You can use the wireframe shader to drive the opacity of a standard_surface.
The wireframe shader itself doesn’t support opacity, and neither do the flat, utility, and ambient_occlusion shaders. They are all color shaders (RGB). In Arnold, only shaders that return closures can support opacity.
In the more general case, where the opacity isn’t coming from the same shader, you can use the emission for the color:
If you need to change the same Arnold attributes on many nodes, use the Attribute Spreadsheet. For example, suppose you wanted to turn off the Opaque attribute on a number of objects in your scene. Here’s how to do it:
Select the objects.
Press the Down arrow (aka Pick Walk) to select the shape nodes.
Open the Attribute Spreadsheet.
Click the All tab, and find the Ai (Arnold) attributes you want to change. Drag across all rows, and then type “off” in the last row.
AOV Composition allows opacity and transparency to carry forward into AOVs. It works only for RGB AOVs, so you won’t see it in the render region (because the xsi display driver always outputs RGBA AOVs).
For example, suppose you have a textured grid with an opacity map:
In the render region, the Main AOV is fine, but the Arnold Direct Diffuse doesn’t have the opacity, even if you enable AOV Composition:
However, if you render out the image (with AOV Composition enabled and the Direct Diffuse format set to RGB), you’ll get what you expected: