OpenGL、SDL_ttf

Question

我在通过 SDL_ttf 在 OpenGL 中显示文本时遇到问题。我编写了 LoadFont() 函数，它加载字体并制作 OpenGL 纹理。

int LoadFont( char *name, GLubyte r, GLubyte g, GLubyte b, char *text, int ptsize)
{
SDL_Color color = {r, g, b, 0};
TTF_Font *font = TTF_OpenFont(name, ptsize);

Uint32 rmask, gmask, bmask, amask;
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
    rmask = 0xff000000;
    gmask = 0x00ff0000;
    bmask = 0x0000ff00;
    amask = 0x000000ff;
#else
    rmask = 0x000000ff;
    gmask = 0x0000ff00;
    bmask = 0x00ff0000;
    amask = 0xff000000;
#endif

SDL_Surface *msg = TTF_RenderText_Blended(font, text, color);

SDL_SetAlpha(msg, 0, 0);

if(!msg) {
    printf("Unable to Render text\n");
    return -1;
}

SDL_Surface *tmp = SDL_CreateRGBSurface(0, msg->w, msg->h, 32, rmask, gmask, bmask, amask);

SDL_BlitSurface(msg, NULL, tmp, NULL);


glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);


glGenTextures(1, &texture[0]);
glBindTexture(GL_TEXTURE_2D, texture[0]);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, msg->w, msg->h, 0, GL_RGBA,
             GL_UNSIGNED_BYTE, msg->pixels);


return 1;
}

我在 initGL() 中使用这个函数:

int initGL( GLvoid )
{
glViewport(0, 0, SCREEN_WIDHT, SCREEN_HEIGHT);
LoadFont("THWACK.TTF", 255, 255, 255, "Hello!", 14);
glEnable(GL_TEXTURE_2D);
glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
glClearDepth(1.0);
glDepthFunc(GL_LESS);
glEnable(GL_DEPTH_TEST);
glShadeModel(GL_SMOOTH);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();

gluPerspective(45.0f, (GLfloat)SCREEN_WIDHT/(GLfloat)SCREEN_HEIGHT, 0.1f, 100.0f);

glMatrixMode(GL_MODELVIEW);

return TRUE;
}

此外，我尝试在多边形上显示此文本:

int drawGLScene( GLvoid )
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();

glTranslatef(0.0f, 0.0f, -5.0f);

glBindTexture(GL_TEXTURE_2D, texture[0]);

glEnable(GL_TEXTURE_2D);
glBegin(GL_POLYGON);
    glTexCoord2f(0.0f, 0.0f); glVertex2f(-1.0f, -1.0f);
    glTexCoord2f(1.0f, 0.0f); glVertex2f(1.0f, -1.0f);
    glTexCoord2f(1.0f, 1.0f); glVertex2f(1.0f, 1.0f);
    glTexCoord2f(0.0f, 1.0f); glVertex2f(-1.0f, 1.0f);
glEnd();
glDisable(GL_TEXTURE_2D);

SDL_GL_SwapBuffers();

return TRUE;
}

但是文本不显示，只有白色多边形。

Answer 1

GL_TEXTURE_MIN_FILTER 默认为 GL_NEAREST_MIPMAP_LINEAR。

如果您请求 mipmap 但不上传完整的 mipmap，您将获得白色纹理。

你有这个:

// disable mipmapping on default texture
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

// create new texture, with default filtering state (==mipmapping on)
glGenTextures(1, &texture[0]);
glBindTexture(GL_TEXTURE_2D, texture[0]);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, msg->w, msg->h, 0, GL_RGBA,
             GL_UNSIGNED_BYTE, msg->pixels);

尝试以下顺序:

// create new texture, with default filtering state (==mipmapping on)
glGenTextures(1, &texture[0]);
glBindTexture(GL_TEXTURE_2D, texture[0]);

// disable mipmapping on the new texture
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, msg->w, msg->h, 0, GL_RGBA,
             GL_UNSIGNED_BYTE, msg->pixels);

纹理过滤状态属于纹理对象而不是纹理单元，因此 glTexParameterf() 位于 glBindTexture() 之后:

OpenGL 2.1 spec ，第 3.8.11 节(第 180 页)和第 3.8.12 节(第 182 页):

3.8.11: ...Next, there are the two sets of texture properties; each consists of the selected minification and magnification filters...

3.8.12: ...The resulting texture object is a new state vector, comprising all the state values listed in section 3.8.11, set to the same initial values...

<小时/>

编辑:如果您想使用TTF_Render*_Blished()，您还必须在渲染之前启用混合:

glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );

glColor3ub( 255, 255, 255 );
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, stringTex );
glBegin(GL_QUADS);
// ...

示例:

#include <SDL.h>
#include <SDL_ttf.h>
#include <SDL_opengl.h>

GLuint TextToTexture( TTF_Font* font, GLubyte r, GLubyte g, GLubyte b, const char* text, int ptsize )
{
    SDL_Color color = { r, g, b };
    SDL_Surface* msg = TTF_RenderText_Blended( font, text, color );

    // create new texture, with default filtering state (==mipmapping on)
    GLuint tex;
    glGenTextures( 1, &tex );
    glBindTexture( GL_TEXTURE_2D, tex );

    // disable mipmapping on the new texture
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
    glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, msg->w, msg->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, msg->pixels );

    SDL_FreeSurface( msg );
    return tex;
}

GLuint stringTex = 0;
void drawGLScene( int winWidth, int winHeight )
{
    glViewport(0, 0, winWidth, winHeight );

    glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
    glClearDepth(1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0f, winWidth / (float)winHeight, 0.1f, 100.0f);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0.0f, 0.0f, -5.0f);

    // this is where the magic happens
    glEnable( GL_BLEND );
    glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );

    glColor3ub( 255, 255, 255 );
    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, stringTex );
    glBegin(GL_QUADS);
        glTexCoord2f( 0.0f, 0.0f ); glVertex2f( -1.0f, -1.0f );
        glTexCoord2f( 1.0f, 0.0f ); glVertex2f(  1.0f, -1.0f );
        glTexCoord2f( 1.0f, 1.0f ); glVertex2f(  1.0f,  1.0f );
        glTexCoord2f( 0.0f, 1.0f ); glVertex2f( -1.0f,  1.0f );
    glEnd();
    glDisable(GL_TEXTURE_2D);
}

int main( int argc, char *argv[] )
{ 
    SDL_Init( SDL_INIT_EVERYTHING );
    SDL_Surface* display = SDL_SetVideoMode( 640, 480, 32, SDL_OPENGL );

    if( -1 == TTF_Init() )
        return -1;

    // http://unifoundry.com/unifont.html
    TTF_Font *font = TTF_OpenFont( "unifont.ttf", 14 );
    stringTex = TextToTexture( font, 255, 255, 255, "Hello!", 14 );

    glDepthFunc(GL_LESS);
    glEnable(GL_DEPTH_TEST);
    glShadeModel(GL_SMOOTH);

    bool running = true;
    while( running )
    {
        SDL_Event event;
        while( SDL_PollEvent( &event ) )
        {
            if( event.type == SDL_QUIT )
            {
                running = false;
                break;
            }
        }

        drawGLScene( display->w, display->h );
        SDL_GL_SwapBuffers();
        SDL_Delay( 16 );
    }

    TTF_CloseFont(font);

    TTF_Quit();
    SDL_Quit();
    return 0;
}