QGLText　核心绘制函数（用于Qt3D中文字显示）

/*-- Author: Xiao Long Teng , shared on 2012-10-10 --*/
void Geotech::QGLText::draw(QGLPainter *painter)
{
    if (_str.isEmpty()) return;

    QFontMetrics fm(_font);
    QRect rect = fm.boundingRect(_str);  // text bounding box
    rect.adjust(0, 0, 1, 1);

    QImage image(rect.size(), QImage::Format_ARGB32);
    image.fill(0);  // set to transparent

    // draw the text on an image
    QPainter p2d(&image);
    p2d.setFont(_font);
    p2d.setPen(QColor(_red * 255, _green * 255, _blue * 255, _alpha * 255));
    p2d.drawText(0, 0, rect.width(), rect.height(), Qt::AlignCenter, _str);
    p2d.end();

    // convert the object coordinate to screen coordinate
    GLdouble winx, winy, winz; // the screen coordinate of the object
    QMatrix4x4 model = painter->modelViewMatrix().top();
    QMatrix4x4 proj = painter->projectionMatrix().top();
    QGLUtils::objectToWindowCoord(_x, _y, _z, model.data(),
                                  proj.data(), &winx, &winy, &winz);
    if (_alignW) winx -= rect.width()/2.0;  // align center of width
    if (_alignH) winy -= rect.height()/2.0;  // align center of height

    // define the font rectangle which is
    // (x, y)                        (x + rect.width(), y)
    //    ------------------------------------
    //    |                                  |
    //    |                                  |
    //    ------------------------------------
    // (x, y + rect.heigth())        (x + rect.width(), y + rect.heigth())
    int x = (int)winx, y = (int)winy;

    QVector2DArray vertices;
    vertices.append(x, y + rect.height());
    vertices.append(x, y);
    vertices.append(x + rect.width(), y);
    vertices.append(x + rect.width(), y + rect.height());

    // texture coordinates
    QVector2DArray texCoord;
    texCoord.append(0.0f, 0.0f);
    texCoord.append(0.0f, 1.0f);
    texCoord.append(1.0f, 1.0f);
    texCoord.append(1.0f, 0.0f);

    // map the image to texture
    QGLTexture2D texture;
    texture.setImage(image);

    // get viewport
    GLint view[4];
    glGetIntegerv(GL_VIEWPORT, &view[0]);

    // use ortho projection to draw the text, because it
    // is easy to find the position of the text under ortho projection.

    // set projection matrix stack
    painter->modelViewMatrix().push();
    painter->modelViewMatrix().setToIdentity();
    QMatrix4x4 projm;
//    projm.ortho(QRect(view[0], view[1], view[2], view[3])); // Znear = -1.0, Zfar = 1.0

    /**
     * Description of glOrtho(left, right, bottom, top, nearVal, farVal) :
     * (left, bottom, -nearVal) and (right, top, -nearVal) specify the points
     * on the near clipping plane that are mapped to the lower left and upper
     * right corners of the window.
     *
     * Note:
     * -nearVal : specifies the location of the near clipping plane
     * -farVal  : specifies the location of the far clipping plane
     */

    projm.ortho(view[0], view[2], view[3], view[1], 0, 1);
    painter->projectionMatrix().push();
    painter->projectionMatrix() = projm;

    // move to the actual position from the screen origin
    painter->modelViewMatrix().translate(0, 0, -winz);

    // enable blend to make the background transaprecy of the text
    glEnable(GL_BLEND);
//    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    painter->clearAttributes();
    painter->setStandardEffect(QGL::FlatReplaceTexture2D);
    texture.bind();
    painter->setVertexAttribute(QGL::Position, vertices);
    painter->setVertexAttribute(QGL::TextureCoord0, texCoord);
    painter->draw(QGL::TriangleFan, 4);
    painter->setStandardEffect(QGL::FlatColor);
    glBindTexture(GL_TEXTURE_2D, 0);
    glDisable(GL_BLEND);
    // restore the matrix stack
    painter->projectionMatrix().pop();
    painter->modelViewMatrix().pop();
}

QGLText 核心绘制函数（用于Qt3D中文字显示）