How to decimate in Blender

The Decimate operation in Blender allows you to simplify a model by reducing the number of faces or triangles in its mesh. One common way to do this is by collapsing edges. In addition to this, Blender also supports two other decimation methods called UnSubdivide and Planar. Decimating a mesh is easy:

  1. Load the model you want to decimate into Blender using File > Open.

  2. In the Properties window on the right, click the Object Modifiers button. This looks like a spanner.

  3. The current modifiers applied on the model are displayed here. Do not remove existing modifiers. Choose Add modifier > Generate > Decimate.

  4. To apply decimate operation, the mesh of your model needs to be a manifold. If it is not, Blender complains that the mesh is a non-manifold. You need to first fix the non-manifold elements of the mesh.

  5. If your mesh is a manifold, then just pick one of the three decimation types: Collapse, Un-subdivide and Planar.The latter two might not be there in versions of Blender that are older than 2.65.

  6. If you chose Collapse, then reduce the decimation Ratio. The number of resulting faces is displayed below in the modifier. But, the decimation is not applied until you click the Apply button. Do that and your mesh is decimated.

Tried with: Blender 2.69 and Ubuntu 12.04 LTS

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How to convert from Blender to OGRE format

Blender does not ship with support for exporting to OGRE mesh and skeleton. However, the Blender2OGRE addon can be used to export a Blender file as OGRE mesh, skeleton and material files.

  1. Download or clone the Blender2OGRE addon repository https://bitbucket.org/MindCalamity/blender2ogre

  2. Make sure that your Blender version is equal or greater than what is required by Blender2OGRE. You can find the minimum version required listed in the bl_info dictionary in the file io_export_ogreDotScene.py. If you are using a version of Blender that is older than what is required, then install a new version.

  3. Open Blender and go to File > User Preferences > Addons > Install from file. Choose the file io_export_ogreDotScene.py from the files you downloaded. Once the addon is added, it will appear as Import-Export: OGRE Exporter and RealXtend. Remember to enable the addon.

  4. If the addon was installed and enabled correctly, you must be able to see a Ogre checkbox at the right in the top toolbar. If your window width is small, you may have to move the top toolbar to the right by pressing and holding the middle mouse button.

  5. Open the model you wish to export. Choose File > Export > OGRE3D. By default, the addon exports mesh, skeleton, texture and material files. Both XML and binary versions of the OGRE mesh are created. You can test if the export worked, by opening the .mesh file using OGREMeshy.

Tried with: Blender 2.69 and Ubuntu 12.04 LTS

How to install new version of Blender in Ubuntu

The version of Blender that ships with Ubuntu is almost always old. This is a problem because a lot of Blender addons available on the web are continuously updated to work with only the most recent versions. To be able to use these addons, you need to install a more recent version of Blender. Thankfully, there are many PPAs that offer recent builds of Blender for most Ubuntu versions.

First, I uninstalled the default version of Blender. Next, I added the PPA ppa:irie/blender maintained by Irie Shinsuke and updated. Finally, I installed Blender from the PPA. The whole process takes only a few steps:

$ sudo apt-get remove blender
$ sudo add-apt-repository ppa:irie/blender
$ sudo apt-get update
$ sudo apt-get install blender

On Ubuntu 12.04 LTS, with this upgrade I went from Blender 2.62 to 2.69 easily 🙂

How to fix non-manifold mesh in Blender

Certain mesh operations in Blender do not work on non-manifold meshes. In such a case, you may have to manually clean up or fix the mesh to remove the non-manifold elements and fill in correct elements.

  1. First, learn what elements of a mesh cause it to be non-manifold.

  2. Next, ask Blender to show you the non-manifold elements of the mesh. You may have to turn around and zoom into the mesh quite a bit to view such elements which are in obscure corners.

  3. Finally, fix the shown non-manifold elements. To fix the mesh, you may have to display the normals in the mesh, remove elements of the mesh and fill in the holes correctly.

Your work is done when the display at the top shows zero non-manifold vertices, edges and faces! 🙂

Tried with: Blender 2.62 and Ubuntu 12.04 LTS

How to view non-manifold elements in Blender

Some mesh operations cannot be performed in Blender if the mesh is non-manifold. To view the non-manifold elements of the mesh:

  1. Switch to Edit Mode. The number of non-manifold elements in the mesh can be seen at the top where the number of vertices, edges and faces is displayed. For example, a non-manifold mesh of 14 faces, where 2 faces are non-manifold would be shown as Fa:2-14.

  2. Strangely, Blender only displays and selects the non-manifold elements in Vertex or Edge select modes. It does not work in Face select mode. So, choose either the Vertex select mode or the Edge select mode. Next, choose Select > Non Manifold. You can also do this by pressing Shift-Ctrl-Alt-M. The non-manifold vertices or edges are now displayed in orange and are selected for use in further operations.

Tried with: Blender 2.62 and Ubuntu 12.04 LTS

What is a non-manifold mesh

The non-manifold mesh arises while you are editing meshes using tools like Blender. Certain mesh operations cannot be performed on non-manifold meshes. A non-manifold mesh might have one or more elements with the following properties:

1. An edge incident to more than two faces.

2. Two or more faces connected only by a vertex and not by an edge.

3. Adjacent faces whose normals are pointing in opposite directions.

The above video explains these concepts beautifully in Maya.

How to fill hole in mesh in Blender

Blender is useful to fill or mend a hole in a mesh. Holes are typically already present in a mesh or you create them while deleting elements of a mesh.

1. Switch to Edit Mode and then click the Vertex select mode button. You are now ready to select vertices.

2. Select the vertices that border the hole. To do this, keep the Shift key pressed down while you select the vertices by using the right mouse button.

3. Press the f key to fill the hole. Blender tries to fill the hole with a suitable polygon. If you want to fill with triangles, then repeat these steps for every 3 vertices that form those triangles.

Tried with: Blender 2.62 and Ubuntu 12.04 LTS

How to delete elements of mesh in Blender

Blender can be used to edit a mesh by deleting vertices, edges or faces from it.

1. Switch to Edit Mode. Switching the Mode can be done from the dropdown in the bottom bar.

2. Depending on whether you want to delete a vertex, edge or face, choose the corresponding Select Mode. These buttons are placed a few buttons to the right of the Mode dropdown. For example, to delete a vertex click the Vertex select mode button.

3. To choose the elements of the mesh you want to delete, click on them using the right mouse button. Keep the Shift key pressed, if you want to select many elements. You can only select the elements of the select mode you picked. For example, in Vertex select mode you can only select vertices.

4. After you have chosen the elements, press the Delete key. A context menu comes up and you can choose what you want to delete: vertices, edges, faces or all. For example, if you chose a vertex, the dialog is asking whether you want to delete that vertex, or the edges incident to that vertex or the faces incident to that vertex or all of the above. Select what you want and those elements will be deleted from the mesh.

Tried with: Blender 2.62 and Ubuntu 12.04 LTS

How to display normals in Blender

The normals of faces and vertices are useful to check for problems in lighting. It can also be useful to diagnose why a mesh is non-manifold.

First, make sure the Properties side bar is visible. This side bar can be toggled by choosing View > Properties. It can also be toggled by pressing the n key.

Next, in the Properties side bar, go to Mesh Display > Normals. Select Face to view face normals and Vertex for vertex normals.

You can also adjust the length of the displayed normals by changing the Normal Size value.

Tried with: Blender 2.62 and Ubuntu 12.04 LTS