通过cocos2d-x的CCGLProgram和CCShaderCache的实现来分析OpenGL ES中的Shader编程

在OpenGL ES中，Shader是着色器，包括两种：顶点着色器（Vertex Shader）和片元着色器（Fragment Shader）。每个program对象有且仅有一个Vertex Shader对象和一个Fragment Shader对象连接到它。 

Shader和Program编程步骤：

1. 创建Shader
      1）编写Vertex Shader和Fragment Shader源码。

      2）创建两个shader 实例：GLuint   glCreateShader(GLenum type);

      3）给Shader实例指定源码。 glShaderSource

      4）在线编译shaer源码 void   glCompileShader(GLuint shader)


2. 创建Program

      1）创建program  GLuint   glCreateProgram(void)

      2）绑定shader到program 。 void   glAttachShader(GLuint program, GLuint shader)。每个program必须绑定一个Vertex Shader 和一个Fragment Shader。

      3）链接program 。 void   glLinkProgram(GLuint program)

      4）使用porgram 。 void   glUseProgram(GLuint program)


在cocos2d-x中使用两个类CCGLProgram和CCShaderCache来完成这些操作，其中CCGLProgram类是基本类，封装了对OpenGL ES接口的调用，而CCShaderCache通过CCGLProgram来完成对shaders的缓存和管理。

bool CCGLProgram::initWithVertexShaderByteArray(const GLchar* vShaderByteArray, const GLchar* fShaderByteArray)
{
    m_uProgram = glCreateProgram(); //创建一个Program，id为m_uProgram
    CHECK_GL_ERROR_DEBUG();

    m_uVertShader = m_uFragShader = 0;

    if (vShaderByteArray)
    {
        //创建顶点着色器并编译，id为m_uVertShader，vShaderByteArray是顶点着色器的源码    
        if (!compileShader(&m_uVertShader, GL_VERTEX_SHADER, vShaderByteArray))
        {
            CCLOG("cocos2d: ERROR: Failed to compile vertex shader");
        }
    }

    // Create and compile fragment shader
    if (fShaderByteArray)
    {
        //创建片元着色器并编译，id为m_uFragShader，fShaderByteArray是片元着色器的源码。    
        if (!compileShader(&m_uFragShader, GL_FRAGMENT_SHADER, fShaderByteArray))
        {
            CCLOG("cocos2d: ERROR: Failed to compile fragment shader");
        }
    }

    if (m_uVertShader)
    {
        //绑定顶点着色器
        glAttachShader(m_uProgram, m_uVertShader);
    }
    CHECK_GL_ERROR_DEBUG();

    if (m_uFragShader)
    {
        //绑定片元着色器    
        glAttachShader(m_uProgram, m_uFragShader);
    }
    m_pHashForUniforms = NULL;
    
    CHECK_GL_ERROR_DEBUG();

    return true;
}

bool CCGLProgram::compileShader(GLuint * shader, GLenum type, const GLchar* source)
{
    GLint status;
 
    if (!source)
    {
        return false;
    }
    
    const GLchar *sources[] = {
#if (CC_TARGET_PLATFORM != CC_PLATFORM_WIN32 && CC_TARGET_PLATFORM != CC_PLATFORM_LINUX && CC_TARGET_PLATFORM != CC_PLATFORM_MAC)
        (type == GL_VERTEX_SHADER ? "precision highp float;\n" : "precision mediump float;\n"),
#endif
        "uniform mat4 CC_PMatrix;\n"
        "uniform mat4 CC_MVMatrix;\n"
        "uniform mat4 CC_MVPMatrix;\n"
        "uniform vec4 CC_Time;\n"
        "uniform vec4 CC_SinTime;\n"
        "uniform vec4 CC_CosTime;\n"
        "uniform vec4 CC_Random01;\n"
        "//CC INCLUDES END\n\n",
        source,
    };

    *shader = glCreateShader(type); //创建shader
    glShaderSource(*shader, sizeof(sources)/sizeof(*sources), sources, NULL);  //指定源码
    glCompileShader(*shader); //编译shader

    glGetShaderiv(*shader, GL_COMPILE_STATUS, &status); //获得shader编译的结果

    if (! status)  //编译失败打印log
    {
        GLsizei length;
        glGetShaderiv(*shader, GL_SHADER_SOURCE_LENGTH, &length);
        GLchar* src = (GLchar *)malloc(sizeof(GLchar) * length);
        
        glGetShaderSource(*shader, length, NULL, src);
        CCLOG("cocos2d: ERROR: Failed to compile shader:\n%s", src);
        
        if (type == GL_VERTEX_SHADER)
        {
            CCLOG("cocos2d: %s", vertexShaderLog());
        }
        else
        {
            CCLOG("cocos2d: %s", fragmentShaderLog());
        }
        free(src);

        abort();
    }
    return (status == GL_TRUE);
}

下面是初始化Shader的过程，通过CCShaderCache::loadDefaultShaders来完成。

void CCShaderCache::loadDefaultShaders()
{
    // Position Texture Color shader
    CCGLProgram *p = new CCGLProgram(); //创建CCGLProgram对象来操作OpenGL的shader
    loadDefaultShader(p, kCCShaderType_PositionTextureColor); //完成该CCGLProgram对象的初始化，如shaders的创建，编译和绑定
                                                              //此处创建的是顶点纹理颜色的program.

    m_pPrograms->setObject(p, kCCShader_PositionTextureColor); //存入字典，key为kCCShader_PositionTextureColor
    p->release();

    // Position Texture Color alpha test
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionTextureColorAlphaTest);

    m_pPrograms->setObject(p, kCCShader_PositionTextureColorAlphaTest);
    p->release();

    //
    // Position, Color shader
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionColor);  //此处创建的是顶点颜色的program.

    m_pPrograms->setObject(p, kCCShader_PositionColor);
    p->release();

    //
    // Position Texture shader
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionTexture);  //此处创建的是顶点纹理的program.

    m_pPrograms->setObject(p, kCCShader_PositionTexture);
    p->release();

    //
    // Position, Texture attribs, 1 Color as uniform shader
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionTexture_uColor);

    m_pPrograms->setObject(p ,kCCShader_PositionTexture_uColor);
    p->release();

    //
    // Position Texture A8 Color shader
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionTextureA8Color);
    
    m_pPrograms->setObject(p, kCCShader_PositionTextureA8Color);
    p->release();

    //
    // Position and 1 color passed as a uniform (to simulate glColor4ub )
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_Position_uColor);
    
    m_pPrograms->setObject(p, kCCShader_Position_uColor);
    p->release();
    
    //
    // Position, Legth(TexCoords, Color (used by Draw Node basically )
    //
    p = new CCGLProgram();
    loadDefaultShader(p, kCCShaderType_PositionLengthTexureColor);
    
    m_pPrograms->setObject(p, kCCShader_PositionLengthTexureColor);
    p->release();
}

void CCShaderCache::loadDefaultShader(CCGLProgram *p, int type)
{
    switch (type) {
        case kCCShaderType_PositionTextureColor:
            //shaders的创建，编译和绑定，不同的program的顶点着色源码和片元着色源码不同。
            p->initWithVertexShaderByteArray(ccPositionTextureColor_vert, ccPositionTextureColor_frag);
            
            //绑定属性名称和索引，属性名称在shader源码中已经定义。
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);
            
            break;
        case kCCShaderType_PositionTextureColorAlphaTest:
            p->initWithVertexShaderByteArray(ccPositionTextureColor_vert, ccPositionTextureColorAlphaTest_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);

            break;
        case kCCShaderType_PositionColor:  
            p->initWithVertexShaderByteArray(ccPositionColor_vert ,ccPositionColor_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);

            break;
        case kCCShaderType_PositionTexture:
            p->initWithVertexShaderByteArray(ccPositionTexture_vert ,ccPositionTexture_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);

            break;
        case kCCShaderType_PositionTexture_uColor:
            p->initWithVertexShaderByteArray(ccPositionTexture_uColor_vert, ccPositionTexture_uColor_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);

            break;
        case kCCShaderType_PositionTextureA8Color:
            p->initWithVertexShaderByteArray(ccPositionTextureA8Color_vert, ccPositionTextureA8Color_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);

            break;
        case kCCShaderType_Position_uColor:
            p->initWithVertexShaderByteArray(ccPosition_uColor_vert, ccPosition_uColor_frag);    
            
            p->addAttribute("aVertex", kCCVertexAttrib_Position);    
            
            break;
        case kCCShaderType_PositionLengthTexureColor:
            p->initWithVertexShaderByteArray(ccPositionColorLengthTexture_vert, ccPositionColorLengthTexture_frag);
            
            p->addAttribute(kCCAttributeNamePosition, kCCVertexAttrib_Position);
            p->addAttribute(kCCAttributeNameTexCoord, kCCVertexAttrib_TexCoords);
            p->addAttribute(kCCAttributeNameColor, kCCVertexAttrib_Color);
            
            break;
        default:
            CCLOG("cocos2d: %s:%d, error shader type", __FUNCTION__, __LINE__);
            return;
    }
    
    p->link(); //调用glLinkProgram链接
    p->updateUniforms();
    
    CHECK_GL_ERROR_DEBUG();
}

上面创建的几种类型的program说明如下：
enum {    
    kCCShaderType_PositionTextureColor, //顶点格式为位置+纹理UV+材质色  
    kCCShaderType_PositionTextureColorAlphaTest, //顶点格式为顶点格式为位置+纹理UV+材质色+用于AlphaTest的ALPHA值  
  
    kCCShaderType_PositionColor, // 顶点格式为位置+材质色  
  
    kCCShaderType_PositionTexture, //顶点格式为位置+纹理UV   
  
    kCCShaderType_PositionTexture_uColor, //顶点格式为位置+纹理UV+材质色  
  
    kCCShaderType_PositionTextureA8Color, //顶点格式为位置+纹理UV+灰度  
  
    kCCShaderType_Position_uColor, //顶点格式为位置+材质色   
      
    kCCShaderType_MAX,  //枚举结束值  
}; 

以类型为kCCShaderType_PositionTextureColor的program为例，它的顶点着色源码和片元着色源码分别为ccShader_PositionTextureColor_vert.h和
ccShader_PositionTextureColor_frag.h。

ccShader_PositionTextureColor_vert.h内容为：

"                                                   \n\
attribute vec4 a_position;                          \n\
attribute vec2 a_texCoord;                          \n\
attribute vec4 a_color;                             \n\
                                                    \n\
#ifdef GL_ES                                        \n\
varying lowp vec4 v_fragmentColor;                  \n\
varying mediump vec2 v_texCoord;                    \n\
#else                                               \n\
varying vec4 v_fragmentColor;                       \n\
varying vec2 v_texCoord;                            \n\
#endif                                              \n\
                                                    \n\
void main()                                         \n\
{                                                   \n\
    gl_Position = CC_MVPMatrix * a_position;        \n\
    v_fragmentColor = a_color;                      \n\
    v_texCoord = a_texCoord;                        \n\
}                                                   \n\
";

ccShader_PositionTextureColor_frag.h内容为：

"                                           \n\
#ifdef GL_ES                                \n\
precision lowp float;                       \n\
#endif                                      \n\
                                            \n\
varying vec4 v_fragmentColor;               \n\
varying vec2 v_texCoord;                    \n\
uniform sampler2D CC_Texture0;              \n\
                                            \n\
void main()                                 \n\
{                                           \n\
    gl_FragColor = v_fragmentColor * texture2D(CC_Texture0, v_texCoord);            \n\
}                                           \n\
";