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\";