OpenGL:绘制矩形后文本不显示

我正在设计一个秤。在我需要有数字显示在白色。目前在下面的代码，我试图只显示"825"。之后，我尝试用灰色画一个矩形框作为比例的背景。但是当我尝试执行时，我看不到这个数字("825，在下面的例子中)是不可见的。只有灰色矩形可以看到。

#define SPD_SCALE_TAPE_STATIC_LINE_COLOR_160        160        // Light GRAY
#define SPD_SCALE_TAPE_STATIC_LINE_COLOR_120        120        // Light GRAY
#define SPD_SCALE_TAPE_STATIC_LINE_COLOR_180        180        // Light GRAY
#define SPD_SCALE_TAPE_STATIC_LINE_COLOR_050        50        // Light GRAY
// Co-ordinates for GRAY box
#define SPD_SCALE_TAPE_GRAY_X1    -8.0
#define SPD_SCALE_TAPE_GRAY_Y1    0.5
#define SPD_SCALE_TAPE_GRAY_X2    -6.0
#define SPD_SCALE_TAPE_GRAY_Y2    6.3
// Method to draw the text with TIMES NEW ROMAN with FONT size = 24
void drawText_TIMES_24(float x, float y, float z, char* text)
{
    int i_len = 0;
    i_len = strlen(text);
    if( i_len > 0)
    {
        glPushMatrix();
        glRasterPos3f(x,y,z);
        while( *text != '' )
        {
        if( i_len == 2 )
        {
            glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, ' ');
            i_len = 0;
        }
        else
        {
            glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, *text);
            text = text+1;
         }
       }
        glPopMatrix();
    }
}

void drawAir_Speed_ReadOut_Val(void)
{
    char a[] = "825";
    glColor3ub(0, 255, 0);
    drawText_TIMES_24(-7.5, 3.0, 0.0, a);
}
// Method to draw Spd Scale Tape Gray Rect
void drawSpd_Scale_Tape_Gray_Rect(void)
{
    glColor3ub(SPD_SCALE_TAPE_STATIC_LINE_COLOR_050,     SPD_SCALE_TAPE_STATIC_LINE_COLOR_050, SPD_SCALE_TAPE_STATIC_LINE_COLOR_050);
// Draw TOP rectangle GRAY box
    glRectf(SPD_SCALE_TAPE_GRAY_X1, SPD_SCALE_TAPE_GRAY_Y1, SPD_SCALE_TAPE_GRAY_X2, SPD_SCALE_TAPE_GRAY_Y2);
// Draw BOTTOM rectangle GRAY box
glRectf(-6.0, -0.5, -8.0, -6.3);
// Draw TOP triangle GRAY box
drawTriangle (-6.5, 0.5, -6.0, 0.0, -6.0, 0.5);
// Draw BOTTOM triangle GRAY box
drawTriangle (-6.5, -0.5, -6.0, 0.0, -6.0, -0.5);
}
void draw(void)
{
  drawAir_Speed_ReadOut_Val();
  drawSpd_Scale_Tape_Gray_Rect();
}