>>A material can be applied to an object in two ways...

a) an item mask - for any item to have that material
b) poly tags - for any polygons to have that material

Hmm, could someone tell me what the difference is between these two trees:


(a poorly done photoshop of Steve's example tree :)

I'm wondering what the extra nodes in the tree are for? Do they do anything, or are they just like folders that hold things (and thus there's no point in having a folder in a folder)?
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If you look at the properties of a shader group in Modo, there are various ways to control what it does. These groups are essentially masks, and Modo applies the materials starting at the bottom of the tree and working upwards. Things higher in the tree override lower items universally unless they are restricted in some way by a mask. The mask can be by Item or by a Polygon Selection. In my example the Chromium Plate Cast is a group that has no restriction (the polygon tag field is set to "All"). It is a convenient way to group a preset. The whole preset can be swapped by deleting the group and adding a new preset. However, it is the child of the Matr: chrome group. This restricts itself and any of it's children to the polygon selection "chrome" (MoIs style applied to objects and faces). A slightly more complex but very useful attribute is the "Apply to Sub Group" option, which then takes the masking from the child groups. This allows you to set one material as the first child of the group and have it apply to all masks that are also children of the group but not to anything else.


Modo's shader tree is quite different to other 3D apps (it is much more like Photoshop layers)and requires a bit of study to fully understand it. One thing that helps enormously in debugging the shader tree is the Render Preview Window. If you right click the preview window, the pixel under the mouse is evaluated and a pop-up shows the items in the shader tree that have contributed to that pixel. You can then click on an item in that list and be taken straight to that item in the shader tree. I have attached a screen shot of the effect of right clicking the preview window.

There are some good tutorials on Luxology's web site. In the on line help for Modo there are a number of videos that explain the shader tree and masks in depth.