Partial Decimation

Reduce overall mesh resolution while keeping details you want.

Use weight painting to indicate which areas of the model are affected more or less the decimate modifier.

Set the whole mesh to a weight of 1

1. Create a vertex group
2. Go into edit mode.
3. A slider called 'weight' appears, with a value of 1.
4. Click the 'assign' button to set the whole model to weight 1.

Paint the areas you want to preserve

1. Switch to weight paint mode. The 3D model looks red. This is weight 1.
2. Paint the areas you want to preserve.
3. Set your brush to 0 to fully preserve the original resolution in those areas.
4. Set your brush to a value between 0 and 1 if you want to reduce the resolution, but not as strongly as the rest of the model (0.4 for example).

Set up a decimate modifier

1. enter the vertex group in the modifier to see the result.