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This tutorial explains a handy trick that both can retrieve valuable geometry erased earlier, as well as conserve the Undo actions stack (100 actions maximum, see below), as well as free up precious memory from RAM.
Though this tutorial is a somewhat lengthy read and may take some effort to fully comprehend, the Bttf tricks will save you time and frustrations.
During modeling in SketchUp you perform many different actions. These actions may change an object in its form or appearance. For example, when you intersect a sphere with a cube to create a custom shape.
SketchUp memorizes these actions until a maximum of 100 are reached. After that SketchUp will continue to memorize your actions but as it can only store 100 it will erase the very first from its memory. For example if during a modeling session you have performed 140 actions then you can undo those until the 40th action.
The Back To The Future (let's refer to it as Bttf) trick uses the Undo command in SketchUp to do its magic. There are two ways to do this trick, normal and reversed. The latter also has a performance enhancement for computers with less RAM or computers with moderate RAM resources yet many other open programs running in the background.
With the Bttf trick you go back (Un do) a number of modeling actions to a state in your model where you can select anything from the scene and copy it to your computer's memory/RAM (also referred to as the Clipboard). Copying something from the scene does not count as an actual action itself, so if you copy the object you will not lose the actions after it.
After you have copied the object you can go forward in actions again (Redo) and paste the copied object in your scene.
The Bttf trick also works in reverse, where you copy something in the last action, go back a number of actions and then paste the copied object in the scene.
Note again that upon pasting the object you will lose all actions that come after that action.
The reversed Bttf trick comes in handy when you want to preserve certain actions but discard the ones that come after those. For example, let's say you have been modeling for some time during a session to create some furniture. You have taken actions 60 to 90 to create some detailed table legs actions and subsequently actions 91 to 100 to create a tabletop. If you would continue modeling without using the Bttf trick eventually you would reach action 161 and lose action 60 from memory. By using the reversed Bttf trick you copy the tabletop to memory (at action 100), undo until action 90 and then paste the tabletop in the scene. This saves you 10 modeling actions, meaning now instead of 60 actions before you reach the limit (from 101 to 160) you now have 70 actions before you reach the limit (from 91 untill 160).
Note 1: Always use the Copy command, not the Cut command. If you use Cut you will subsequently lose all steps afterwards. Subsequently you cannot undo the Cut action!
Note 2: Pasting an object is counted as an action. So you will lose any actions that come after the action where you paste the object. For example if you go back from action 134 to 56 and copy something there, then go forward to action 80 and paste it there, you will subsequently lose actions 81 to 134.
There is another great advantage to the reversed Bttf trick, it frees up memory. When you copy an object and then undo the actions and paste it in the scene you will free up every action that existed from that point forward. So if you have no use for those steps you will lose them after the Paste command, but it does free up memory.
It may take a while to see the benefit of this trick but avid modelers will quickly see its use especially when involved with complex intersections of geometry. The reversed Bttf trick for example will allow you to go back before a destructive intersection was made while preserving the final (intersected) geometry by copying it to memory and pasting it to the action where the intersect action was taken.
Likewise the normal Bttf trick allows you to go back to a step and retrieve something which you deleted from the scene before.
Of course it is possible to create Components out of everything you think may be useful for later. But you will have to create many Components if you are to record each action you perform on an object. This in turn takes up more storage space and slows down the loading and performance of your entire model. Using the Bttf tricks you can avoid this and you're also a lot quicker.