.0f,1.0f);

    shaderManager.InitializeStockShaders();

    glGenTextures(TEXTURE_COUNT, textures);
    for(iLoop = 0; iLoop < TEXTURE_COUNT; iLoop++)
    {
        glBindTexture(GL_TEXTURE_2D, textures[iLoop]);

        pBytes = gltReadTGABits(szTextureFiles[iLoop],&iWidth, &iHeight,
                                &iComponents, &eFormat);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

        glTexImage2D(GL_TEXTURE_2D, 0, iComponents, iWidth, iHeight, 0, eFormat, GL_UNSIGNED_BYTE, pBytes);

        glGenerateMipmap(GL_TEXTURE_2D);//為紋理生成一組完整的mipmap，即多級紋理(解析度遞減的同一紋理)

        free(pBytes);
    }

    //繪製地板幾何圖形
    GLfloat z;
    floorBatch.Begin(GL_TRIANGLE_STRIP, 28, 1);
    for(z = 60.0f; z >= 0.0f; z -=10.0f)
    {
        floorBatch.MultiTexCoord2f(0, 0.0f, 0.0f);
        floorBatch.Vertex3f(-10.0f, -10.0f, z);

        floorBatch.MultiTexCoord2f(0, 1.0f, 0.0f);
        floorBatch.Vertex3f(10.0f, -10.0f, z);

        floorBatch.MultiTexCoord2f(0, 0.0f, 1.0f);
        floorBatch.Vertex3f(-10.0f, -10.0f, z - 10.0f);

        floorBatch.MultiTexCoord2f(0, 1.0f, 1.0f);
        floorBatch.Vertex3f(10.0f, -10.0f, z - 10.0f);
    }
    floorBatch.End();
    //繪製天花板幾何圖形
    ceilingBatch.Begin(GL_TRIANGLE_STRIP, 28, 1);
    for(z = 60.0f; z >= 0.0f; z -=10.0f)
    {
        ceilingBatch.MultiTexCoord2f(0, 0.0f, 1.0f);
        ceilingBatch.Vertex3f(-10.0f, 10.0f, z - 10.0f);

        ceilingBatch.MultiTexCoord2f(0, 1.0f, 1.0f);
        ceilingBatch.Vertex3f(10.0f, 10.0f, z - 10.0f);

        ceilingBatch.MultiTexCoord2f(0, 0.0f, 0.0f);
        ceilingBatch.Vertex3f(-10.0f, 10.0f, z);

        ceilingBatch.MultiTexCoord2f(0, 1.0f, 0.0f);
        ceilingBatch.Vertex3f(10.0f, 10.0f, z);
    }
    ceilingBatch.End();
    //繪製左面牆幾何圖形
    leftWallBatch.Begin(GL_TRIANGLE_STRIP, 28, 1);
    for(z = 60.0f; z >= 0.0f; z -=10.0f)
    {
        leftWallBatch.MultiTexCoord2f(0, 0.0f, 0.0f);
        leftWallBatch.Vertex3f(-10.0f, -10.0f, z);

        leftWallBatch.MultiTexCoord2f(0, 0.0f, 1.0f);
        leftWallBatch.Vertex3f(-10.0f, 10.0f, z);

        leftWallBatch.MultiTexCoord2f(0, 1.0f, 0.0f);
        leftWallBatch.Vertex3f(-10.0f, -10.0f, z - 10.0f);

        leftWallBatch.MultiTexCoord2f(0, 1.0f, 1.0f);
        leftWallBatch.Vertex3f(-10.0f, 10.0f, z - 10.0f);
    }
    leftWallBatch.End();

    //繪製右面牆幾何圖形
    rightWallBatch.Begin(GL_TRIANGLE_STRIP, 28, 1);
    for(z = 60.0f; z >= 0.0f; z -=10.0f)
    {
        rightWallBatch.MultiTexCoord2f(0, 0.0f, 0.0f);
        rightWallBatch.Vertex3f(10.0f, -10.0f, z);

        rightWallBatch.MultiTexCoord2f(0, 0.0f, 1.0f);
        rightWallBatch.Vertex3f(10.0f, 10.0f, z);

        rightWallBatch.MultiTexCoord2f(0, 1.0f, 0.0f);
        rightWallBatch.Vertex3f(10.0f, -10.0f, z - 10.0f);

        rightWallBatch.MultiTexCoord2f(0, 1.0f, 1.0f);
        rightWallBatch.Vertex3f(10.0f, 10.0f, z - 10.0f);
    }
    rightWallBatch.End();
}


//退出，刪除紋理物件
void ShutdownRC(void)
{
    glDeleteTextures(TEXTURE_COUNT, textures);
}


//前後移動視口
void SpecialKeys(int key, int x, int y)
{
    if(key == GLUT_KEY_UP)
        viewZ += 0.5f;

    if(key == GLUT_KEY_DOWN)
        viewZ -= 0.5f;

    glutPostRedisplay();
}


void ChangeSize(int w, int h)
{
    GLfloat fAspect;

    if(h == 0)
        h = 1;

    glViewport(0, 0, w, h);

    fAspect = (GLfloat)w/(GLfloat)h;

    viewFrustum.SetPerspective(80.0f,fAspect,1.0,120.0);
    projectionMatrix.LoadMatrix(viewFrustum.GetProjectionMatrix());
    transformPipeline.SetMatrixStacks(modelViewMatrix, projectionMatrix);
}


//繪製場景
void RenderScene(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    modelViewMatrix.PushMatrix();
    modelViewMatrix.Translate(0.0f, 0.0f, viewZ);

    shaderManager.UseStockShader(GLT_SHADER_TEXTURE_REPLACE, transformPipeline.GetModelViewProjectionMatrix(), 0);

    glBindTexture(GL_TEXTURE_2D, textures[TEXTURE_FLOOR]);
    floorBatch.Draw();

    glBindTexture(GL_TEXTURE_2D, textures[TEXTURE_CEILING]);
    ceilingBatch.Draw();

    glBindTexture(GL_TEXTURE_2D, textures[TEXTURE_BRICK]);
    leftWallBatch.Draw();
    rightWallBatch.Draw();

    modelViewMatrix.PopMatrix();

    glutSwapBuffers();
}


int main(int argc, char *argv[])
{
    gltSetWorkingDirectory(argv[0]);

    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowSize(800, 600);
    glutCreateWindow("Anisotropic Tunnel");
    glutReshapeFunc(ChangeSize);
    glutSpecialFunc(SpecialKeys);
    glutDisplayFunc(RenderScene);

    //右擊滑鼠彈出選單
    glutCreateMenu(ProcessMenu);
    glutAddMenuEntry("GL_NEAREST",0);
    glutAddMenuEntry("GL_LINEAR",1);
    glutAddMenuEntry("GL_NEAREST_MIPMAP_NEAREST",2);
    glutAddMenuEntry("GL_NEAREST_MIPMAP_LINEAR", 3);
    glutAddMenuEntry("GL_LINEAR_MIPMAP_NEAREST", 4);
    glutAddMenuEntry("GL_LINEAR_MIPMAP_LINEAR", 5);
    glutAddMenuEntry("Anisotropic Filter", 6);
    glutAddMenuEntry("Anisotropic Off", 7);

    glutAttachMenu(GLUT_RIGHT_BUTTON);

    GLenum err = glewInit();
    if (GLEW_OK != err) {
        fprintf(stderr, "GLEW Error: %s\n", glewGetErrorString(err));
        return 1;
    }

    init();
    glutMainLoop();
    ShutdownRC();

    return 0;
}