How to substitute Keepout with SpeedLimits in nav2

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OS:Ubuntu 20.04 Distribution: ROS2 galactic

I am using this as a reference to understand speed limits and and keepout. https://navigation.ros.org/tutorials/docs/navigation2_with_speed_filter.html

I have two questions.

1. is it possible to substitute keepout_filter with speedlimit_filter? I have confirmed that keepout_filter and speedlimit_filter both work. However, I noticed that if I set the grayscale value to 100 (px:255), the global plan avoids 100 pixels. I am thinking that if the pixel grayscale can be avoided by setting the pixel grayscale to 100, then keepout can be achieved without using keepout_filter. Is this correct?

2. I found a procedure on the nav2 wiki to remove the black area when creating the speedlimit_filter mask image. Is this a necessary step? I felt it was not necessary to remove the black area, since the robot will not be able to enter the area.