OpenGL ES2.0编程步骤
OpenGL ES (OpenGL for Embedded Systems) 是 OpenGL 三维图形 API 的子集，针对手机、PDA和游戏主机等嵌入式设备而设计。该API由Khronos集团定义推广，Khronos是一个图形软硬件行业协会，该协会主要关注图形和多媒体方面的开放标准。
1. 保存全局变量的数据结构
以下例子程序均基于Linux平台。
typedef struct _escontext
{
void* userData; // Put your user data here...
GLint width; // Window width
GLint height; // Window height
EGLNativeWindowType hWnd; // Window handle
EGLDisplay eglDisplay; // EGL display
EGLContext eglContext; // EGL context
EGLSurface eglSurface; // EGL surface
// Callbacks
void (ESCALLBACK *drawFunc) ( struct _escontext * );
void (ESCALLBACK *keyFunc) ( struct _escontext *, unsigned char, int, int );
void (ESCALLBACK *updateFunc) ( struct _escontext *, float deltaTime );
}ESContext;
typedef struct
{
// Handle to a program object
GLuint programObject;
// Atrribute Location
GLint positionLoc;
GLint textureLoc;
// Uniform location
GLint matrixModeLoc;
GLint matrixViewLoc;
GLint matrixPerspectiveLoc;
// Sampler location
GLint samplerLoc;
// texture
GLuint texture;
} UserData;
2. 初始化EGL渲染环境和相关元素（第一步曲）
int InitEGL(ESContext * esContext)
{
NativeWindowType eglWindow = NULL;
EGLDisplay display;
EGLContext context;
EGLSurface surface;
EGLConfig configs[];
EGLBoolean eRetStatus;
EGLint majorVer, minorVer;
EGLint context_attribs[] = {EGL_CONTEXT_CLIENT_VERSION, , EGL_NONE};
EGLint numConfigs;
EGLint cfg_attribs[] = {EGL_BUFFER_SIZE, EGL_DONT_CARE,
EGL_DEPTH_SIZE, ,
EGL_RED_SIZE, ,
EGL_GREEN_SIZE, ,
EGL_BLUE_SIZE, ,
EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
EGL_NONE};
// Get default display connection
display = eglGetDisplay((EGLNativeDisplayType)EGL_DEFAULT_DISPLAY);
if ( display == EGL_NO_DISPLAY )
{
return EGL_FALSE;
}
// Initialize EGL display connection
eRetStatus = eglInitialize(display, &majorVer, &minorVer);
if( eRetStatus != EGL_TRUE )
{
return EGL_FALSE;
}
//Get a list of all EGL frame buffer configurations for a display
eRetStatus = eglGetConfigs (display, configs, , &numConfigs);
if( eRetStatus != EGL_TRUE )
{
return EGL_FALSE;
}
// Get a list of EGL frame buffer configurations that match specified attributes
eRetStatus = eglChooseConfig (display, cfg_attribs, configs, , &numConfigs);
if( eRetStatus != EGL_TRUE || !numConfigs)
{
return EGL_FALSE;
}
// Create a new EGL window surface
surface = eglCreateWindowSurface(display, configs[], eglWindow, NULL);
if (surface == EGL_NO_SURFACE)
{
return EGL_FALSE;
}
// Set the current rendering API (EGL_OPENGL_API, EGL_OPENGL_ES_API,EGL_OPENVG_API)
eRetStatus = eglBindAPI(EGL_OPENGL_ES_API);
if (eRetStatus != EGL_TRUE)
{
return EGL_FALSE;
}
// Create a new EGL rendering context
context = eglCreateContext (display, configs[], EGL_NO_CONTEXT, context_attribs);
if (context == EGL_NO_CONTEXT)
{
return EGL_FALSE;
}
// Attach an EGL rendering context to EGL surfaces
eRetStatus = eglMakeCurrent (display, surface, surface, context);
if( eRetStatus != EGL_TRUE )
{
return EGL_FALSE;
}
//If interval is set to a value of 0, buffer swaps are not synchronized to a video frame, and the swap happens as soon as the render is complete.
eglSwapInterval(display,);
// Return the context elements
esContext->eglDisplay = display;
esContext->eglSurface = surface;
esContext->eglContext = context;
return EGL_TRUE;
}
3. 生成Program （第二步曲）
3.1 LoadShader
LoadShader主要实现以下功能：
1) 创建Shader对象
2) 装载Shader源码
3) 编译Shader
其实现参考代码如下：
/* type specifies the Shader type: GL_VERTEX_SHADER or GL_FRAGMENT_SHADER */
GLuint LoadShader ( GLenum type, const char *shaderSrc )
{
GLuint shader;
GLint compiled;
// Create an empty shader object, which maintain the source code strings that define a shader
shader = glCreateShader ( type );
)
;
// Replaces the source code in a shader object
glShaderSource ( shader, , &shaderSrc, NULL );
// Compile the shader object
glCompileShader ( shader );
// Check the shader object compile status
glGetShaderiv ( shader, GL_COMPILE_STATUS, &compiled );
if ( !compiled )
{
GLint infoLen = ;
glGetShaderiv ( shader, GL_INFO_LOG_LENGTH, &infoLen );
)
{
char* infoLog = malloc (sizeof(char) * infoLen );
glGetShaderInfoLog ( shader, infoLen, NULL, infoLog );
esLogMessage ( "Error compiling shader:\n%s\n", infoLog );
free ( infoLog );
}
glDeleteShader ( shader );
;
}
return shader;
}
1）glCreateShader
它创建一个空的shader对象，它用于维护用来定义shader的源码字符串。支持以下两种shader:
（1） GL_VERTEX_SHADER: 它运行在可编程的“顶点处理器”上，用于代替固定功能的顶点处理；
（ 2） GL_FRAGMENT_SHADER: 它运行在可编程的“片断处理器”上，用于代替固定功能的片段处理；
2）glShaderSource
shader对象中原来的源码全部被新的源码所代替。
3）glCompileShader
编译存储在shader对象中的源码字符串，编译结果被当作shader对象状态的一部分被保存起来，可通过glGetShaderiv函数获取编译状态。
4）glGetShaderiv
获取shader对象参数，参数包括：GL_SHADER_TYPE, GL_DELETE_STATUS, GL_COMPILE_STATUS, GL_INFO_LOG_LENGTH, GL_SHADER_SOURCE_LENGTH.
3.2 LoadProgram
其参考代码如下：
GLuint LoadProgram ( const char *vShaderStr, const char *fShaderStr )
{
GLuint vertexShader;
GLuint fragmentShader;
GLuint programObject;
GLint linked;
// Load the vertex/fragment shaders
vertexShader = LoadShader ( GL_VERTEX_SHADER, vShaderStr );
fragmentShader = LoadShader ( GL_FRAGMENT_SHADER, fShaderStr );
// Create the program object
programObject = glCreateProgram ( );
)
;
// Attaches a shader object to a program object
glAttachShader ( programObject, vertexShader );
glAttachShader ( programObject, fragmentShader );
// Bind vPosition to attribute 0
glBindAttribLocation ( programObject, , "vPosition" );
// Link the program object
glLinkProgram ( programObject );
// Check the link status
glGetProgramiv ( programObject, GL_LINK_STATUS, &linked );
if ( !linked )
{
GLint infoLen = ;
glGetProgramiv ( programObject, GL_INFO_LOG_LENGTH, &infoLen );
)
{
char* infoLog = malloc (sizeof(char) * infoLen );
glGetProgramInfoLog ( programObject, infoLen, NULL, infoLog );
esLogMessage ( "Error linking program:\n%s\n", infoLog );
free ( infoLog );
}
glDeleteProgram ( programObject );
return GL_FALSE;
}
// Free no longer needed shader resources
glDeleteShader ( vertexShader );
glDeleteShader ( fragmentShader );
return programObject;
}
1）glCreateProgram
建立一个空的program对象，shader对象可以被连接到program对像
2）glAttachShader
program对象提供了把需要做的事连接在一起的机制。在一个program中，在shader对象被连接在一起之前，必须先把shader连接到program上。
3）glBindAttribLocation
把program的顶点属性索引与顶点shader中的变量名进行绑定。
4）glLinkProgram
连接程序对象。如果任何类型为GL_VERTEX_SHADER的shader对象连接到program,它将产生在“可编程顶点处理器”上可执行的程
序；如果任何类型为GL_FRAGMENT_SHADER的shader对象连接到program,它将产生在“可编程片断处理器”上可执行的程序。
5）glGetProgramiv
获取program对象的参数值，参数有：GL_DELETE_STATUS, GL_LINK_STATUS,
GL_VALIDATE_STATUS, GL_INFO_LOG_LENGTH, GL_ATTACHED_SHADERS,
GL_ACTIVE_ATTRIBUTES, GL_ACTIVE_ATTRIBUTE_MAX_LENGTH,
GL_ACTIVE_UNIFORMS, GL_ACTIVE_UNIFORM_MAX_LENGTH.
3.3 CreateProgram
在3.1中只实现了Shader的编译，在3.2中只实现了Program的链接，现在还缺少真正供进行编译和链接的源码，其参考代码如下：
int CreateProgram(ESContext * esContext)
{
GLuint programObject;
GLbyte vShaderStr[] =
"attribute vec4 vPosition; \n"
"void main() \n"
"{ \n"
" gl_Position = vPosition; \n"
"} \n";
GLbyte fShaderStr[] =
"precision mediump float;\n"\
"void main() \n"
"{ \n"
" gl_FragColor = vec4 ( 1.0, 0.0, 0.0, 1.0 );\n"
"} \n";
// Create user data
esContext->userData = malloc(sizeof(UserData));
UserData *userData = esContext->userData;
// Load the shaders and get a linked program object
programObject = LoadProgram ( (const char*)vShaderStr, (const char*)fShaderStr );
)
{
return GL_FALSE;
}
// Store the program object
userData->programObject = programObject;
// Get the attribute locations
userData->positionLoc = glGetAttribLocation ( g_programObject, "v_position" );
glClearColor ( 0.0f, 0.0f, 0.0f, 1.0f );
;
}
4. 安装并执行Program（第三步）
void Render ( ESContext *esContext )
{
UserData *userData = esContext->userData;
GLfloat vVertices[] = { 0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f };
// Set the viewport
glViewport ( , , esContext->width, esContext->height );
// Clear the color buffer
glClear ( GL_COLOR_BUFFER_BIT );
// Use the program object
glUseProgram ( userData->programObject );
// Load the vertex data
glVertexAttribPointer ( , , GL_FLOAT, GL_FALSE, , vVertices );
glEnableVertexAttribArray ( );
glDrawArrays ( GL_TRIANGLES, , );
eglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);
4.1 glClear
清除指定的buffer到预设值。可清除以下四类buffer:
1）GL_COLOR_BUFFER_BIT
2）GL_DEPTH_BUFFER_BIT
3）GL_ACCUM_BUFFER_BIT
4）GL_STENCIL_BUFFER_BIT
预设值通过glClearColor, glClearIndex, glClearDepth, glClearStencil, 和glClearAccum来设置。
1）gClearColor
指定color buffer的清除值，当调用glClear(GL_COLOR_BUFFER_BIT)时才真正用设定的颜色值清除color buffer。参数值的范围为:0~1。
void glClearColor( GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
2）glClearIndex
指定color index buffer清除值。void glClearIndex( GLfloat c);
3）glClearDepth
指定depth buffer的清除值，取值范围为:0~1，默认值为1。
void glClearDepth( GLclampd depth);
4）glClearStencil
指定stencil buffer清除值的索引，初始值为0。void glClearStencil( GLint s);
5）glClearAccum
指定accumulation buffer的清除值，初始值为0，取值范围为：-1~1
void glClearAccum( GLfloat red,GLfloat green,GLfloat blue,GLfloat alpha);
4.2 glUseProgram
安装一个program object，并把它作为当前rendering state的一部分。
1) 当一个可执行程序被安装到vertex processor，下列OpenGL固定功能将被disable:
- The modelview matrix is not applied to vertex coordinates.
- The projection matrix is not applied to vertex coordinates.
- The texture matrices are not applied to texture coordinates.
- Normals are not transformed to eye coordinates.
- Normals are not rescaled or normalized.
- Normalization of GL_AUTO_NORMAL evaluated normals is not performed.
- Texture coordinates are not generated automatically.
- Per-vertex lighting is not performed.
- Color material computations are not performed.
- Color index lighting is not performed.
- This list also applies when setting the current raster position.
2) 当一个可执行程序被安装到fragment processor，下列OpenGL固定功能将被disable:
- Texture environment and texture functions are not applied.
- Texture application is not applied.
- Color sum is not applied.
- Fog is not applied.
4.3 glVertexAttribPointer
定义一个通用顶点属性数组。当渲染时，它指定了通用顶点属性数组从索引index处开始的位置和数据格式。其定义如下：
void glVertexAttribPointer(
GLuint index, // 指示将被修改的通用顶点属性的索引
GLint size, // 指点每个顶点元素个数(1~4)
GLenum type, // 数组中每个元素的数据类型
GLboolean normalized, //指示定点数据值是否被归一化(归一化<[-1,1]或[0,1]>：GL_TRUE,直接使用:GL_FALSE)
GLsizei stride, // 连续顶点属性间的偏移量，如果为0，相邻顶点属性间紧紧相邻
const GLvoid * pointer);//顶点数组
//注:其index应该小于#define GL_MAX_VERTEX_ATTRIBS 0x8869
4.4 glEnableVertexAttribArray
Enable由索引index指定的通用顶点属性数组。
void glEnableVertexAttribArray( GLuint index);
void glDisableVertexAttribArray( GLuint index);
默认状态下，所有客户端的能力被disabled，包括所有通用顶点属性数组。如果被Enable，通用顶点属性数组中的值将被访问并被用于rendering，通过调用顶点数组命令：glDrawArrays, glDrawElements, glDrawRangeElements, glArrayElement,
glMultiDrawElements, or glMultiDrawArrays.
4.5 glDrawArrays
void glDrawArrays( GLenum mode,
GLint first,
GLsizei count);
1) mode:指明render原语，如：GL_POINTS,
GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_TRIANGLE_STRIP,
GL_TRIANGLE_FAN, GL_TRIANGLES, GL_QUAD_STRIP, GL_QUADS, 和 GL_POLYGON。
2) first: 指明Enable数组中起始索引。
3) count: 指明被render的原语个数。
可以预先使用单独的数据定义vertex、normal和color，然后通过一个简单的glDrawArrays构造一系列原语。当调用
glDrawArrays时，它使用每个enable的数组中的count个连续的元素，来构造一系列几何原语，从第first个元素开始。
4.6 eglSwapBuffers
把EGL surface中的color buffer提交到native window进行显示。
EGLBoolean eglSwapBuffers(EGLDisplay display,EGLSurface surface)
5. 协调组织
在前面的描述中，三步曲已经完成了：
1）初始化EGL环境，为绘图做好准备
2）生成Program
3）安装并执行Program
只有这三个关键人物，还不能运行，还需要一个协调组织者。其参考代码如下：
int main(int argc, char** argv)
{
ESContext esContext;
UserData userData;
int iFrames;
unsigned long iStartTime,iEndTime;
int iDeltaTime;
memset( &esContext, , sizeof( ESContext) );
esContext.userData = &userData;
esContext.width = ;
esContext.height = ;
// Init EGL display, surface and context
if(!InitEGL(&esContext))
{
printf("Init EGL fail\n");
return GL_FALSE;
}
// compile shader, link program
if(!CreateProgram(&esContext))
{
printf("Create Program fail\n");
return GL_FALSE;
}
iStartTime = GetCurTime();
iFrames = ;
)
{ // render a frame
Render(&esContext);
iFrames++;
iEndTime = GetCurTime();
iDeltaTime = iEndTime - iStartTime;
)
{
iStartTime = iEndTime;
float fFrame = iFrames * 1000.0 / iDeltaTime;
iFrames = ;
printf("Frame: %f\n", fFrame);
}
}
glDeleteProgram (esContext.userData->programObject);
return GL_TRUE;
}