Xcode+opengl學習（3）顏色混合的理解應用

 **目標顏色**：已經儲存在顏色緩衝區中的顏色值
 **源顏色**：作爲當前渲染命令的結果進入顏色緩衝區的顏色值
     本篇主要介紹源顏色和目標顏色的混合
 如：

   GLfloat vRed[]={1.0f,0.0f,0.0f,0.5f};
   glEnable(GL_BLEND);
   glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
   shaderManager.UseStockShader(GLT_SHADER_IDENTITY,vRed);
   squareBatch.Draw();
   glDisablu(GL_BLEND);

glBlendFunc(GLenum S,GLenum D);

注意，如果取上述例子中的S和D，計算方程爲：
最終顏色=（源顏色✖️自身alpha值）+（緩衝區裏的顏色✖️（1-剛纔的alpha值））
例如：
目標顏色=（1.0f,0.0f,0.0f,0.0f）
源顏色=（0.0f,0.0f,1.0f,0.6f）
源alpha值=0.6
那麼最終顏色=（源顏色✖️0.6）+（目標顏色✖️0.4）

下例是利用矩形塊的移動體現blend的實現。

#include<GLTools.h>
#include<GLShaderManager.h>
#include<GLUT/GLUT.h>

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();
    
    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();
}

void SpecialKeys(int key, int x, int y)
{
    GLfloat stepSize = 0.025f;
    
    GLfloat blockX = vVerts[0];
    GLfloat blockY = vVerts[7];
    
    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;
    
    
    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;
    
  
    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();
}

void RenderScene(void)
{
    
    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);
    
    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;
}