Hi
Since recent versions the Undo behaviour has changed in the sense that it now removes undo history to free up Memory.
This is great but also really annoying as previously I could complete a lengthy zbrush sessions and never have to worry about Undos, but now it tries to remove always at the most inconvenient of times.
Is there a way to revert back or disable this feature.
I am sure i read somewhere it was possible but i cannot recall the location of the information!
Cheers
OK, I’m not sure we’re talking about the same thing, but ZB doesn’t “remove” undos–when the undo history line reaches the right of the screen, the bar compresses so each action occupies less space.
Zbrush will also only save the undo history with a project file if it is enabled in both the preferences and in the file menu. At least this was true of the previous version.
Otherwise you’re referring to a behavior I’m not familiar with. In any event, you can set the minimum number of undos you want Zbrush to retain in Preferences > Mem.
Remember to store your configuration after making changes in the preferences so ZB remembers it on startup.
Its strange
So i grabbed a sphere ,made about 400 undo actions on it.
Added multiple layers
then added noise maker and my undo history went from 400+ to 17 where i could only go back pre layers , with none of the previous sculpting
I remember i never had this problem unless it explicity told me that “this is an undo-able” action.
OK, I cant say what may have happened there, but just keep in mind that Undos are stored contextually. So each subtool has its own history, as does just the blank 2.5 d canvas. So if at any point something in your process created or switched you to another active subtool, or out of edit mode, you would be on an entirely different undo history.
I am pretty sure this gives me back the older 4R8 functionality of the Undo system.
Obviously proceed with caution “This is not advised” but i would rather choose where i delete my History rather than Zbrush choosing it at a really annoying time.
For example i can be working on something for an hour, scultpting away with Dynamesh, and what appears to be a random time it will remove all undos and leave me with just the Dynamesh. it will throw up a warning about removing undos but i cannot choose to not perform the action once the message appears.
I think its a memory issue as it happens much more frequently when i have say a 16k map in the texture slot.
I have 64Gb of ram so in theory i should be ok!
it doesnt feel anything like 4R8 in that regard!!
Maybe i am getting old!! 
Came here via a google search-- can confirm, so far as I can tell, ZBrush will occasionally just decide to “throw out” the entire undo history of a project. I make time-lapses using the undo history functions, and just today, opened up a nearly finished project to make a video, and the undo history is at zero. It had an intact undo history when I saved it last night, and have never deleted the undo history on this project at any time, because I was making it as a time-lapse.
Not only are my current and backup save purged of undo data, but all the quicksaves of that project likewise have no undo history at all. Which is pretty bizarre. Other projects and quicksaves have their undo history intact, up into the thousands for one long video I had laying around.
I am an advanced ZBrush user, I understand about subtools having their own histories and all that, yet here we are. No video for me.
It’s a rare phenomenon-- only happened to me twice before over several years, but it does happen. The first time it happened, I likewise assumed that you guys had changed the program to automatically purge undos on shutdown or something. But yes, Undo histories do sometimes disappear into the ether.
If there an obscure macro I’m accidentally typing that switches off the undos? I do not know. I have found nothing so far in my own behavior thus far that causes this to happen. Not looking for a fix necessarily, I assume the data is lost, but this is a real thing that happens sometimes. No switch to 2.5D, same polysphere, same mode, for the entire sculpt.
Edit-- the problem seems to be tied to the file, not the subtool. Copying the subtool and re-pasting it into a new project saves the undo history from then on normally, but does not restore the old lost undo history.
I also now notice that the “Skip Quicksave” toggle is on for my file, which explains the lack of backups, but I swear I did not toggle it that way. This was the first time I ever even opened preferences on this project. Checked the default polysphere projects to see if it somehow got checked there in those files, but couldn’t find anything there.
Enable saving is toggled on for the main file undo history however, and it still reliably deletes the undo history after every close now. Happy to forward you the file if you like.
Undo settings are default and untouched.
I don’t know if you solved your issue yet considering you replied here a year ago, but I just came across this thread due to the same issue Chris is having. (I randomly get a popup that says “some undo history has been deleted” and tells me to go to preferences to set how much history I want to keep. Sometimes this happens after only two actions, which is absolutely insane if it’s a memory issue, as I have 64 gigs of ram, and my harddrive is nowhere near full)
Anyway, in trying to figure out if there’s an actual solution for my issue, I came across a solution for yours!
According to this page, there’s a toggle-able setting to turn off the loading of history when you open a ZPR. Seems like a silly option as I’m not sure why you’d want to load a ZPR over a ZTL if you’re not loading history, but I’m sure there are other reasons to use a ZPR that I’m not aware of. (I just use ZTLs)
The part relating to your issue is at the very bottom of the page.
http://docs.pixologic.com/user-guide/3d-modeling/undo-history/
