File optimization: Duplicate or linked duplicate?

Question

I'm currently in the process of optimizing my blend file. I am still relatively new at blender and would like some documentation on file size and render time reduction using linked or unlinked duplicates.

For Example: I have a architecture scene wich is currently 2GB in file size with a LOT of trees and bushes. Should I best use duplicates or linked duplicates? and what are the advantages/disatvantages? could I cut down on render time and/or file size?

As a former 3ds Max user I was quite familiar with proxys but I don't have the knowledge to apply this technique in blender (yet).

Therefore I'm reaching out to this awesome blender community for a little help.

Answer 1

Especially in the case of things like trees you could use linked duplicates. You can create those with Alt+D.

But if you have already created the normal duplicates in your scene you can still link them by selecting all objects that you want to convert to a linked duplicate, then select-click the object that you want to link them to and press Ctrl+L in object mode and choose "Data". That will turn all these objects into instances. That means they share the same mesh. However, you can still transform the objects, by moving, rotating, scaling so that they look a bit different.

A common problem when using Linked Duplicates is that this will usually also mean that they have the same material. However, you can also have different materials on linked duplicates by changing the material link in the material tab in the properties editor from "Data" (which means the material is linked to the mesh) to "Object". By doing so you can have objects that share the same mesh and still use different materials on them.

That way can decrease the file size A LOT.

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Another way to add many objects (like e.g. trees) to your scene is to use Group Instances. Usually this is done by having a separate blendfile for the object that you want to use as an instance, or, so to say, as a library object.

Example

Create a file with the tree. Select everything that belongs to the tree (but not lamp or camera) and create a group by pressing Ctrl+G. The outlines and wireframe will turn green to indicate that these objects belong to a group. Give the group a proper name ("tree" e.g.). Save the file in your project folder. In your main scene file, open the file menu (F1) and choose "Link" (or use Ctrl+Alt+O right away). In the now open file browser you navigate to the tree file and click it. That will bring you inside the blendfile directory, where you can go to a folder named "Groups". There you can choose your group ("tree") and link it to the scene. The tree is now attached to an Empty object. You can duplicate, move, rotate and scale this group instance, but you cannot edit the mesh or material, because all the data is linked in from the original file.

This is probably the cleanest way to add library objects to your scene.

Answer 2

I'm not sure that the benchmarks you seem to be asking for exist, that is, what the cost in render time and file size is for various options that affect file size. And some of this may be of more important earlier in the workflow. For example, if you are contemplating a stand of trees, you can decide earlier on whether you're going to have a more natural stand of trees, containing a half dozen or dozen trees (like a wild grove, in nature), or whether the trees are going to be more like an orchard, consisting all of a single type of tree. In the latter case, you can use linked duplicates of a single mesh; in the former, you might need to duplicate multiple types. Depending upon the specific trees, you may be able to use duplicates of several objects. On the other hand, in a "native" grove, even consisting of a few species of tree, you might find use of duplicates much more limited on account of taking to account the different sizes of trees on account of their diverse ages. A similar situation might apply to an interior scene. You might need only one type of seat in a model of the interior of a movie theater, for example, where you need dozens of type of individual chairs if you are modeling a furniture warehouse.

There are other factors that can affect the file size, too. For example the best decisions you make about the exterior of buildings in the scene can have an impact. Consider two nearly identical buildings to be seen very close up. A brick wall, up close, may have a vastly larger size because of the much greater number of bricks needed to cover the surface of the wall than a wall of the same size which is concrete construction, with a wall sided with wood somewhere in between. So, if you're going to have three buildings, one with concrete walls, one with wood siding, and one of masonry construction, your choice of how to place these buildings will have a bearing on the file size. Placing the masonry building closest to the camera may well result in a much larger file size than if the concrete structure is closest, because in the latter case, the masonry building can be suitably modeled with image textures, where in the former one, each brick may have to contain a eight vertices.