The need for a semi-transparent map overlay.

I read somewhere (but not on the Roll20 forums) that Roll20 GMs (Pro) can use a macro that allows a single-button toggle to activate a semi-transparent map overlay that allows markings/text.   I'll be disappointed if some other VTTs can do this but our beloved Roll20 cannot. 

HI CrimsonRaven! You can likely do this with the foreground layer, among other ways. If you can be more specific about what you are trying to accomplish, we can make more specific suggestions.

Thank you Keith the Legend, and even you LordG for your responses.  Ok, picture a transparent film (like an Avery Sheet Protector, but with a soft texture to please the eye) placed over a table top map.  The film, can be "edited by the GM" to mark parts of the underlying map, or assign numbers to each square, or a chess-like coordinates (files/ranks) at the borders.  This is so useful I'm sure someone has thought of this already.  And yes, I bet the foreground layer will be my answer, even if its "static" and I have to edit it outside of Roll20 using image software. 

When using other layers you can add annotations without editing the original image - just add the elements on top of the base image. Also - and I may be missing something here - unless you need to have this overlay sometimes visible to the players, why not use the GM layer? This is a screen shot of the landing page of my last campaign. The monochrome image, the white map symbols, a few other symbols, the black text and the red "You are Here" are all that's visible to the players. The rest is on the GM layer and is only visible to me. The map icons are tokens with tool-tips that the players can view on mouse-over. All the other colored text entries are separate items, not part of the main map.

Here is how I that sort of thing in my game. I prepare the overlays using image editing software. I design the overlay so that it matches the map size. I might prepare several for a single map. Then I assemble them into a rollable token on the map layer. The base image for the Rollable token is an invisible png (a png with no visible pixels). I position it on top of the map. Then I use token mod and a dedicated Chat Menu to control which face of the overlay token is shown. I also make the chat menu accessible to players. Here is how it looks in practice:

I have also used Encounter Helper (available in One Click) to move entire groups of objects to and from the GM layer. Not quite the same thing, but maybe easier to handle a lot of info that changes frequently. You can also look into the Fade script (One Click) to control an overlay's opacity, fading it in or out for cinematic effect.

I run The One Ring 2e and annotate the Player’s Map using the Foreground Layer all the time. Usually with Text and Line pointing to a Landmark not already on the map, For Landmarks themselves, I annotate with transparent tokens with Nameplates & Auras, and I move them to the Foreground Layer to show to players.