# Grasshopper Tutorials Part 1.7

These are grasshopper recreations of a design pattern 'Placeholder' found in Robert Woodbury's Elements of Parametric Design. In both definitions, a component is populated onto a surface through different techniques.

The problem of replicating a unit on a surface is a common one. These 2 definitions solve that problem in two different ways. The first is based on the technique described in Lift Architect's 'Grasshopper Primer' and uses a unit modeled in Rhino first. The second uses a technique based on substituting components with the same inputs (for users familiar with Catia, this is akin to 'Power Copying') - the unit is built in Grasshopper.

Here a unit is copied multiple times onto a surface using a 'Morph' component. The component transforms the unit by matching its bounding box to a series of twisted boxes created on the patches of the surface. These patches are generated by feeding in subdivided 2-dimensional domain information into the 'Surface Box' component.

In the second definition, the unit is defined in Grasshopper ('Component in Placeholder' group). The key to instantiation (repeating a unit) is to build the definition in such a way that inputs match. The unit here (an arbitrary shading device) is built initially from a set of 4 points - PtCollection. This matches the structure of the Vertices data being output from the 'Brep Components (or Explode)' component - each branch is a list of 4 points. Therefore we can feed either 'PtCollection' or 'PtsFromSrf' into 'PtsInput' without breaking the definition.

The grasshopper definitions can be downloaded here.

Videos of Placeholder 1 and 2: