widget can not render in right position with FullScreenUserWidgetActor

I have use the FullScreenUserWidgetActor with UMG widget overlay on the gameview windows and mediaoutput , with “ viewport” the widget postion display is correct, but the “postprocess” output the positon display is not correct . I have change the widget draw size,and we found that the resolution is not right, and the also the widget lost qulity. we think that may be the widget render problem that resize the target?