opengl|es - 2D绘制

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本文关注的是基于SDL的OPENGL|ES(下文简称ES)的2D绘制的实现方式。

ES中常见的实现2D的方法是使用正投影，然后绘制2D纹理

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glDisable( GL_DEPTH_TEST ) ;
glMatrixMode( GL_PROJECTION ) ;
glLoadIdentity() ;
/*
* Upside-down square viewport: it maps the screen as if the (arbitrary-set) resolution were
* 1000x1000 pixels.
*
*/
glOrtho( /* left */ 0, /* right */ 1000, /* bottom */ 1000, /* top */ 0,
/* near */ 0, /* far */ 1 ) ;
- or, preferably to keep the 4/3 ratio like 800x600, 640x480, 1024x768, etc. -
// Non-reversed 4/3 viewport:
glOrtho( /* left */ -320.0f, /* right */ 320.0f, /* bottom */ -240.0f, /* top */ 240.0f,
/* near */ -1, /* far */ 1 )

glDisable( GL_DEPTH_TEST ) ;glMatrixMode( GL_PROJECTION ) ;glLoadIdentity() ;/* * Upside-down square viewport: it maps the screen as if the (arbitrary-set) resolution were * 1000x1000 pixels. * */glOrtho( /* left */ 0, /* right */ 1000, /* bottom */ 1000, /* top */ 0, /* near */ 0, /* far */ 1 ) ;- or, preferably to keep the 4/3 ratio like 800x600, 640x480, 1024x768, etc. -// Non-reversed 4/3 viewport:glOrtho( /* left */ -320.0f, /* right */ 320.0f, /* bottom */ -240.0f, /* top */ 240.0f, /* near */ -1, /* far */ 1 )

这里有三个问题，

其一SDL与ES的坐标系统不同，SDL的（0，0）在屏幕的左上角，而ES的在屏幕的左下角，这里可以使用上下反转的正投影来解决，或者在每次绘制前使用如下的变换，

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glMatrixMode( GL_TEXTURE ) ;
glLoadIdentity() ;
glScalef( 1, -1, 1 ) ;

glMatrixMode( GL_TEXTURE ) ; glLoadIdentity() ; glScalef( 1, -1, 1 ) ;

其二OPENGL认为自己拥有整个屏幕，对于显示缓存，只能使用OPENGL进行控制

其三在OPENGL与SDL之间的BLIT，当SDL选择OPENGL模式的时候（OPENGLBLIT模式是不被推荐的）是不能之间进行的，这里使用了技巧，是将SDL的缓存转换成了OPENGL的材质，然后供OPENGL的2D显示使用，如下

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/* Use the surface width and height expanded to powers of 2，OPENGL 的纹理大小必须是2的倍数 */
w = gy_powerOfTwo(surface->w);
h = gy_powerOfTwo(surface->h);
texcoord[0] = 0; /* Min X */
texcoord[1] = 0; /* Min Y */
texcoord[2] = surface->w / (gy_real32)w; /* Max X */
texcoord[3] = surface->h / (gy_real32)h; /* Max Y */
image = SDL_CreateRGBSurface(
SDL_SWSURFACE,
w, h,
32,
#if SDL_BYTEORDER != SDL_BIG_ENDIAN /* OpenGL RGBA masks */
0x000000ff,
0x0000ff00,
0x00ff0000,
0xff000000
#endif
);
if ( image == NULL ) {
return 0;
}
/* Save the alpha blending attributes */
saved_flags = surface->flags&(SDL_SRCALPHA|SDL_RLEACCELOK);
saved_alpha = surface->format->alpha;
if ( (saved_flags & SDL_SRCALPHA) == SDL_SRCALPHA ) {
}
SDL_SetAlpha(surface, SDL_SRCALPHA, saved_alpha);//记得在正投影后设置纹理的环境时，使用GL_MODULATE模式
/* Copy the surface into the GL texture image */
area.x = 0;
area.y = 0;
area.w = surface->w;
area.h = surface->h;
SDL_BlitSurface(surface, &area, image, &area);
/* Restore the alpha blending attributes */
if ( (saved_flags & SDL_SRCALPHA) == SDL_SRCALPHA ) {
SDL_SetAlpha(surface, saved_flags, saved_alpha);
}
/* Create an OpenGL texture for the image */
glGenTextures(1, &texture); //生成纹理
glBindTexture(GL_TEXTURE_2D, texture); //选择纹理
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexImage2D(
GL_TEXTURE_2D,
0,
GL_RGBA,
w, h,
0,
GL_RGBA,
GL_UNSIGNED_BYTE/*每种颜色8位*/,
image->pixels);
SDL_FreeSurface(image); /* No longer needed */

/* Use the surface width and height expanded to powers of 2，OPENGL 的纹理大小必须是2的倍数 */w = gy_powerOfTwo(surface->w);h = gy_powerOfTwo(surface->h); texcoord[0] = 0;/* Min X */texcoord[1] = 0;/* Min Y */texcoord[2] = surface->w / (gy_real32)w;/* Max X */texcoord[3] = surface->h / (gy_real32)h;/* Max Y */image = SDL_CreateRGBSurface(SDL_SWSURFACE,w, h,32,#if SDL_BYTEORDER != SDL_BIG_ENDIAN /* OpenGL RGBA masks */ 0x000000ff,0x0000ff00,0x00ff0000,0xff000000#endif);if ( image == NULL ) {return 0;} /* Save the alpha blending attributes */ saved_flags = surface->flags&(SDL_SRCALPHA|SDL_RLEACCELOK); saved_alpha = surface->format->alpha; if ( (saved_flags & SDL_SRCALPHA) == SDL_SRCALPHA ) { }SDL_SetAlpha(surface, SDL_SRCALPHA, saved_alpha);//记得在正投影后设置纹理的环境时，使用GL_MODULATE模式 /* Copy the surface into the GL texture image */ area.x = 0; area.y = 0; area.w = surface->w; area.h = surface->h; SDL_BlitSurface(surface, &area, image, &area); /* Restore the alpha blending attributes */ if ( (saved_flags & SDL_SRCALPHA) == SDL_SRCALPHA ) { SDL_SetAlpha(surface, saved_flags, saved_alpha); }/* Create an OpenGL texture for the image */glGenTextures(1, &texture); //生成纹理glBindTexture(GL_TEXTURE_2D, texture); //选择纹理glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,w, h,0,GL_RGBA,GL_UNSIGNED_BYTE/*每种颜色8位*/,image->pixels);SDL_FreeSurface(image); /* No longer needed */

最后值得注意的是，ES在纹理绘制时选择使用glDrawElements函数，其最后一个参数是索引值，由于OPENGL是状态机驱动的，整个坐标系统是提前将状态设置好的，不用总是在困扰与这个东西是如何绘制出来，呵呵，如下是glDrawElements的使用方式

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glBindTexture(GL_TEXTURE_2D, uiTexture);
glEnableClientState( GL_VERTEX_ARRAY );
glEnableClientState( GL_NORMAL_ARRAY );
glEnableClientState( GL_TEXTURE_COORD_ARRAY );
glNormalPointer( GL_FLOAT, 0, rnormal );
glVertexPointer(3, GL_FLOAT, 0, vertex); //注意纹理坐标体系和点坐标体系是不同的
glTexCoordPointer(2,GL_FLOAT,0,textures );
glDrawElements( GL_TRIANGLES,6,GL_UNSIGNED_BYTE,trivert);
glDisableClientState( GL_TEXTURE_COORD_ARRAY );
glDisableClientState( GL_VERTEX_ARRAY );

glBindTexture(GL_TEXTURE_2D, uiTexture);glEnableClientState( GL_VERTEX_ARRAY );glEnableClientState( GL_NORMAL_ARRAY );glEnableClientState( GL_TEXTURE_COORD_ARRAY );glNormalPointer( GL_FLOAT, 0, rnormal ); glVertexPointer(3, GL_FLOAT, 0, vertex); //注意纹理坐标体系和点坐标体系是不同的glTexCoordPointer(2,GL_FLOAT,0,textures );glDrawElements( GL_TRIANGLES,6,GL_UNSIGNED_BYTE,trivert); glDisableClientState( GL_TEXTURE_COORD_ARRAY );glDisableClientState( GL_VERTEX_ARRAY );