public class SpyglassGui extends Gui {

  private final static ResourceLocation overlay = new ModelResourceLocation("glassworks:textures/gui/spyglass.png");

  private final static int OVERLAY_WIDTH = 256;
  private final static int OVERLAY_HEIGHT = 256;

  private Minecraft mc;

  public SpyglassGui(Minecraft mc) {
    this.mc = mc;
  }

  public void renderOverlay(int screenWidth, int screenHeight) {
    ScaledResolution scaledresolution = new ScaledResolution(mc);
    int width = scaledresolution.getScaledWidth();
    int height = scaledresolution.getScaledHeight();

    float scalefact = (float)screenHeight/(float)OVERLAY_HEIGHT;

    int xPos = width/2 -(int)((scalefact*OVERLAY_WIDTH)/2);
    int yPos = height/2 -OVERLAY_HEIGHT/2;


    GL11.glPushMatrix();
    mc.renderEngine.bindTexture(overlay);
    GL11.glScalef(scalefact,scalefact, 1);

    this.drawTexturedModalRect(xPos, 0, 0, 0, 256, 256);

    this.drawRect(0, 0, xPos, screenHeight, 0);
    this.drawRect((int) (xPos+scalefact*256), 0, screenWidth, screenHeight, 0);
    GL11.glPopMatrix();
  }
}



Called in

public SpyglassHandler(SpyglassGui overlay) {
          spyglassOverlay = overlay;
    }

    private SpyglassGui spyglassOverlay;
@SubscribeEvent
    public void preRenderOverlay(RenderGameOverlayEvent.Pre event)
    {
        //condition code
        spyglassOverlay.renderOverlay(event.getResolution().getScaledWidth(), event.getResolution().getScaledHeight());
    }


set in
@EventHandler
    public void init(FMLInitializationEvent event)
    {
        MinecraftForge.EVENT_BUS.register(new SpyglassHandler(new SpyglassGui(Minecraft.getMinecraft())));
    }