SDL2 + OpenGL + SDL2_TTF: Displaying text

Question

I'm having trouble getting a TTF font to draw in OpenGL plus SDL2.0.

I remember that before SDL version 2 I had no problems, but there seems to be a lack of documentation on the subject, perhaps due to the new standard.

I have included the code below to show generally what I am doing.

This code is very inefficient as I recreate the Texture every frame, take this into consideration if copy+pasting :)

void main_render() {
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  //Set our color back to white (Full texture color)
  glColor3f(1.0f, 1.0f, 1.0f);

  mainCamera->perspective();
  mainCamera->translate();

  //Load the font
  TTF_Font *font = TTF_OpenFont("../resource/font.ttf", 10);
  if (font == nullptr) {
    std::cout << "TTF_OpenFont error: " << std::endl;
    return;
  }

  //Render font to a SDL_Surface
  SDL_Color color = {0,0,255,80};
  SDL_Surface *surface = TTF_RenderText_Blended(font, "Hi!", color);
  if (surface == nullptr) {
    TTF_CloseFont(font);
    std::cout << "TTF_RenderText error: " << std::endl;
    return;
  }

  //Create a SDL_Texture * from the surface
  SDL_Texture * text = SDL_CreateTextureFromSurface(fontRender, surface);
  if (text == nullptr){
    std::cout << "SDL_CreateTextureFromSurface error: " << std::endl;
    return;
  }

  //Bind the SDL_Texture in OpenGL
  SDL_GL_BindTexture(text, NULL, NULL);

  //Draw the SDL_Texture * as a Quad
  glEnable(GL_TEXTURE_2D);
  glBegin(GL_QUADS); {
    glTexCoord2d(0, 0); glVertex3f(0,              0,              0);
    glTexCoord2d(1, 0); glVertex3f(0 + surface->w, 0,              0);
    glTexCoord2d(1, 1); glVertex3f(0 + surface->w, 0 + surface->h, 0);
    glTexCoord2d(0, 1); glVertex3f(0,              0 + surface->h, 0); 
  } glEnd();
  glDisable(GL_TEXTURE_2D);

  //Cleanup
  TTF_CloseFont(font);
  SDL_DestroyTexture(text);
  SDL_FreeSurface(surface);

  //Swap the buffers to refresh the window
  mainWindow->swap_buffers();
}

Tavurth · Accepted Answer · 2015-05-04T21:51:08.970

So OpenGL requires that all textures have dimensions of Base2 on my system (2,4,16,32,64...)

I fixed this problem by doing an incremental search for the closest power of two to the original dimension:

  unsigned int power_two_floor(unsigned int val) {
    unsigned int power = 2, nextVal = power*2;

    while((nextVal *= 2) <= val)
      power*=2;

    return power*2;
  }

I then ran into a snag: The texture was the correct color, yet the pixels were scrambled. This was fixed by copying the image to a RGB SDL_Surface, using the adjusted OpenGL dimensions.

//Find the first power of two for OpenGL image 
  int w = power_two_floor(surface->w)*2;
  int h = power_two_floor(surface->h)*2;

  //Create a surface to the correct size in RGB format, and copy the old image
  SDL_Surface * s = SDL_CreateRGBSurface(0, w, h, 32, 0x00ff0000,0x0000ff00,0x000000ff,0xff000000);
  SDL_BlitSurface(surface, NULL, s, NULL);

Adding filter information was required to correct OpenGL from utilizing mipmaps:

 //Avoid mipmap filtering
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

Here is my answer:

unsigned int power_two_floor(unsigned int val) {
  unsigned int power = 2, nextVal = power*2;
  while((nextVal *= 2) <= val)
    power*=2;
  return power*2;
}

void main_render() {
  //Load the font
  TTF_Font *font = TTF_OpenFont("../resource/font.ttf", 10);
  if (font == nullptr) {
    std::cout << "TTF_OpenFont error: " << std::endl;
    return;
  }

  SDL_Color colorFg = {0,0,255};
  SDL_Surface *surface;

  //Render font to a SDL_Surface
  if ((surface = TTF_RenderText_Blended(font, "Hi!", colorFg)) == nullptr) {
    TTF_CloseFont(font);
    std::cout << "TTF_RenderText error: " << std::endl;
    return;
  }

  GLuint texId;

  //Generate OpenGL texture
  glEnable(GL_TEXTURE_2D);
  glGenTextures(1, &texId);
  glBindTexture(GL_TEXTURE_2D, texId);

  //Avoid mipmap filtering
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

  //Find the first power of two for OpenGL image 
  int w = power_two_floor(surface->w)*2;
  int h = power_two_floor(surface->h)*2;

  //Create a surface to the correct size in RGB format, and copy the old image
  SDL_Surface * s = SDL_CreateRGBSurface(0, w, h, 32, 0x00ff0000,0x0000ff00,0x000000ff,0xff000000);
  SDL_BlitSurface(surface, NULL, s, NULL);

  //Copy the created image into OpenGL format
  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, s->pixels);

  //Draw the OpenGL texture as a Quad
  glBegin(GL_QUADS); {
    glTexCoord2d(0, 1); glVertex3f(0,              0,              0);
    glTexCoord2d(1, 1); glVertex3f(0 + surface->w, 0,              0);
    glTexCoord2d(1, 0); glVertex3f(0 + surface->w, 0 + surface->h, 0);
    glTexCoord2d(0, 0); glVertex3f(0,              0 + surface->h, 0); 
  } glEnd();
  glDisable(GL_TEXTURE_2D);

  //Cleanup
  TTF_CloseFont(font);
  SDL_FreeSurface(s);
  SDL_FreeSurface(surface);
  glDeleteTextures(1, &texId);
}

SDL2 + OpenGL + SDL2_TTF: Displaying text

1 Answers1