how do you split without propagation
 

Imagine three side by side blocks, I want to create an O-ring in the middle one without splitting the other two. What should I do?
In general how do you split a 3d block without splitting the other adjacent ones.
Thanks.

Implied
 

These are two questions because OGrid splits are different from regular splits...

However, they are the same in that behind the scenes, the mesh mapping must propagate (hexas must still be mapped), even if the blocks appear unsplit... We call this hidden splits "implied splits". Generally, I let my regular splits go all the way thru my model because I would rather deal with splits I can see than with "implied splits". You can expose an implied split by extending the visible split... You can also merge blocks to hide an edge after the fact... Also, if your concern is the number of output blocks, you can merge in output mode so that you have the control during blocking, but fewer blocks on export.

For regular HGrid splits, To split thru only one block and not the block next to it, I usually use INDEX control. A split will only go thru visible blocks. A slower way to do it (which I rarely use) is to change the method pull down to "selected blocks". Then it will first ask for an edge and then the blocks to split...

For Ogrid, the implied splits are more complicated... Therefore, an Open OGrid (such as a CGRID or quarter OGrid (YGRID) must extend to the outer boundary. If you wanted an Ogrid in one block, but not its neighbor, just don't select the neighbor. Ogrid will "turn around" in its block and avoid any propagation issues with its neighbor.

If you want a CGRID and select the face of the block between the two blocks, you will find that it selects the second block instead. Basically, selecting the face actually means that the Ogrid passes thru that face into the next block. Once there, it must turn around. If you want it to continue thru that block also, select the face on the opposite side. If you have another adjacent block, it will then be selected and so on. If that block is adjacent to the VORFN boundary, then a VORFN boundary block will be selected and the OGRID will appear to terminate as a CGRID at that boundary...

I hope that rough explanation is enough to get you on your way...

Thanks for the comprehensive response. However I'm still not clear about the strategy to avoid creating the O-Grid in the neighboring boxes. So let me use an example.

Suppose we have three blocks that are lying side-by-side in a horizonal row. Say I want to create an o-grid within the middle one. Plus I want the grid to have only four radial branches
*the ones that extend from the box in the o-grid* so I select the two edges in the back however this creates additional o-grids in the two neighboring boxes. Is there a way I could avoid this?

Also if I don't select the edges the o-grid is created in the middle one BUT the radial branches are two short and editing doesn't make much of a difference either.

Would appreciate the help.

Start with Offset control...
 

4 radial branches? Sorry, I don't understand. Maybe take a few screen shots or make a sketch to show what you want and then I can tell you how to get it.

As for not liking the offset of your OGrid, there are several ways to control/change it.

1) When you first create the OGrid, you can set the Offset. The default (1) is usually idea based on your edge lengths (About 1/3rd the diagonal length when done on a cube). But some users may want to set this to 2 (double) or so. By default this number is "relative" to ideal, but you could also check the "absolute box", which would mean that "1" would be "1 unit". Then you could set it to how ever many units thick you want the boundary to be...

2) Once you already have an OGRID, you can go to Blocking (tab) => Edit Block => Modify Ogrid => Rescale OGrid... By default, this applies to Ogrid Edges parallel and visible to a selected edge, so use your index control to limit it if you want. Again you have the same options of offset and absolute.

I usually use this one when I want to change a lot of edges at once.

3) Another option is under "Blocking (tab) => Move Vertex => Set Edge Length". I usually click on one edge first and MMB so it tells me the current length, then I select all the edges I want to adjust and type in a new length and apply. The end with the higher association (point is higher than curve is higher than surface is higher than volume) usually stays still. If they have the same association, the lower ogrid index stays still. If you don't like the result, use the freeze vertex to pick which side stays still. You only need to pick one and it will be matched on the other pairs based on index.

I usually use this option when I want to change just a few offsets or the offsets are not easily separated out with index control.

Hi Simon,

Your knowledge is amazing. Thanks to your comments I managed to create an O-grid in the middle block. However what I'm asking is slightly different. As you can see in the picture I've attached, there are branches coming from the box in the O-Grid. I want the four back vertices of the box to be connected directly to the top and bottom vertices of the block (so those connecting branches would disappear). What I do is, when creating the O-grid I select those two back edges, but this makes the O-grid propagate thru the two neighboring blocks. Is there a way I could avoid the propagation?
Nick

Sketch?
 

Sorry Nick, still not getting what you want to do...

Maybe just use an old fashioned piece of paper and draw me a sketch, hopefully with black and blue ink so I can see the difference between internal and external edges... if it is complicated, use numbers on the verts to help me understand your description. Then scan it or take a crude photo and use the the paper-clip to attach it to a post...