TO REDUCE NUMBER OF VERTICES IN A MESH

Blender version 2.82a or above.

 Look at the number of vertices in the image mesh1.png and mesh3.png.

 

                                                            mesh1.png

     mesh3.png

          

To reduce the number of vertices as in mesh3.png from mesh1.png, follow these steps.

1. Add a plane, go to top view. Select plane and scale it up by pressing S then 4, then 'Enter" key.Change to edit mode, select all the vertices. At the top see the list of mesh options like View,Select,Add,Mesh Vertex, Edge, Face, UV.

Select "Mesh" -> "Clean Up"-> "Dissolve Geometry". In the pop up window at the left bottom corner, gradually reduce the Ratio value to get the number  of vertices as in mesh2.png. Make sure the shape of the plane is not changed while adjusting the Ratio value.  Now the vertices/edges should look as in mesh2.png.

                                                            mesh2.png
 

Next, in Edit mode, select edge select option. Select the edges which you want to remove then click on "Mesh"->"Delete"->"Dissolve Edges" to get a clean mesh as shown in image mesh3.png.

Save the file. 

 

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DECIMATE MODIFIER

  Let us say we created a mesh object, but ended up with a lot more number of vertices than you think is the optimal number. To maintain the overall shape and still reduce the vertex count, decimate modifier can be used. The way to use it is explained below:

  Add a Monkey object and change to edit mode. In the info editor header the number of vertices is shown as 507. Change to solid mode. Having the monkey object selected, go to Properties Editor and click on the Modifier icon(Spanner), then click on Add modifier button.  In the drop down menu under "Generate" click on "Decimate". Under the "Collapse-Un-Subdivide-Planar" buttons,  "Ratio    1.0000" is visible  which is a slider control. Click LMB and drag to change the value from 1.0000 to say 0.700. Next click on Apply button. The number of vertices would have reduced to 360.

  As you reduce the ratio value, the vertex count will reduce, but beyond a point the details of the mesh shape will tend to be lost.