【OpenGL】照相机管理案例

案例代码主要部分在RenderScene和SpecialKeys，其中SpecialKeys接收上下左右键使用GLFrame的MoveForward和RotateWorld方法进行平移和旋转GLFrame

接着在RenderScene用GetCameraMatrix(mCamera)从照相机的GLFrame获取照相机变换矩阵，接着是放入模型视图矩阵堆栈的栈顶进行参与变换。

RenderScene函数中模型视图矩阵堆栈的变化情况和渲染流程：

1、堆栈默认有单位矩阵，先拷贝出一份栈顶矩阵（单位矩阵）放置于栈顶，此时栈内有2个单位矩阵
2、从照相机GLFrame获取到照相机变换矩阵，存入栈顶，栈有3个元素
3、绘制背景
4、堆栈栈顶矩阵进行平移变换(0,0,-2.5)（即栈顶矩阵是照相机变换矩阵+平移①）
5、拷贝栈顶矩阵元素（照相机变换矩阵+平移①），放置于栈顶，此时栈有4个元素。
6、对栈顶元素进行旋转变换①（即栈顶元素变为照相机变换矩阵+平移①+旋转①）
7、绘制花托
8、栈顶矩阵出栈，栈顶矩阵变为5操作的栈顶元素（即照相机变换矩阵+平移①），此时栈有3个元素。
9、对栈顶元素进行旋转②和平移②，此时栈顶元素为 照相机变换矩阵+平移①+平移②+旋转②。
10、绘制球体
11、出栈，此时栈有2个元素 （单位矩阵、单位矩阵）【此处出栈是出栈第二步Push的照相机矩阵，虽然此时的照相机矩阵已经变成了第九步操作的矩阵了】
12、出栈，此时栈剩下原本的1个元素 即单位矩阵。【此处出栈是出栈第一步Push的单位矩阵】
这么看可能很复杂，实际非常简单，只要你有即将对栈顶的操作，且希望保留原有栈顶，那就应该先进行PushMatrix，对新栈顶进行各种变换，然后渲染，之后再PopMatrix恢复到操作之前的栈顶元素，就这样，比如 第一步Push是为了保护原有的单位矩阵，让下一次渲染也是从单位矩阵开始进行计算；第五步保护的是平移①操作 在进行了花托旋转① 并渲染花托后 恢复到第四部的栈顶元素；一共进行了3次Push，那也一定是有对应的3次Pop进行恢复。 其实第一步Push是Push照相机变换，在这里之后才真正开始的。

// SphereWorld.cpp
// OpenGL SuperBible
// New and improved (performance) sphere world
// Program by Richard S. Wright Jr.

#pragma comment(lib, "gltools.lib")

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

#include <math.h>
#include <stdio.h>

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


GLShaderManager		shaderManager;			// Shader Manager
GLMatrixStack		modelViewMatrix;		// Modelview Matrix
GLMatrixStack		projectionMatrix;		// Projection Matrix
GLFrustum			viewFrustum;			// View Frustum
GLGeometryTransform	transformPipeline;		// Geometry Transform Pipeline

GLTriangleBatch		torusBatch;
GLBatch				floorBatch;
GLTriangleBatch     sphereBatch;
GLFrame             cameraFrame;

//
// This function does any needed initialization on the rendering
// context. 
void SetupRC()
{
	// Initialze Shader Manager
	shaderManager.InitializeStockShaders();

	glEnable(GL_DEPTH_TEST);
	glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

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

	// This makes a torus
	gltMakeTorus(torusBatch, 0.4f, 0.15f, 30, 30);

	// This make a sphere
	gltMakeSphere(sphereBatch, 0.1f, 26, 13);

	floorBatch.Begin(GL_LINES, 324);
	for (GLfloat x = -20.0; x <= 20.0f; x += 0.5) {
		floorBatch.Vertex3f(x, -0.55f, 20.0f);
		floorBatch.Vertex3f(x, -0.55f, -20.0f);

		floorBatch.Vertex3f(20.0f, -0.55f, x);
		floorBatch.Vertex3f(-20.0f, -0.55f, x);
	}
	floorBatch.End();
}


///
// Screen changes size or is initialized
void ChangeSize(int nWidth, int nHeight)
{
	glViewport(0, 0, nWidth, nHeight);

	// Create the projection matrix, and load it on the projection matrix stack
	viewFrustum.SetPerspective(35.0f, float(nWidth) / float(nHeight), 1.0f, 100.0f);
	projectionMatrix.LoadMatrix(viewFrustum.GetProjectionMatrix());

	// Set the transformation pipeline to use the two matrix stacks 
	transformPipeline.SetMatrixStacks(modelViewMatrix, projectionMatrix);
}


// Respond to arrow keys by moving the camera frame of reference
void SpecialKeys(int key, int x, int y)
{
	float linear = 0.1f;
	float angular = float(m3dDegToRad(5.0f));

	if (key == GLUT_KEY_UP)
		cameraFrame.MoveForward(linear);

	if (key == GLUT_KEY_DOWN)
		cameraFrame.MoveForward(-linear);

	if (key == GLUT_KEY_LEFT)
		cameraFrame.RotateWorld(angular, 0.0f, 1.0f, 0.0f);

	if (key == GLUT_KEY_RIGHT)
		cameraFrame.RotateWorld(-angular, 0.0f, 1.0f, 0.0f);
}

// Called to draw scene
void RenderScene(void)
{
	// Color values
	static GLfloat vFloorColor[] = { 0.0f, 1.0f, 0.0f, 1.0f };
	static GLfloat vTorusColor[] = { 1.0f, 0.0f, 0.0f, 1.0f };
	static GLfloat vSphereColor[] = { 0.0f, 0.0f, 1.0f, 1.0f };

	// Time Based animation
	static CStopWatch	rotTimer;
	float yRot = rotTimer.GetElapsedSeconds() * 60.0f;

	// Clear the color and depth buffers
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);


	// Save the current modelview matrix (the identity matrix)
	modelViewMatrix.PushMatrix();

	M3DMatrix44f mCamera;
	cameraFrame.GetCameraMatrix(mCamera);
	modelViewMatrix.PushMatrix(mCamera);

	// Draw the ground
	shaderManager.UseStockShader(GLT_SHADER_FLAT,
		transformPipeline.GetModelViewProjectionMatrix(),
		vFloorColor);
	floorBatch.Draw();

	// Draw the spinning Torus
	modelViewMatrix.Translate(0.0f, 0.0f, -2.5f);

	// Save the Translation
	modelViewMatrix.PushMatrix();

	// Apply a rotation and draw the torus
	modelViewMatrix.Rotate(yRot, 0.0f, 1.0f, 0.0f);
	shaderManager.UseStockShader(GLT_SHADER_FLAT, transformPipeline.GetModelViewProjectionMatrix(),
		vTorusColor);
	torusBatch.Draw();
	modelViewMatrix.PopMatrix(); // "Erase" the Rotation from before

	// Apply another rotation, followed by a translation, then draw the sphere
	modelViewMatrix.Rotate(yRot * -2.0f, 0.0f, 1.0f, 0.0f);
	modelViewMatrix.Translate(0.8f, 0.0f, 0.0f);
	shaderManager.UseStockShader(GLT_SHADER_FLAT, transformPipeline.GetModelViewProjectionMatrix(),
		vSphereColor);
	sphereBatch.Draw();

	// Restore the previous modleview matrix (the identity matrix)
	modelViewMatrix.PopMatrix();
	modelViewMatrix.PopMatrix();
	// Do the buffer Swap
	glutSwapBuffers();

	// Tell GLUT to do it again
	glutPostRedisplay();
}




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

	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(800, 600);

	glutCreateWindow("OpenGL SphereWorld");

	glutSpecialFunc(SpecialKeys);
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);

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

	SetupRC();
	glutMainLoop();
	return 0;
}