Importing PDF module maps - hiding secrets (secret doors, pits, room numbers, etc.)

I am very new to Roll20 and am planning to use it for a live in-person game with a module I own in PDF.  I've converted the maps to PNG, followed the instructions for importing them, and have been very successful with the grid alignment tool.  However, like most module maps, they show secret doors, pits, room numbers, etc.  I would like to hide these.  So far, I've found two approaches: 1) One detailed in  this "Maps from Modules" forum post  - if I understand this approach correctly, how it works is you put the map into the map layer temporarily, only to do grid alignment.  Once you've done that, you then move it to the GM layer, and basically re-trace the entire map on top of it.  The advantages of this approach are successfully hiding the secret stuff, allowing the GM to still see the secret stuff via the GM layer, and being able to do dynamic lighting (which I'm not as concerned about, I'm fine with plain old fog of war.)  The disadvantage is you basically have to re-draw the entire map, which takes a long time and may not look as good as the original. 2) Edit the PNG before import - I came up with this myself.  In this approach, you use an image editor program to erase the secret stuff, and then once you're done, you import the resulting PNG into Roll20 as a map.  The advantages are successfully hiding the secret stuff, and having a nice looking map.  The biggest disadvantage I see here is that the process can be very labor-intensive, especially since you often end up erasing portions of the original grid lines, which, unless you fix, will be a tip-off to players.  Then too, you need to remember where the secret stuff is (no problem, I'll keep a printout of the real map in my lap or behind the GM screen.)  Finally of course there's no dynamic lighting / line of sight but again I am less concerned about that. Do I have these two correct, and/or is there another way that would be less work and would still look pretty good?  Again I don't care about dynamic lighting, will just do fog of war. Thanks!

For some PDFs, the labels are rendered separately from the actual maps. This is only the case in official PDFs, so games with only illegal OCR scans like 5e content won't work. If it's an official PDF, you should be able to right click and copy the image, pasting into an image editor, and it will come without the labels. Alternatively, there are programs that can extract all the images. I used this technique a lot for many pathfinder PDFs I purchased.

Very good write-up on 2 of the top options! I can add 3 more related options, Just Fog Of War... When they enter the room, as GM you reveal everything in the room except the small area that has GM-information. Just leave a black box of Fog covering the secret information. The downside, players know there is some kind of clue there, and a spot of blackness that interrupts the clear full view of the map. Tokens/objects/drawing... Place a different piece of art on the token layer over the secret information. Throw down a rug, chair, a stone floor tile, or a refrigerator. The downside, every time the players see a refrigerator parked in the middle of a room or pushed up against a wall they suspect a secret is written there. Another related technique: Scribble with the drawing tool, obfuscate the secret information, cross out the words with a black or gray line. GM deciding that certain secrets are not actually revealing anything that impacts the game so don't worry about it.... Okay it says this is room 92. Allow the players to glean the fact they are in room 92. Still only the GM knows what events/adversaries/traps are in room 92, so, you can simply let your players see "92" on the floor. They still don't get any special information about what this means. When I do the "Edit in image editor" method.... or the refrigerator method.... I make a decision of which bits to hide, and which bits to allow.  If it is 92 different room numbers then I usually won't take the time to edit/cover all of those, if they don't impact the gameplay.  But when it comes to a secret trap, code word, command-word, secret door, hidden treasure, things like that are worth the effort to hide, so that GM can spring it at the appropriate time. Secret Doors, Secret Rooms, Secret Tunnels are the hardest to hide, for me. If it is a small note or number of writing on the floor, it's easier to cover up with a refrigerator or a block of Fog, or to edit with image-editor programs. But if it is a tunnel then you need to close-up the wall so it looks like a normal wall, and you must trim your Fog Of War closely (give it a sharp haircut) so that they don't notice a tunnel/hallway extending away on the other side of "that solid-looking wall".  The double-hardest situation is maps that put a giant $ on the secret door location and part of the "$" sticks out into the floor of the main room. If a map was built ideally for Virtual Tabletops in the first place, you would actually separate the secret tunnel with a small gap of blackness, instead of making the architecture realistically abut the adjoining room. There should ideally be enough room there to "reveal" with fog of war (or to allow with Dynamic lighting) so that it looks convincingly like there is certainly NOT another tunnel there, and only shows the secret tunnel once the GM choose to extend the lighting / reveal the Fog going out an extra few feet.

To add to what Gold said about hiding details on the maps with tokens or other objects. I will sometimes use a screen capture of part of a wall or whatever and then put that on top of whatever is on the map to hide information or features I don't want the players seeing or that I haven't taken the time to edit out in a graphics editor.  Doing this you can also drop walls and things on the maps you buy in the Marketplace to completely change the look for the players; get more use out of your purchases.

Thanks so much everyone for these very helpful responses!  Looks like I will use a mix of all the solutions above.

Gold: > Throw down a rug, chair, a stone floor tile, or a refrigerator. I like this idea! I'll need a lot more items to place in rooms, but that adds some depth to the map anyways.  A win for everyone :-)

I will add one to Gold's list. Often you can 'draw' on the map lay a white rectangle to cover the object. For example, a pit trap can easily be covered with a white rectangle. When the PCs fall into it/find it, just select the rectangle and move it to the GM layer. The pit becomes visible. This can work with secret doors fairly well. If you have a $ in a smooth wall, draw a white rectangle on both sides to cut off the S potions leaving only the wall line. If you have a rough wall, use the free-line tool do draw in an irregular 'wall' on top of the $. It gets filled in and covers the $. Again, select and move to GM layer if/when the players find it.