核心内容：
gltMakeCube(cubeBatch, 20.0); //创建一个半径为20个单位长度的立方体批次数据填充到cubeBatch （GLBatch对象）
该函数会将2D纹理坐标分配到GLT_ATTRIBUTE_TEXTURE0属性槽，但我们需要的并不是2D纹理坐标，而是3D纹理坐标。

3D纹理坐标获取方法是在顶点着色器里直接将归一化的顶点坐标作为输出到片段着色器，此时片段着色器插值得到的顶点坐标就是三维纹理坐标了，接着在片段着色器使用texture(samplerCube纹理对象, 三维纹理坐标);采样得到颜色作为输出，下面列出所有代码：

天空盒顶点着色器：

// Skybox Shader
// Vertex Shader
// Richard S. Wright Jr.
// OpenGL SuperBible
#version 130

// Incoming per vertex... just the position
in vec4 vVertex;

uniform mat4   mvpMatrix;  // Transformation matrix

// Texture Coordinate to fragment program
out vec3 vVaryingTexCoord;


void main(void) 
    {
    // Pass on the texture coordinates 
    vVaryingTexCoord = normalize(vVertex.xyz);

    // Don't forget to transform the geometry!
    gl_Position = mvpMatrix * vVertex;
    }

天空盒片段着色器：

// Skybox Shader
// Fragment Shader
// Richard S. Wright Jr.
// OpenGL SuperBible
#version 130

out vec4 vFragColor;

uniform samplerCube  cubeMap;

in vec3 vVaryingTexCoord;

void main(void)
    { 
    vFragColor = texture(cubeMap, vVaryingTexCoord);
    }
    

 注意：天空盒两个面之间的缝隙问题 解决方法  glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS); 即可

在GLSL代码里，需注意的是天空盒顶点着色器使用到的MVP矩阵，其中MV矩阵只有摄像机旋转矩阵生效的。

紧接着需渲染反射环境的球体解释，主要是反射球体的顶点着色器，它进行了计算出视觉空间的法线向量、视角向量，再用reflect函数获取反射向量，并进行了反向的摄像机旋转矩阵变换（即转置摄像机旋转矩阵），最后归一化反射向量传给片段着色器作为三维纹理坐标，进行采样立方体贴图输出颜色。

注意：反射向量进行了一个反向的摄像机旋转变换，书上解释说是如果不进行的话就会导致当摄像机在场景中移动时，立方体贴图将不能正确地反射围绕它的天空盒。
可亲自测试不进行这个变换，即vCoords = mInverseCamera * vCoords; 会如何，我猜测是没有影响的，因为平移摄像机不会导致摄像机旋转矩阵变化，其转置矩阵也不会变化，而产生不了任何效果，只有当旋转摄像机时才会出现问题，可能作者是想球体反射环境是绝对不变化的吧，例如：摄像机旋转时，球体反射的景象依然是一样的，这就需要进行一个反向旋转来保持一致了。

// Reflection Shader
// Vertex Shader
// Richard S. Wright Jr.
// OpenGL SuperBible
#version 130

// Incoming per vertex... position and normal
in vec4 vVertex;
in vec3 vNormal;

uniform mat4   mvpMatrix;
uniform mat4   mvMatrix;
uniform mat3   normalMatrix;
uniform mat4   mInverseCamera;

// Texture coordinate to fragment program
smooth out vec3 vVaryingTexCoord;

void main(void) 
    {
    // Normal in Eye Space
    vec3 vEyeNormal = normalMatrix * vNormal;
    
    // Vertex position in Eye Space
    vec4 vVert4 = mvMatrix * vVertex;
    vec3 vEyeVertex = normalize(vVert4.xyz / vVert4.w);
    
    // Get reflected vector
    vec4 vCoords = vec4(reflect(vEyeVertex, vEyeNormal), 1.0);
   
    // Rotate by flipped camera
    vCoords = mInverseCamera * vCoords;
    vVaryingTexCoord.xyz = normalize(vCoords.xyz);

    // Don't forget to transform the geometry!
    gl_Position = mvpMatrix * vVertex;
    }

 反射球体的片段着色器：

// Reflection Shader
// Fragment Shader
// Richard S. Wright Jr.
// OpenGL SuperBible
#version 130

out vec4 vFragColor;

uniform samplerCube cubeMap;
smooth in vec3 vVaryingTexCoord;

void main(void)
    { 
    vFragColor = texture(cubeMap, vVaryingTexCoord.stp);
    }
    

// CubeMapped.cpp
// OpenGL SuperBible
// Demonstrates applying a cube map to an object (sphere) using
// and using the same map for the skybox.
// Program by Richard S. Wright Jr.
#pragma comment(lib,"gltools.lib")
#include <GLTools.h>	// OpenGL toolkit
#include <GLMatrixStack.h>
#include <GLFrame.h>
#include <GLFrustum.h>
#include <GLGeometryTransform.h>
#include <StopWatch.h>

#include <math.h>
#include <stdlib.h>

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


GLFrame             viewFrame;
GLFrustum           viewFrustum;
GLTriangleBatch     sphereBatch;
GLBatch             cubeBatch;
GLMatrixStack       modelViewMatrix;
GLMatrixStack       projectionMatrix;
GLGeometryTransform transformPipeline;
GLuint              cubeTexture;
GLint               reflectionShader;
GLint               skyBoxShader;

GLint               locMVPReflect, locMVReflect, locNormalReflect, locInvertedCamera;
GLint				locMVPSkyBox;


// Six sides of a cube map
const char *szCubeFaces[6] = { "pos_x.tga", "neg_x.tga", "pos_y.tga", "neg_y.tga", "pos_z.tga", "neg_z.tga" };

GLenum  cube[6] = { GL_TEXTURE_CUBE_MAP_POSITIVE_X,
					 GL_TEXTURE_CUBE_MAP_NEGATIVE_X,
					 GL_TEXTURE_CUBE_MAP_POSITIVE_Y,
					 GL_TEXTURE_CUBE_MAP_NEGATIVE_Y,
					 GL_TEXTURE_CUBE_MAP_POSITIVE_Z,
					 GL_TEXTURE_CUBE_MAP_NEGATIVE_Z };


//
// This function does any needed initialization on the rendering
// context. 
void SetupRC()
{
	GLbyte *pBytes;
	GLint iWidth, iHeight, iComponents;
	GLenum eFormat;
	int i;

	// Cull backs of polygons
	glCullFace(GL_BACK);
	glFrontFace(GL_CCW);
	glEnable(GL_DEPTH_TEST);

	glGenTextures(1, &cubeTexture);
	glBindTexture(GL_TEXTURE_CUBE_MAP, cubeTexture);

	// Set up texture maps        
	glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
	glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);
	glPixelStorei(GL_UNPACK_ALIGNMENT, 1);


	// Load Cube Map images
	for (i = 0; i < 6; i++)
	{
		// Load this texture map
		pBytes = gltReadTGABits(szCubeFaces[i], &iWidth, &iHeight, &iComponents, &eFormat);
		glTexImage2D(cube[i], 0, iComponents, iWidth, iHeight, 0, eFormat, GL_UNSIGNED_BYTE, pBytes);
		free(pBytes);
	}
	glGenerateMipmap(GL_TEXTURE_CUBE_MAP);

	viewFrame.MoveForward(-4.0f);
	gltMakeSphere(sphereBatch, 1.0f, 52, 26);
	gltMakeCube(cubeBatch, 20.0f);

	reflectionShader = gltLoadShaderPairWithAttributes("Reflection.vp", "Reflection.fp", 2,
		GLT_ATTRIBUTE_VERTEX, "vVertex",
		GLT_ATTRIBUTE_NORMAL, "vNormal");

	locMVPReflect = glGetUniformLocation(reflectionShader, "mvpMatrix");
	locMVReflect = glGetUniformLocation(reflectionShader, "mvMatrix");
	locNormalReflect = glGetUniformLocation(reflectionShader, "normalMatrix");
	locInvertedCamera = glGetUniformLocation(reflectionShader, "mInverseCamera");


	skyBoxShader = gltLoadShaderPairWithAttributes("SkyBox.vp", "SkyBox.fp", 2,
		GLT_ATTRIBUTE_VERTEX, "vVertex",
		GLT_ATTRIBUTE_NORMAL, "vNormal");

	locMVPSkyBox = glGetUniformLocation(skyBoxShader, "mvpMatrix");


}

void ShutdownRC(void)
{
	glDeleteTextures(1, &cubeTexture);
}


// Called to draw scene
void RenderScene(void)
{
	// Clear the window
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	M3DMatrix44f mCamera;
	M3DMatrix44f mCameraRotOnly;
	M3DMatrix44f mInverseCamera;

	viewFrame.GetCameraMatrix(mCamera, false);
	viewFrame.GetCameraMatrix(mCameraRotOnly, true);
	m3dInvertMatrix44(mInverseCamera, mCameraRotOnly);

	modelViewMatrix.PushMatrix();
	// Draw the sphere
	modelViewMatrix.MultMatrix(mCamera);
	glUseProgram(reflectionShader);
	glUniformMatrix4fv(locMVPReflect, 1, GL_FALSE, transformPipeline.GetModelViewProjectionMatrix());
	glUniformMatrix4fv(locMVReflect, 1, GL_FALSE, transformPipeline.GetModelViewMatrix());
	glUniformMatrix3fv(locNormalReflect, 1, GL_FALSE, transformPipeline.GetNormalMatrix());
	glUniformMatrix4fv(locInvertedCamera, 1, GL_FALSE, mInverseCamera);

	glEnable(GL_CULL_FACE);
	sphereBatch.Draw();
	glDisable(GL_CULL_FACE);
	modelViewMatrix.PopMatrix();

	modelViewMatrix.PushMatrix();
	modelViewMatrix.MultMatrix(mCameraRotOnly);
	glUseProgram(skyBoxShader);
	glUniformMatrix4fv(locMVPSkyBox, 1, GL_FALSE, transformPipeline.GetModelViewProjectionMatrix());
	cubeBatch.Draw();
	modelViewMatrix.PopMatrix();

	// Do the buffer Swap
	glutSwapBuffers();
}



// Respond to arrow keys by moving the camera frame of reference
void SpecialKeys(int key, int x, int y)
{
	if (key == GLUT_KEY_UP)
		viewFrame.MoveForward(0.1f);

	if (key == GLUT_KEY_DOWN)
		viewFrame.MoveForward(-0.1f);

	if (key == GLUT_KEY_LEFT)
		viewFrame.RotateLocalY(0.1);

	if (key == GLUT_KEY_RIGHT)
		viewFrame.RotateLocalY(-0.1);

	// Refresh the Window
	glutPostRedisplay();
}


void ChangeSize(int w, int h)
{
	// Prevent a divide by zero
	if (h == 0)
		h = 1;

	// Set Viewport to window dimensions
	glViewport(0, 0, w, h);

	viewFrustum.SetPerspective(35.0f, float(w) / float(h), 1.0f, 1000.0f);

	projectionMatrix.LoadMatrix(viewFrustum.GetProjectionMatrix());
	transformPipeline.SetMatrixStacks(modelViewMatrix, projectionMatrix);
}

int main(int argc, char* argv[])
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(800, 600);
	glutCreateWindow("OpenGL Cube Maps");
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);
	glutSpecialFunc(SpecialKeys);

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


	SetupRC();

	glutMainLoop();

	ShutdownRC();

	return 0;
}