Question

I’ve been attempting to render text onto an openGL window using SDL and the SDL_TTF library on windows XP, VS2010.
Versions:
SDL version 1.2.14
SDL TTF devel 1.2.10
openGL (version is at least 2-3 years old).

I have successfully created an openGL window using SDL / SDL_image and can render lines / polygons onto it with no problems.

However, moving onto text it appears that there is some flaw in my current program, I am getting the following result when trying this code here

for those not willing to pastebin here are only the crutial code segments:

void drawText(char * text) {
    glLoadIdentity();
    SDL_Color clrFg = {0,0,255,0}; // set colour to blue (or 'red' for BGRA)
    SDL_Surface *sText = TTF_RenderUTF8_Blended( fntCourier, text, clrFg );
    GLuint * texture = create_texture(sText);
    glBindTexture(GL_TEXTURE_2D, *texture);
    // draw a polygon and map the texture to it, may be the source of error
    glBegin(GL_QUADS); {
        glTexCoord2i(0, 0); glVertex3f(0, 0, 0);
        glTexCoord2i(1, 0); glVertex3f(0 + sText->w, 0, 0);
        glTexCoord2i(1, 1); glVertex3f(0 + sText->w, 0 + sText->h, 0);
        glTexCoord2i(0, 1); glVertex3f(0, 0 + sText->h, 0);
    } glEnd();
    // free the surface and texture, removing this code has no effect
    SDL_FreeSurface( sText );
    glDeleteTextures( 1, texture );
}

segment 2:

// create GLTexture out of SDL_Surface
GLuint * create_texture(SDL_Surface *surface) {
    GLuint texture = 0;

    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    // The SDL_Surface appears to have BGR_A formatting, however this ends up with a 
    // white rectangle no matter which colour i set in the previous code.
    int Mode = GL_RGB;

    if(surface->format->BytesPerPixel == 4) {
        Mode = GL_RGBA;
    }

    glTexImage2D(GL_TEXTURE_2D, 0, Mode, surface->w, surface->h, 0, Mode, 
                 GL_UNSIGNED_BYTE, surface->pixels);

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

    return &texture;
}

Is there an obvious bit of code I am missing?
Thank you for any help on this subject.
I’ve been trying to learn openGL and SDL for 3 days now, so please forgive any misinformation on my part.

---

EDIT:

I notice that using
TTF_RenderUTF8_Shaded
TTF_RenderUTF8_Solid

Throw a null pointer exception, meaning that there is an error within the actual text rendering function (I suspect), I do not know how this means TTF_RenderUTF8_Blended returns a red square but I suspect all troubles hinge on this.

Answer 1

I think the problem is in the glEnable(GL_TEXTURE_2D) and glDisable(GL_TEXTURE_2D) functions which must be called every time the text is painted on the screen.And maybe also the color conversion between the SDL and GL surface is not right.
I have combined create_texture and drawText into a single function that displays the text properly. That’s the code:

void drawText(char * text, TTF_Font* tmpfont) {
SDL_Rect area;
SDL_Color clrFg = {0,0,255,0};
SDL_Surface *sText = SDL_DisplayFormatAlpha(TTF_RenderUTF8_Blended( tmpfont, text, clrFg ));
area.x = 0;area.y = 0;area.w = sText->w;area.h = sText->h;
SDL_Surface* temp = SDL_CreateRGBSurface(SDL_HWSURFACE|SDL_SRCALPHA,sText->w,sText->h,32,0x000000ff,0x0000ff00,0x00ff0000,0x000000ff);
SDL_BlitSurface(sText, &area, temp, NULL);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, sText->w, sText->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, temp->pixels);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glEnable(GL_TEXTURE_2D);
glBegin(GL_QUADS); {
    glTexCoord2d(0, 0); glVertex3f(0, 0, 0);
    glTexCoord2d(1, 0); glVertex3f(0 + sText->w, 0, 0);
    glTexCoord2d(1, 1); glVertex3f(0 + sText->w, 0 + sText->h, 0);
    glTexCoord2d(0, 1); glVertex3f(0, 0 + sText->h, 0); 
} glEnd();
glDisable(GL_TEXTURE_2D);
SDL_FreeSurface( sText );
SDL_FreeSurface( temp );
} 

screenshot

I’m initializing OpenGL as follows:

int Init(){
glClearColor( 0.1, 0.2, 0.2, 1);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho( 0, 600, 300, 0, -1, 1 );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
if( glGetError() != GL_NO_ERROR ){
    return false;
}
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_COLOR, GL_ONE_MINUS_SRC_ALPHA);
}