create_doublesided_rendermaterial.literate

# Creating a double-sided material

_written by Nathan 'jesterKing' Letwory_

## Introduction

To create a double-sided material type we are going to use `RenderMaterial` from
the `Rhino.Render` namespace.

A GUID is needed to create a material of a specific type. One can find many GUIDs
in the [`RenderMaterial`
properties](https://developer.rhino3d.com/api/RhinoCommon/html/Properties_T_Rhino_Render_RenderMaterial.htm),
but unfortunately the double-sided material GUID is not exposed through the API
yet.

To figure out what the GUID of the double-sided material type is I create one
manually in Rhino, then save the material as a file. The resulting `.rmtl` file
can be opened in a text editor. In the `<material>` tag you'll find the
`type-id` attribute that holds the GUID
we need.

Following these steps give us the information we need: a double-sided material
type has the GUID `E6CD1973-B739-496E-AB69-32957FA48492`.

A new instance we create with one of the static method [`RenderContent.Create()`
overloads](https://developer.rhino3d.com/api/RhinoCommon/html/Overload_Rhino_Render_RenderContent_Create.htm).
Using one of these overloads will result in the material being created _and_
added to the persistent content list. In other words it will show up in the
material editor.

A double-sided material references two other materials. This means we need to
create these materials as well. Such materials will be set as child content to
the double-sided material.

To create render materials as the child content for the double-sided material in
the front and back slots we'll be using [`RenderContent.Create(Guid type,
RenderContent parent, string childSlotName,
RenderContent.ShowContentChooserFlags flags, RhinoDoc
doc)`](https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Render_RenderContent_Create.htm).


## Create and setup the material

We'll create and assign the material only when there is an object selection.

```py : <<if objects are selected create material and assign>>=
# only if we have a selection do the work
if object_selection:
    <<create a double-sided material>>
    <<assign material to object selection>>
```

To create the double-sided parent material we use `RenderContent.Create`. We
don't need much else besides the render content GUID and the document.

Once we have a material we need to bracket any changes we want to make between
`BeginChange()` and `EndChange()` calls. Otherwise Rhino will ignore any
programmatical changes to the render content because it is already in the
document after creation.

We set a name suffixed with a new GUID each time this material is created by the
script. This so we don't end up with materials that have the same name. While it
is possible to do so with a script names should really be unique.

Note that the `front_material` and `back_material` instances are created within
the `BeginChange()` and `EndChange()` bracket of `render_material`. The creation
of these materials is done using the
[`RenderContent.Create()`](https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Render_RenderContent_Create.htm)
function that creates the material as child to a parent content using the given
child slot name.

```py : <<create a double-sided material>>=
# create material
render_material = Rhino.Render.RenderContent.Create(
    System.Guid("E6CD1973-B739-496E-AB69-32957FA48492"),
    Rhino.Render.RenderContent.ShowContentChooserFlags.None,
    scriptcontext.doc)
render_material.BeginChange(Rhino.Render.RenderContent.ChangeContexts.Program)
render_material.Name = "Double-Sided Material " + System.Guid.NewGuid().ToString()

front_material = Rhino.Render.RenderContent.Create(
    Rhino.Render.RenderMaterial.MetalMaterialGuid,
    render_material,
    "front",
    Rhino.Render.RenderContent.ShowContentChooserFlags.None,
    scriptcontext.doc)

back_material = Rhino.Render.RenderContent.Create(
    Rhino.Render.RenderMaterial.PlasterMaterialGuid,
    render_material,
    "back",
    Rhino.Render.RenderContent.ShowContentChooserFlags.None,
    scriptcontext.doc)
random_color1 = Rhino.Display.Color4f(
    random.random(),
    random.random(),
    random.random(),
    1.0)
random_color2 = Rhino.Display.Color4f(
    random.random(),
    random.random(),
    random.random(),
    1.0)
# set color for front material
front_material.BeginChange(Rhino.Render.RenderContent.ChangeContexts.Program)
front_material.SetParameter("color", random_color1)
# set color for back material
back_material.BeginChange(Rhino.Render.RenderContent.ChangeContexts.Program)
back_material.SetParameter("color", random_color2)
render_material.EndChange()

```

## Assign material to objects

Now that we have a material we can assign it to each object in our selection.
For Rhino to notice changes to objects in the document we need to do something
similar like the method above for changing render content that is already in the
document. We have to `CommitChanges()` on the object for the changes to stick.

```py : <<assign material to object selection>>=
# assign
for ob in object_selection:
    print("Adding material", render_material.Name, "to", ob)
    ob.RenderMaterial = render_material
    ob.CommitChanges()
```

## The imports

We need access to the `Rhino.Render` namespace for the material and render
content related classes. Further we want to access the document. For that we are
going to use `scriptcontext`. We'll import just the namespaces or modules
meaning we'll have to include these when using classes and functionality they
provide. We're doing that for this script to make it clear where the used bits
and pieces come from.


```py : <<import libraries>>=
import System
import Rhino.Display
import Rhino.Render

```

To create a random color we'll need the `random` module along with access to
`Color4f` from the `Rhino.Display` namespace.

```py : <<import libraries>>=+
import scriptcontext
import random

```

## Main structure of the script

The script will be implemented as a script, not a command. It will import the
necessary modules and namespaces. It'll take note of the currently selected
objects. Then it will create a double-sided material as we already explained.
Finally the new material will be assigned to each of the selected objects.

```py : <<Double-Sided Material Creation Script.*>>= ./create_doublesided_rendermaterial.py
<<import libraries>>
<<determine selected objects>>
<<if objects are selected create material and assign>>
```

## Retrieve object selection

We're not going to ask the user to select anything. Instead the script relies on
the selection already being made. We still create a material, but without a
selection no assignment will be made.

```py : <<determine selected objects>>=
# get object selection
object_selection = [ob for ob in scriptcontext.doc.Objects if ob.IsSelected(False)]

```


## Conclusion

It is relatively straightforward to programmatically create new render content
in Rhino. There are a few mechanisms the programmer needs to be mindful of:
bracketing of changes to render content, and explicitely committing changes made
to document objects.

Note that this particular implementation does not really check whether it is
useful to even have a `RenderMaterial` assigned. It would be good to add that
extra security.

Furthermore the script could be improved with a piece of code that actually
queries the user for object selection if none has been made.

These improvements are left to the reader to implement, as the main goal here
has been achieved: show how to create and assign a new material of a specific
type.

The generated script is in the repository [here](https://github.com/jesterKing/rhipy/blob/master/create_doublesided_rendermaterial.py)