__init__.py

def view_draw(engine, depsgraph, region, space_data, region_data):
    #print(">>> view_draw [%f]:" % time.clock(), engine)

    try:
        region = bpy.context.region
        v3d = bpy.context.space_data
        rv3d = bpy.context.region_data

        data = {}
        _camera = {}
        ipr = engine._ipr
        width = region.width
        height = region.height

        view_matrix = rv3d.view_matrix
        if view_matrix != ipr.view_matrix:
            ipr.view_matrix = view_matrix.copy()
            _camera['matrix'] = ('MATRIX', numpy.reshape(view_matrix.inverted().transposed(), -1))

        view_perspective = rv3d.view_perspective
        if view_perspective != ipr.view_perspective:
            ipr.view_perspective = view_perspective
            if view_perspective == 'CAMERA':
                ipr.camera_data = _view_update_camera(region.width / region.height, v3d, rv3d, _camera)
            elif view_perspective == 'PERSP':
                ipr.camera_data = _view_update_persp(v3d, _camera)
            else:
                # TODO: orpho
                return
        elif view_perspective == 'CAMERA':
            cdata = v3d.camera.data
            fit = cdata.sensor_fit
            sensor = cdata.sensor_height if fit == 'VERTICAL' else cdata.sensor_width
            offset_x, offset_y = rv3d.view_camera_offset
            camera_data = (rv3d.view_camera_zoom, fit, sensor, cdata.lens,
                           offset_x, offset_y, cdata.shift_x, cdata.shift_y)
            if camera_data != ipr.camera_data:
                ipr.camera_data = _view_update_camera(region.width / region.height, v3d, rv3d, _camera)
        elif view_perspective == 'PERSP':
            lens = v3d.lens
            if lens != ipr.camera_data[0]:
                _camera['fov'] = ('FLOAT', math.degrees(2 * math.atan(64.0 / (2 * lens))))
                ipr.camera_data = (lens, )
        else:
            # TODO: orpho
            return

        if _camera:
            data.setdefault('nodes', {})['__camera'] = _camera

        (width, height), rect = ipr.update(width, height, data)

        # Draw IPR OpenGL to Viewport
        OpenGL.draw(width, height, rect)

    except:
        print("~" * 30)
        traceback.print_exc()
        print("~" * 30)