You can use this value to interactively vary the amount of polygon reduction that takes place. The reduction is expressed as a percentage of the original number of polygons (triangles, remember) in the object so that if you choose 50% for an object with 500 triangle polygons, the tool will attempt to reduce the object to 250 triangle polygons.
Do not expect this to be an exact science; there may be further reductions due to, for example, any co-planar areas (i.e. faces with polygons all in the same plane) which are always reduced as far as possible. But the Reduction Strength is a great way to quickly reduce or increase the poly count.
With a Reduction Strength of 0%, all coplanar neighboring polygons will be fused together and then triangulated. A Reduction Strength value of 100% will result in no change to the object.
There are a number of options that allow you to adjust the trade-off between the speed and the accuracy of the reduction. These are now described.
The value assigned here affects the final result of the optimization by determining how much fold-over checking is done; a low value reduces the number of checks made while the mesh is reduced, a high value increases the number of checks.
You can enter any value between 0 and 20000; use a high value if you have a complex mesh with a high reduction strength, a low value should suffice for simple meshes and/or low reduction values. In general, the value you choose does not affect the speed of the reduction process with the exception that a value of 0 will lead to a faster reduction, since this turns off any checks.
As with other options the checks work during the entire reduction process and monitor the appearance of overlapping polygons in the mesh of the object. Here is an example that shows how an overlapped (i.e. folded over) mesh can result from a bad reduction decision and therefore why such checks are necessary.
In Figure 1, below, the triangle (polygon) C has overlapped the A and B polygons of the mesh.
Or, perhaps more clearly, with the neighbor triangles of the collapsed edge being vi, vj in Figure 2.
A normal reduction should result in Figure 3 (or Figure 4 in 3D).
So, start with the default value for the Mesh Quality Factor, which should work fine for the majority of objects and, if you encounter problems, increase the value until the problems disappear.
This option, when enabled, will speed up the work of the reduction process for objects, such as cubes, that have many co-planar areas. When you enable this option Polygon Reduction employs a special, fast method to optimize co-planar areas (areas that have polygons in the same plane) irrespective of the reduction strength factor.
If this option is disabled, a different method is used for co-planar areas. As an example, create a cube with 50 segments along each side and then use Polygon Reduction on the cube, once with Co-Planar Optimization enabled and again with it disabled. You should find that the cube reduces considerably faster with the option enabled.
This option attempts to preserve the original surface continuity of the object. It detects boundaries in the original object that occur either on polygon edges that belong to only one polygon, or on polygon edges that are shared between two polygons with a large difference in surface Normals.
Surfaces enclosed by these boundaries are reduced in such a way as to preserve as closely as possible the original boundaries, while keeping the resulting boundary and the resulting surface continuous. However, the boundaries of differently reduced surfaces may not meet perfectly: there may be overlaps and gaps.
For instance, consider a cylinder. This option will ensure that the cylinder wall will be continuous, and have a continuous boundary at the top and bottom of the side walls that approximates the original boundary. The cylinder will also retain its caps, consisting of polygons that define a continuous boundary that approximates the original boundary. However, the resulting wall and caps may not meet perfectly at their boundaries.
Without this option, the polygon reduction process may introduce gaps into the boundaries when reducing the surfaces. As a result, the resulting wall surfaces and wall boundaries will be discontinuous, and the caps may be removed entirely.
This option, when enabled, monitors the appearance of sliver polygons (i.e. triangles with an angle smaller than 15˚) within the object mesh during the reduction process and tries to eliminate them, if possible.
Here is an example of the appearance of sliver triangles as a result of reduction:
Enabling this option usually leads to a much better distributed mesh. On the other hand be aware that this check will preserve any already existing sliver triangles (from the original mesh) to the highest levels of reduction.
A more thorough explanation and some advice on how to work with these options is given in the Hints and Tips section below.