将OpenGL代码移植成OpenGLES
首先是一些基础概念和经验分享：
1、OpenGL是一个跨平台的图形渲染标准
2、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。
OpenGL ES 是从 OpenGL 裁剪定制而来的，去除了 glBegin/glEnd，四边形（GL_QUADS）、多边形（GL_POLYGONS）等复杂图元等许多非绝对必要的特性。现在主要有两个版本，OpenGL ES 1.1针对固定管线硬件的，是以 OpenGL 1.5 规范为基础的。OpenGL ES 2.x 针对可编程管线硬件，参照 OpenGL 2.0 规范定义。
3、egl提供了opengles和本地窗口之间的关联
4、OpenGLES模拟器，这里的模拟器仅仅是一套api和dll（要与android模拟器区分开）。主要用于非嵌入式平台（如windows）下运行opengles代码。
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1、头文件：
#if !defined(HAVE_GLES) #include <GL/gl.h> #else #include <GLES/gl.h> #endif
2、windows下可以使用eglport(http://sourceforge.net/p/eglport/code-0/3/tree/trunk/)，android和ios下各有其对应实现，如果使用了SDL的话，基础框架如下：
#if !defined(HAVE_GLES)
#include <GL/gl.h>
#else
#include <GLES/gl.h>
#include "eglport.h"
#endif
#include <SDL/SDL.h>
int main( void )
{
// other stuff here
SDL_Init( SDL_INIT_VIDEO );
#if defined(HAVE_GLES)
if (EGL_Open())
exit(1);
#endif
// some more initialisations
SDL_Surface* screen;
#if !defined(HAVE_GLES)
screen = SDL_SetVideoMode( 800, 600, 0, SDL_HWSURFACE | SDL_OPENGL | SDL_FULLSCREEN);
#else
screen = SDL_SetVideoMode( 800, 480, 0, SDL_SWSURFACE | SDL_FULLSCREEN );
EGL_Init();
#endif
// the event loop
while( quit == 0 ) {
// management of the even and the game
#if !defined(HAVE_GLES)
SDL_GL_SwapBuffers();
#else
EGL_SwapBuffers();
#endif
}
// probably some more clean-ups here
#if defined(HAVE_GLES)
EGL_Close();
#endif
SDL_Quit();
return 0;
}
注意EGL_xxx系列函数的位置。
3、一些宏定义：
gles只支持浮点型
#define GLdouble GLfloat #define GL_CLAMP GL_CLAMP_TO_EDGE #define glClearDepth glClearDepthf #define glOrtho glOrthof #define glFrustum glFrustumf
#define glColor4fv(a) glColor4f(a[0], a[1], a[2], a[3]) #define glColor3fv(a) glColor4f(a[0], a[1], a[2], 1.0f) #define glColor3f(a,b,c) glColor4f(a, b, c, 1.0f)
4、具体代码移植示例，因为gles去除了glBegin等函数，所以贴图、顶点绘制、画线等代码都有可能需要进行修改
a、没有颜色的普通矩形：
#if !defined(HAVE_GLES)
glBegin(GL_QUADS);
glVertex2f(-10,-10);
glVertex2f(10,-10);
glVertex2f(10,10);
glVertex2f(-10,10);
glEnd();
#else
GLfloat q3[] = {
-10,-10,
10,-10,
10,10,
-10,10
};
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, 0, q3);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_VERTEX_ARRAY);
#endif
b、贴图
glBindTexture(GL_TEXTURE_2D, carac->TextureName);
#if !defined(HAVE_GLES)
glBegin(GL_QUADS);
glTexCoord2f(0,0);
glVertex3f(pos[0]-tailleX/2, pos[1]-tailleY/2, 0);
glTexCoord2f(1,0);
glVertex3f(pos[0]+tailleX/2, pos[1]-tailleY/2, 0);
glTexCoord2f(1,1);
glVertex3f(pos[0]+tailleX/2, pos[1]+tailleY/2, 0);
glTexCoord2f(0,1);
glVertex3f(pos[0]-tailleX/2, pos[1]+tailleY/2, 0);
glEnd();
#else
GLfloat vtx1[] = {
pos[0]-tailleX/2, pos[1]-tailleY/2, 0,
pos[0]+tailleX/2, pos[1]-tailleY/2, 0,
pos[0]+tailleX/2, pos[1]+tailleY/2, 0,
pos[0]-tailleX/2, pos[1]+tailleY/2, 0
};
GLfloat tex1[] = {
0,0,
1,0,
1,1,
0,1
};
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, vtx1);
glTexCoordPointer(2, GL_FLOAT, 0, tex1);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
#endif
c、颜色渐变：
#if !defined(HAVE_GLES)
glBegin(GL_QUADS);
glColor3d( .1, .1, .7);
glVertex3d(0, 0, 0);
glVertex3d(0, -pbarheight, 0);
glColor3d( 0, 0, 0.5);
glVertex3d((startupProgress / startupProgressSteps) * pbarwidth, -pbarheight, 0);
glVertex3d((startupProgress / startupProgressSteps) * pbarwidth, 0, 0);
glEnd();
#else
GLfloat vtx1[] = {
0, 0, 0,
0, -pbarheight, 0,
(startupProgress / startupProgressSteps) * pbarwidth, -pbarheight, 0,
(startupProgress / startupProgressSteps) * pbarwidth, 0, 0
};
GLfloat col1[] = {
.1, .1, .7, 1.0f,
.1, .1, .7, 1.0f,
0, 0, 0.5, 1.0f,
0, 0, 0.5, 1.0f
};
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, vtx1);
glColorPointer(4, GL_FLOAT, 0, col1);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
#endif
d、三角形扇面：
#if !defined(HAVE_GLES)
glBegin(GL_QUAD_STRIP);
// Front
glVertex3f(-left, 0, front); // bottom left
glVertex3f(-left-wider, height, front+wider); // top left
glVertex3f( right, 0, front); // bottom right
glVertex3f( right+wider, height, front+wider); // top right
// Right
glVertex3f( right, 0,-back); // bottom r
glVertex3f( right+wider, height,-back-wider); // top r
// Back
glVertex3f(-left, 0, -back); // bottom right
glVertex3f(-left-wider, height, -back-wider); // top right
// Left
glVertex3f(-left, 0, front); // bottom r
glVertex3f(-left-wider, height, front+wider); // top r
glEnd();
#else
{
GLfloat vtx1[] = {-left, 0, front, -left-wider, height, front+wider, right, 0, front, right+wider, height, front+wider};
GLfloat vtx2[] = {right, 0, front, right+wider, height, front+wider, right, 0,-back, right+wider, height,-back-wider};
GLfloat vtx3[] = {right, 0,-back, right+wider, height,-back-wider, -left, 0, -back, -left-wider, height, -back-wider};
GLfloat vtx4[] = {-left, 0, -back, -left-wider, height, -back-wider, -left, 0, front, -left-wider, height, front+wider};
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, vtx1);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glVertexPointer(3, GL_FLOAT, 0, vtx2);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glVertexPointer(3, GL_FLOAT, 0, vtx3);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glVertexPointer(3, GL_FLOAT, 0, vtx4);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_VERTEX_ARRAY);
}
#endif
e、使用int/short
#if !defined(HAVE_GLES)
glBegin(GL_QUADS);
glTexCoord2i(0, 1); glVertex2i(-256, -256);
glTexCoord2i(0, 0); glVertex2i(-256, 256);
glTexCoord2i(1, 0); glVertex2i(256, 256);
glTexCoord2i(1, 1); glVertex2i(256, -256);
glEnd();
#else
GLshort vtx1[] = { -256, -256, -256, 256, 256, 256, 256, -256 };
GLshort tex1[] = { 0, 1, 0, 0, 1, 0, 1, 1 };
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glVertexPointer(2, GL_SHORT, 0, vtx1);
glTexCoordPointer(2, GL_SHORT, 0, tex1);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
#endif
f、glRect
#ifndef HAVE_GLES
glRecti ( dx + min[0], dy + min[1],
dx + max[0], dy + max[1] ) ;
#else
GLshort vtx1[] = { dx + min[0], dy + min[1], dx + min[0], dy + max[1], dx + max[0], dy + max[1], dx + max[0], dy + min[1]};
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_SHORT, 0, vtx1);
glDrawArrays(GL_TRIANGLE_FAN,0,4);
glDisableClientState(GL_VERTEX_ARRAY);
#endif
5、glDrawArrays渲染模式对应关系
GL_POINTS -> GL_POINTS GL_TRIANGLES -> GL_TRIANGLES (or GL_LINE_LOOP if glPolygonMode is set to GL_LINE) GL_QUADS -> GL_TRIANGLE_FAN GL_LINE_STRIP -> GL_LINE_STRIP GL_LINE -> GL_LINE GL_POLYGON -> GL_TRIANGLE_FAN
6、所有贴图必须是2的整次幂，某些特殊情况会有更高的需求（如powervr显卡使用pvr贴图必须是正方形以及2的整次幂大小）。