lol to the wat response
to see what I mean......give the func_rotating (jk2 symbol) a targetname in gtk.
make something to target the func_rotating.
This will create a line from the target to the targetname (the jk2 symbol)
This line will show you what piece of the Jk2symbol is the main piece of the rotating object.
The func_rotating object will be centered on this piece without an origin brush.
If you have an origin brush AND an origin value, with the func_rotating object will be centered on the main brush and not the origin brush unless of course the main brush of the object is in fact the origin brush
Making the correct piece of the rotating object the main piece, eliminates the guess work needed to place it exactly where you want it to be. The question is....how can you make sure that the object will be exactly where you want it to be? You may not experience this problem yourself with the JK2 symbol. If you copied and pasted it from another map without doing anything to it but changing the origin the piece that should be the main piece IS the main piece.
Lets say you selected the entire Jk2 symbol and made it into a func_rotating yourself. but when you turned it into a func rotating, the j or k was selected as the main piece by the editor, the rotating object will rotate on the j or k and you will observe it wobbling in your map. You have multiple brushes for a func_rotating object, Gtk will only select ONE of those pieces to be the main piece of the object if an origin brush is not used. The object as a whole is not the main piece. (computers are stupid, they don't know what you want to do exactly, you have to tell it exactly what you want it to do). If you do the target name target thing you should see what I mean.
Or another example. You make a door and a trigger for the door. You will see a light blue line going from the trigger to the door. Now, the door is one entity and doesn't need an origin so the trick I am speaking of doesn't apply unless of course you personaly have a preference what part of the multiple brush door is targeted however, you can see which brush is being targeted by the door by following the line that is made from the trigger to the door.
Func_rotating stuff generally does not need or use a trigger but giving it a targetname and making a trigger or target for it will show you that light blue line pointing straight toward the exact brush gtk has identified as the main brush of the object.
The previous description I provided was to help out to tell GTK exactly which piece of the object will rotate on. I will try to explain it again.
make a box around the jk2 symbol (or whatever needs to be rotated) then > select inside command with GTK.
Click the outside or inside main ring of the symbol holding shift. - this will deselect the inside or outside ring.
Click the same ring you just deselected - still holding shift - this will re-select that ring you deselected in the previous step.
Then right click and select func_rotating - this creates the entire object as a rotating object
At this point you're JK2symbol does not have an origin value.
to get the correct (exact) origin value.......
deselect everything
copy and paste the ring you deselected then reselected in the previous step, then immediatly delete the ring you copied and pasted (doing this is a little trick with Gtk. the copy and paste trick, works with much more than finding origin values.)
switch to the overhead view of the grid and find the center of the ring you copied, pasted and deleted in a previous step.
now right click and add an info_notnull. The info_notnull should be placed exactly in the center of the ring, click n for the entity viewer and you can grab the origin for the info_notnull from there. The func_rotating object will be exactly where you want it to be using this method.
One other object that is helpful in using this method are func_trains.
example : You have a multiple brush object and you create is as a train. the origin (main brush) of that train, even if u do use an origin brush for it, it will be the center point for where it will appear at a path_corner. For the temple of mechanism in one example for the tram. if I select the whole thing without setting the correct brush with the correct origin point, I'm in for a lot of headaches and guess work. The tram would not be where I wanted it if some random brush of the train was selected as the main brush.
A little more about the copy and paste trick for other uses. Lets say you are working on one area of the map and you move into another area. From the overhead view in the new area you are wotking, if you create a brush, it could be above or below where you want it to be then you will have to move it into place. to save a bit of time and instead of having to move the brush you created to where you want it to go, copy and paste something in that area, then delete it, then create a new brush. That new brush will be where you want it to be. This is specifically helpful when adding a new wall or a hint brush or most anything really.
I have observed, gtk remembers where you put stuff and when you put something new in one spot, it remembers what you were doing previously. The best way to control this yourself, is to copy paste and delete something and it's useful to save some time when building a map. Gtk will remember the height and general location on the z axis of the piece that is copied, pasted then deleted. The next brush you create will be the exact same height and location of the z (up and down) axis of the piece you just copied, pasted and deleted.
I hope I explained this enough for you to understand. The only way to explain further would be to make a video tutorial. I also hope this is helpful and will save you some time.