c++ - OpenGL 自由类型文本，y 偏移到字符的最上边界

Question

在 OpenGL 中绘制文本我使用的是 freetype。我现在已经成功渲染文本，但我仍然遇到字符未指向正确的 y 坐标的问题(这些字符不是平面的)。

看下面的图片，我制作了“正在加载...”文本。

如您所见，y 坐标的位置不正确。这让我很困惑。这是我所做的代码:

class FontBitmap{
    private: 
    struct character_info {
      float bw; // bitmap.width;
      float bh; // bitmap.rows;
    } c[256];  
    int next_p2 (int a )
    {
        int rval=1;
        while(rval<a) rval<<=1;
        return rval;
    }
      GLuint  texture[256]; //character textures
    public:
      FT_Library  library;   /* handle to library     */
      FT_Face     face;      /* handle to face object */
      FontBitmap(){}

      void Print(char * str,GLfloat x,GLfloat y){ //the function where to render the text
          glEnable(GL_TEXTURE_2D);
          glEnable(GL_BLEND);
          glPushMatrix();
          glTranslatef(x,y,0.0f);
          for(int i = 0;i < strlen(str);i++)
          {       
              glPushMatrix(); 
              glBindTexture(GL_TEXTURE_2D,texture[str[i]]);
              glTranslatef(x,0,0);  //the placement of per character
              glBegin(GL_QUADS);
              glTexCoord2f(0,0); glVertex3f(0,0,0.0f);
              glTexCoord2f(1,0);glVertex3f(c[str[i]].bw,0,0.0f);
              glTexCoord2f(1,1); glVertex3f(c[str[i]].bw, c[str[i]].bh ,0.0f);
              glTexCoord2f(0,1); glVertex3f(0, c[str[i]].bh,0.0f);            
              glEnd();
              x +=c[str[i]].bw;
              glPopMatrix();
          }
          glPopMatrix();
          glDisable(GL_TEXTURE_2D);
          glDisable(GL_BLEND);
      }

        FontBitmap(char * filePath, int size) //the constructor
        {
            glEnable(GL_TEXTURE_2D);    

          if (FT_Init_FreeType (&library)) 
          {
              //on error
              return;
          }
         if (FT_New_Face(library,filePath,0,&face))
         {
             //on error
             return;
         } 
         FT_Set_Pixel_Sizes (face, 0, size);
         for (unsigned long i = 33; i < 256; i++) 
         {
           if(FT_Load_Char(face, i, FT_LOAD_RENDER))
           {
               //on error;
               return;
           }
           int width = next_p2( face->glyph->bitmap.width );
           int height = next_p2( face->glyph->bitmap.rows );

           c[i].bw = width;
           c[i].bh = height;

            GLubyte* expanded_data = new GLubyte[ 2 * width * height];

            for(int j=0; j <height;j++) {
                    for(int i=0; i < width; i++){
                            expanded_data[2*(i+j*width)]= expanded_data[2*(i+j*width)+1] = 
                                    (i>=face->glyph->bitmap.width || j>=face->glyph->bitmap.rows) ?
                                    0 : face->glyph->bitmap.buffer[i + face->glyph->bitmap.width*j];
                    }
            }
            glGenTextures(1,&texture[i]);
            glBindTexture( GL_TEXTURE_2D, texture[i]);
            glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
            glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
        glPixelStorei(GL_UNPACK_ALIGNMENT,1);
            glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, width, height,
                      0, GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, expanded_data );
            delete [] expanded_data;
         }

        }

    };

Answer 1

关于您的问题，Ascent 和 Descent 是您感兴趣的两个 FreeType 指标。

字符之间的水平间距由 Advance 和 Kerning 控制