opengl超级宝典（第五版）阅读笔记 5 混合

这里只提一种最基本的混合，其他的混合方式可以参考书中的表格
混合代码如下：

	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vRed);
	squareBatch.Draw();
	glDisable(GL_BLEND);

大部分代码前面我们都已经接触过，主要解释一下glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
最终显示的颜色为 源颜色的alpha * 源颜色 + 目标颜色的颜色*(1-alpha)
如下图所示：

完整代码如下：

#include <GLTools.h>  // OpenGL toolkit   
#define FREEGLUT_STATIC  //在windows和linux上，使用freeglut静态版本，需要添加这一行，否则会出现错误
#include <glut.h>
#pragma comment(lib,"gltools.lib")//要加上这一行链接一下gltools库
GLBatch squareBatch;
GLBatch greenBatch;
GLBatch redBatch;
GLBatch blueBatch;
GLBatch blackBatch;	
GLShaderManager shaderManager;
GLfloat blockSize = 0.2f;
GLfloat vVerts[] = { -blockSize, -blockSize, 0.0f,	
blockSize, -blockSize, 0.0f,
blockSize,  blockSize, 0.0f,
-blockSize,  blockSize, 0.0f };

void SetupRC()
{
	glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
	shaderManager.InitializeStockShaders();
	// Load up a triangle fan   
	squareBatch.Begin(GL_TRIANGLE_FAN, 4);
	squareBatch.CopyVertexData3f(vVerts);
	squareBatch.End();

	GLfloat vBlock[] = { 0.25f, 0.25f, 0.0f,
		0.75f, 0.25f, 0.0f,
		0.75f, 0.75f, 0.0f,
		0.25f, 0.75f, 0.0f };

	greenBatch.Begin(GL_TRIANGLE_FAN, 4);
	greenBatch.CopyVertexData3f(vBlock);
	greenBatch.End();


	GLfloat vBlock2[] = { -0.75f, 0.25f, 0.0f,
		-0.25f, 0.25f, 0.0f,
		-0.25f, 0.75f, 0.0f,
		-0.75f, 0.75f, 0.0f };

	redBatch.Begin(GL_TRIANGLE_FAN, 4);
	redBatch.CopyVertexData3f(vBlock2);
	redBatch.End();


	GLfloat vBlock3[] = { -0.75f, -0.75f, 0.0f,
		-0.25f, -0.75f, 0.0f,
		-0.25f, -0.25f, 0.0f,
		-0.75f, -0.25f, 0.0f };

	blueBatch.Begin(GL_TRIANGLE_FAN, 4);
	blueBatch.CopyVertexData3f(vBlock3);
	blueBatch.End();


	GLfloat vBlock4[] = { 0.25f, -0.75f, 0.0f,
		0.75f, -0.75f, 0.0f,
		0.75f, -0.25f, 0.0f,
		0.25f, -0.25f, 0.0f };

	blackBatch.Begin(GL_TRIANGLE_FAN, 4);
	blackBatch.CopyVertexData3f(vBlock4);
	blackBatch.End();
}

// Respond to arrow keys by moving the camera frame of reference   
void SpecialKeys(int key, int x, int y)
{
	GLfloat stepSize = 0.025f;
	GLfloat blockX = vVerts[0];   // Upper left X   
	GLfloat blockY = vVerts[7];  // Upper left Y   
	if (key == GLUT_KEY_UP)
		blockY += stepSize;
	if (key == GLUT_KEY_DOWN)
		blockY -= stepSize;
	if (key == GLUT_KEY_LEFT)
		blockX -= stepSize;
	if (key == GLUT_KEY_RIGHT)
		blockX += stepSize;
	// Collision detection   
	if (blockX < -1.0f) blockX = -1.0f;
	if (blockX >(1.0f - blockSize * 2)) blockX = 1.0f - blockSize * 2;;
	if (blockY < -1.0f + blockSize * 2)  blockY = -1.0f + blockSize * 2;
	if (blockY > 1.0f) blockY = 1.0f;
	// Recalculate vertex positions   
	vVerts[0] = blockX;
	vVerts[1] = blockY - blockSize * 2;
	vVerts[3] = blockX + blockSize * 2;
	vVerts[4] = blockY - blockSize * 2;
	vVerts[6] = blockX + blockSize * 2;
	vVerts[7] = blockY;
	vVerts[9] = blockX;
	vVerts[10] = blockY;
	squareBatch.CopyVertexData3f(vVerts);
	glutPostRedisplay();
}
///////////////////////////////////////////////////////////////////////////////   
// Called to draw scene   
void RenderScene(void)
{
	// Clear the window with current clearing color   
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

	GLfloat vRed[] = { 1.0f, 0.0f, 0.0f, 0.5f };
	GLfloat vGreen[] = { 0.0f, 1.0f, 0.0f, 1.0f };
	GLfloat vBlue[] = { 0.0f, 0.0f, 1.0f, 1.0f };
	GLfloat vBlack[] = { 0.0f, 0.0f, 0.0f, 1.0f };

	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vGreen);
	greenBatch.Draw();

	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vRed);
	redBatch.Draw();

	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlue);
	blueBatch.Draw();
	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vBlack);
	blackBatch.Draw();
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	shaderManager.UseStockShader(GLT_SHADER_IDENTITY, vRed);
	squareBatch.Draw();
	glDisable(GL_BLEND);

	// Flush drawing commands   
	glutSwapBuffers();
}
void ChangeSize(int w, int h)
{
	glViewport(0, 0, w, h);
}
int main(int argc, char* argv[])
{
	gltSetWorkingDirectory(argv[0]);
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
	glutInitWindowSize(800, 600);
	glutCreateWindow("Move Block with Arrow Keys to see blending");
	GLenum err = glewInit();
	if (GLEW_OK != err)
	{
		fprintf(stderr, "Error: %s\n", glewGetErrorString(err));
		return 1;
	}
	glutReshapeFunc(ChangeSize);
	glutDisplayFunc(RenderScene);
	glutSpecialFunc(SpecialKeys);
	SetupRC();
	glutMainLoop();
	return 0;
}