Question

我想在GL中绘制的内容上绘制文字。我希望符号本身是不透明的，而其余部分是透明的，允许看到绘制的内容。下面的代码生成一个文本，这是正确的文本，但具有完美的白色背景。我绘制的内容完全不存在。

我如何解决这个问题？我正在使用SDL 2.0，VSC

   glPushMatrix();

   /*Content drawn in GL*/

   GLuint TextureID = 0;
   SDL_Color Color = {30, 30, 30, 0};
   TTF_Font * Font = TTF_OpenFont("Times.ttf", 30);
   SDL_Surface * Message = TTF_RenderText_Blended(Font, "ASDASDASD", Color);

   glGenTextures(1, &TextureID);
   glBindTexture(GL_TEXTURE_2D, TextureID);

   glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, Message->w, Message->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, Message->pixels);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

   glBindTexture(GL_TEXTURE_2D, TextureID);

   glBegin(GL_QUADS);
   {
        glColor4f(.5, .5, .5, 1);
        glTexCoord2f(0,0); glVertex2f(MouseX/10, MouseY/10);
        glTexCoord2f(1,0); glVertex2f((MouseX/10) + (Message->w / 10), MouseY/10);
        glTexCoord2f(1,1); glVertex2f((MouseX/10) + (Message->w / 10), (MouseY/10) + (Message->h / 10));
        glTexCoord2f(0,1); glVertex2f(MouseX/10, (MouseY/10) + (Message->h / 10));
   }
   glEnd();

   glPopMatrix();

   SDL_FreeSurface(Message);
   SDL_GL_SwapWindow(GameWindow);

Answer 1

以这种方式思考：当您在上面创建纹理时，您正在针对dark grey fully transparent（0,0,0,0）绘制fully transparent background（30,30,30,0）文本由于TTF_RenderText_Blended）。然后你使用立即模式颜色混合来光栅相同大小的四边形（提示：没有混合功能，也没有提到z缓冲区禁用！）。

P.S。你在例子中疯狂地泄漏：

glGenTexture每一帧没有glDeleteTextures（你只需要一个新的，如果它改变了）
每个都没有TTF_CloseFont的TTF_OpenFont 框架（仅在您不需要渲染该字体之前保留它）
TTF_RenderText_Blended表面没有泄漏，但您可以立即释放它 glTexImage2D

我认为您是否意味着使用字体作为蒙版从纯色背景中减去？我通常创建白色字体纹理（完整alpha）然后在着色器中应用颜色或蒙版。要在即时模式下实现类似效果，您可以：

// startup
glEnable(GL_BLEND);
glDisable(GL_DEPTH_TEST);

SDL_Color Color = {255, 255, 255, 255};
TTF_Font * Font = TTF_OpenFont("Times.ttf", 30);
SDL_Surface * Message = TTF_RenderText_Blended(Font, "ASDASDASD", Color);
TTF_CloseFont(Font);

glGenTextures(1, &TextureID);
glBindTexture(GL_TEXTURE_2D, TextureID);

glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, Message->w, Message->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, Message->pixels);
SDL_FreeSurface(Message);

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



// frame render
glClear(GL_COLOR_BUFFER_BIT);

// you'll see this in many (immediate mode) 2D rendering examples
// without the blend eq below, this would render the white text on top of the quad's background color
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// use text as a mask to cut away from what is below
glBlendEquation(GL_FUNC_REVERSE_SUBTRACT);

glBindTexture(GL_TEXTURE_2D, TextureID);


glBegin(GL_QUADS);
{
    glColor4f(.5, .5, .5, 1);
    glTexCoord2f(0,0); glVertex2f(MouseX/10, MouseY/10);
    glTexCoord2f(1,0); glVertex2f((MouseX/10) + (Message->w / 10), MouseY/10);
    glTexCoord2f(1,1); glVertex2f((MouseX/10) + (Message->w / 10), (MouseY/10) + (Message->h / 10));
    glTexCoord2f(0,1); glVertex2f(MouseX/10, (MouseY/10) + (Message->h / 10));
}
glEnd();

// if done every frame, make sure to: glDeleteTextures(1, &TextureID);

SDL_GL_SwapWindow(GameWindow);

非常方便的网站：http://www.andersriggelsen.dk/glblendfunc.php

如何将SDL TTF曲面转换为具有透明背景的GL纹理

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