This page presents the main concepts which are specific to MDL graphs and should be well understood for making the most of this graph type in Substance 3D Designer.

When creating or loading an MDL graph, the first unpinned 3D view panel found by Designer will automatically switch to the Iray renderer. If no 3D view is available, a new 3D view panel will be created and switched to the Iray renderer to host the rendering of the MDL material being edited.

When the Iray renderer is selected in a 3D view panel, that panel’s Materials menu lets you switch between available MDL materials, which include materials loaded in the Explorer panel and materials in Designer’s MDL library. Learn more about working with MDL materials in Iray in the Iray section of this documentation.

Root node

The result of an MDL graph is defined by the Root node. Any node of the graph can be set as root as long as its outputs data of type material, i.e., a material definition. An MDL graph may have only one Root node.

Generally, a node which can be set as Root can be self-sufficient, as it already contains a material definition which may be customized by passing data to its inputs.
For instance, if you want to work on a glass-like material, you may want to use a Glass material definition as Root node as a starting point, but that is not mandatory. Many material nodes are templated which can be turned into any complex material using the extensive list of MDL nodes.

The Root node includes a thumbnail displaying a preview of its current output.

Root node in an MDL graph and its properties displayed in the Properties panel

Connectors and types

As there are far more data types in MDL graphs than other graphs in Designer, you may witness unique appearances of node connectors. The important concepts to understand are list below.

Connector shape

The shape of the connector indicates whether the data type is uniform (circle) or varying (square).

"A variable of a uniform type can only be set to a uniform value. A variable of a varying type can be set to a varying value as well as to a uniform value. The resulting value in the variable is then always considered varying." (Source: Section 6.3 of the MDL Specification)

Here are some examples:

• a Texture sample is varying as values are impacted by the sampled pixel
• a Color value is uniform as it is passed equally regardless of the context
• a BRDF is varying as values are impacted by the angle of incidence
• a Float or Boolean value is uniform as it is passed equally regardless of the context

Connector color

The type of data going from an output connector or expected by an input connector is color-coded and displayed between parentheses after the identifier/label when hovering the connector with the mouse.

Aspect of connectors vary according to I/O value type, which is displayed in parentheses after I/O identifier

Filtered node creation

You can add any node available in the mdl category of the Library in the graph by dragging the node from the Library view into the Graph view, or by pressing Spacebar to open the Node menu in the Graph view when nothing is selected. In this case, an unfiltered list of nodes is displayed.

However, there are cases where the list of nodes in the Node menu is filtered to only display nodes of matching data type for the target input or output:

• if a node is selected in the Graph view and Spacebar is pressed
• if clicking LMB, holding and dragging a link out of a node connector

You may want to keep in mind the rules applied for filtering:

• if the Node menu is displayed by pressing Spacebar when a single node is selected, the list includes nodes where the data type of the first input matches the selected node’s output data type
• if the Node menu is displayed by pressing Spacebar when multiple nodes are selected, the list includes nodes where the data type of the first input matches the last selected node’s output data type
• if the Node menu is displayed by dragging a link out of an output connector, the list includes nodes where the data type of the first input matches the selected output data type
• if the Node menu is displayed by dragging a link out of an input connector, the list includes nodes where the data type of the output matches the selected input data type

Filtered node creation in MDL graph, note the list changes according to the value type for the connector

Graph inputs and textures

MDL materials can receive data from external sources, in the form of values and textures for instance. This is achieved by exposing a node, contrary to the Substance graph where dedicated input nodes exist for this purpose.

Data can be passed to the exposed node depending on its type. For instance, Float values can be passed to an exposed float node, and a texture can be passed to an exposed color node (in this case, the sampled pixel’s RGBA values are passed as a color value).

Exposed nodes create graph inputs which are both raw value inputs and samplers for textures