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When you first add a cylinder, you have the option to change the number of vertices, etc, but once you do anything else in blender, it seems you no longer can edit those parameters.

Is there a setting anywhere or an add-on that allows these parameters to be edited later on? If not, how easy would such an addon/feature be to create for anyone reading this?

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Blender does not have a construction history. You can only edit the parameters of the last action, nothing earlier than that. To edit the parameters of the last action you can use F6 or the Operator Tab in the Tool PanelT

enter image description here enter image description here

If you want to modify the last tool you have to do it before using another tool.

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    $\begingroup$ It seems like that could be something that could be implemented maybe through the same mechanism the Undo history is recorded, but then again, I'm not a developer. $\endgroup$ – Thom Blair III Mar 18 '14 at 13:04
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There is a workflow solution to this. While you're making a model, if you think that you may want to change one or more parts of the model, make that set of parts separate bits of the mesh. This can be achieved with a vertex group, or just by leaving the part of the mesh unconnected to the rest of the object.

When you decide to select on part, select it by using the defined vertex group, or if you've left it unconnected to the rest of the mesh, by selecting a point on the part and pressing CTRL - L, or by pressing CTRL [numpad] + until the part is selected. Get the necessary parameters of the part (especially the median point location), or create the new one, if not at the desired location, in a way that you can move it there, and when it is in the desired place delete the original.

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With Blender 2.8 at least, you can always hit the F9 key and you´ll see the operator panel again.

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    $\begingroup$ Thanks! This is exactly what I was looking for. I would add that you cannot bring up the operator panel again once you go into edit mode with that object or move it with the mouse. It would be nice to maybe have a parametric object type available that can be converted into a mesh later, or maybe a given a mesh modifier for non-destructive editing. $\endgroup$ – Coby Randal Jan 23 at 4:48
  • $\begingroup$ I am new to blender and this seems a bit insane to be because, it also seems that I cannot make a new primitive at anywhere other than the origin. So what happens if my origin is already occupied by other geometry and I cannot see the new shape? I can do the "Adjust Last Operation" right after creating the object, but then I can't easily see what effect changing the parameters has because the geometry is not visible. Is there a workaround for this? $\endgroup$ – uglycoyote Feb 8 at 21:39
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As already mentioned in other answers you can't edit object parameters after they are created in Blender. After the creation process they become regular "static" objects and loose any "parametric properties" they had.

In an attempt to not leave you empty handed I'll try to present an alternative workflow: Be smart, use modifiers to your advantage.

If you foresee the need to adjust geometry after the fact, or edit parameters frequently, plan ahead, build your objects in a way that allows tweaking needed parameters easily.

This is where modifiers come in, they provide for what is often called "non-destructive modeling."

If you need to, say, easily change the number segments of a cylinder, or frequently adjust its height then it would be smart to rather than of adding a regular mesh cylinder build one with modifiers instead.

Create a simple single edge mesh with no actual faces, add a Screw Modifier to it, then a Solidify Modifier. You now have a "Parametric Cylinder" you can easily adjust the number of segments and height of. Just control parameters from the modifier stack, replace single vertex with an edge and now you have top and bottom caps.

enter image description here

This is what is often called a "non-destructive workflow", where modifiers are kept "live" so they can be easily edited.

This will obviously not work every time nor for everything, and you can't always achieve every end goal without some destructive steps, but be inventive and you can go a long way.

Use modifiers to your advantage, extend this to other tools like constrains, drivers, shapekyes etc., and you can go a long way in creating easily editable "parametric objects".

Here is a "Parametric Sphere" made from a single vertex mesh and two Screw modifiers.

enter image description here

You could even control radius with an additional Displace Modifier"

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    $\begingroup$ I am very new to blender. I love the globe example using the screw modifier, There isn't a hope in Hades I would have thought of that myself. Thankyou $\endgroup$ – DeveloperChris Feb 26 at 0:53

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