This is a feature that's been enhanced in PD Howler 9.5.... Learn more about Howler 9.5. Learn more about other recently added features here.
 

Light Table aka Onion Skins

Dogwaffle has supported Onion skins with 2 frames back and forth for a while now. See Onion skins in v1.6.

With version 9.5, we've expanded it to 3 levels each side of the current frame. This will let you see a little more into the past and the future frames.

We've also added the ability to change the opacity level of the frames. Sometimes, you just want to see a single prior frame and not get distracted by others ahead of that one. This is particularly the case when working with video. Other times, it gets difficult to recognize which one is the prior frame, and which one is the one prior to that one, and one more before that. The ability to adjust the opacity levels will hopefully make it possible for you to dynamically adjust the visibility levels as needed.

Additionally, you can colorize the frames, so that it gets easier to distinguish which ones are the prior frames and which ones are the future frames. The older frames receive a red'ish tint, sort of like a redshift effect as they're moving away from you in time, while the newer frames turn blue'ish, as they're coming to you.

Watch this video for more details: