Billboard (Биллборд/Спрайт) (комментарии)

void gld_StartDrawScene(void)
{
  inv_yaw=180.0f-yaw;
  cos_inv_yaw = (float)cos(inv_yaw * M_PI / 180.f);
  sin_inv_yaw = (float)sin(inv_yaw * M_PI / 180.f);
  billboard_pitch = ((pitch > 87.0f && pitch <= 90.0f) ? 87.0f : pitch);
  cos_billboard_pitch = (float)cos(billboard_pitch * M_PI / 180.f);
  sin_billboard_pitch = (float)sin(billboard_pitch * M_PI / 180.f);
}

void gld_DrawSprite(GLSprite *sprite)
{
    float x1, x2, x3, x4, z1, z2, z3, z4;
    float y1, y2, cy, ycenter, y1c, y2c;
    float y1z2_y, y2z2_y;

    ycenter = (float)fabs(sprite->y1 - sprite->y2) * 0.5f;
    y1c = sprite->y1 - ycenter;
    y2c = sprite->y2 - ycenter;
    cy = sprite->y + ycenter;

    y1z2_y = -(y1c * sin_billboard_pitch);
    y2z2_y = -(y2c * sin_billboard_pitch);

    x1 = +(sprite->x1 * cos_inv_yaw - y1z2_y * sin_inv_yaw) + sprite->x;
    x2 = +(sprite->x2 * cos_inv_yaw - y1z2_y * sin_inv_yaw) + sprite->x;
    x3 = +(sprite->x1 * cos_inv_yaw - y2z2_y * sin_inv_yaw) + sprite->x;
    x4 = +(sprite->x2 * cos_inv_yaw - y2z2_y * sin_inv_yaw) + sprite->x;

    y1 = +(y1c * cos_billboard_pitch) + cy;
    y2 = +(y2c * cos_billboard_pitch) + cy;

    z1 = -(sprite->x1 * sin_inv_yaw + y1z2_y * cos_inv_yaw) + sprite->z;
    z2 = -(sprite->x2 * sin_inv_yaw + y1z2_y * cos_inv_yaw) + sprite->z;
    z3 = -(sprite->x1 * sin_inv_yaw + y2z2_y * cos_inv_yaw) + sprite->z;
    z4 = -(sprite->x2 * sin_inv_yaw + y2z2_y * cos_inv_yaw) + sprite->z;

    glBegin(GL_TRIANGLE_STRIP);
      glTexCoord2f(sprite->ul, sprite->vt); glVertex3f(x1, y1, z1);
      glTexCoord2f(sprite->ur, sprite->vt); glVertex3f(x2, y1, z2);
      glTexCoord2f(sprite->ul, sprite->vb); glVertex3f(x3, y2, z3);
      glTexCoord2f(sprite->ur, sprite->vb); glVertex3f(x4, y2, z4);
    glEnd();
}

  float xcenter = (sprite->x1 + sprite->x2) * 0.5f;
  float ycenter = (sprite->y1 + sprite->y2) * 0.5f;
  glTranslatef(sprite->x + xcenter, sprite->y + ycenter, sprite->z);
  glRotatef(inv_yaw, 0.0f, 1.0f, 0.0f);
  glRotatef(billboard_pitch, 1.0f, 0.0f, 0.0f);
  glTranslatef(-xcenter, -ycenter, 0);

  glBegin(GL_TRIANGLE_STRIP);
    glTexCoord2f(sprite->ul, sprite->vt); glVertex3f(sprite->x1, sprite->y1, 0.0f);
    glTexCoord2f(sprite->ur, sprite->vt); glVertex3f(sprite->x2, sprite->y1, 0.0f);
    glTexCoord2f(sprite->ul, sprite->vb); glVertex3f(sprite->x1, sprite->y2, 0.0f);
    glTexCoord2f(sprite->ur, sprite->vb); glVertex3f(sprite->x2, sprite->y2, 0.0f);
  glEnd();

 float modelview[16];
  if(BILLBOARDED)
  {
    // get the current modelview matrix
    glGetFloatv(GL_MODELVIEW_MATRIX , modelview);
    

    // undo all rotations
    // beware all scaling is lost as well 
    for(int i=0; i<3; i++ ) 
      for(int j=0; j<3; j++ ) {
        if ( i==j )
          modelview[i*4+j] = 1.0;
        else
          modelview[i*4+j] = 0.0;
      }

    // set the modelview with no rotations and scaling
    glLoadMatrixf(modelview);    
  }

static void l3dBillboardCheatSphericalBegin(void)
{
  float modelview[16];
  int i,j;

  // save the current modelview matrix
  glPushMatrix();

  // get the current modelview matrix
  glGetFloatv(GL_MODELVIEW_MATRIX , modelview);

  // undo all rotations
  // beware all scaling is lost as well 
  for(i = 0; i < 3; i++)
  {
    for(j = 0; j < 3; j++ )
    {
      if (i == j)
        modelview[i * 4 + j] = 1.0;
      else
        modelview[i * 4 + j] = 0.0;
    }
  }

  // set the modelview with no rotations
  glLoadMatrixf(modelview);
}

static void gld_DrawSprite(GLSprite *sprite)
{
    glPushMatrix();
    glTranslatef(sprite->x, sprite->y, sprite->z);
    l3dBillboardCheatSphericalBegin();
    glBegin(GL_TRIANGLE_STRIP);
    glTexCoord2f(sprite->ul, sprite->vt); glVertex3f(-sprite->x1, sprite->y1, 0.0f);
    glTexCoord2f(sprite->ur, sprite->vt); glVertex3f(-sprite->x2, sprite->y1, 0.0f);
    glTexCoord2f(sprite->ul, sprite->vb); glVertex3f(-sprite->x1, sprite->y2, 0.0f);
    glTexCoord2f(sprite->ur, sprite->vb); glVertex3f(-sprite->x2, sprite->y2, 0.0f);
    glEnd();
    glPopMatrix();
    glPopMatrix();
}

modelview[0] = 1.0;
modelview[1] = 0.0;
......
modelview[15] = 1.0;