Simple Examples

All paths:

 //*

All paths that end in "room":

 //room

All paths that end in "room" and start with "renderpass":

 /renderpass//room

All paths that end in "room/chair" and start with "renderpass":

 /renderpass//room/chair

An exact path:

 /renderpass/street/house/room

Starts with renderpass, followed by one path element of anything, and then ends with "house/room":

 /renderpass/*/house/room

Ends in "room/chair":

 room/chair

Any node anywhere under a node called "room":

 //room//*

Odd numbered chairs, but only those inside "room":

 //*[substring(name(),6) mod 2 = 1 and parent::*[name() = 'room']]

More Complex Examples

The following examples, referring to attributes, are somewhat more complex.

Select objects beginning with the name paperclip:

 //*[starts-with(name(),'paperclip')]

Select objects in the, and only in the "wood" set:

 //*[@sets=',wood,']

Select objects beginning with the name pencil_box in the set "wood":

 //*[starts-with(name(),'pencil_box')]//*[@sets=',wood,']

Select desk_1 and desk_2:

 //desk_1 | //desk_2

Select of the objects the belong to the study_area_1 in the set "paintedmetal":

 //study_area_1//*[contains(@sets, ',paintedmetal,')]

Select obects that don't belong to study_area_3:

 //*[not(contains(name(),'study_area_3'))]

Select all object except desk_1, desk_2, blotter_1, and paperclip_2:

 //*[not(contains(name(),'desk_1')) and not(contains(name(),'desk_2'))
 and not(contains(name(),'blotter_1')) and not(contains(name(),'paperclip_1'))]

Select all object except desk_1`, desk_2, blotter_1, and paperclip_2 in set "shiny" but not set "burnt":

 //*[not(contains(name(),'desk_1')) and not(contains(name(),'desk_2'))
 and not(contains(name(),'blotter_1')) and not(contains(name(),'paperclip_1'))
 and contains(@sets,',shiny,') and not(contains(@sets,',burnt'))]

Select all of the pencil_boxes that are objects with "study" in the name:

 //*[contains(name(),'study')//*[contains(name(),'pencil_box')]

Select the "bolts" in all study_areas except study_area_2:

 //*[starts-with(name(),'study_area')
 and not(contains(name(),'3'))]//*[starts-with(name(),'bolt')]

Select all of the even numbered study_areas:

 //*[substring-after(name(),'study_areas') mod 2 = 0]

Select all object that are in set "red" and set "chair":

 //*[contains(@sets,',red,') and contains(@sets,'chair')]

Select all object that are in set "red" or set "chair":

 //*[contains(@sets,',red,') or contains(@sets,'chair')]

Select all objects that are in set "red" but not "chair":

 //*[contains(@sets,',red,') and not(contains(@sets,'chair'))]

Select all objects in sets that begin with "paint":

 //*[starts-with(@sets,',paint')]

Multipass Binding Rules

Beyond simple path expressions, RLF rules can contain references to the renderpass node, which contains information about the current renderpass. These expressions will include /renderpass[@attrName="..."]/.

For example if you wish to apply a shader to all of the geometry in the scene for all passes, except shadow, where you would apply the default shader you would write the following rules:

Rule one will find all geometry and stop matching when the pass_class is shadow, otherwise rule two will match all geometry.

You could also have written the rules in the following manner:

Here we match all objects that aren't in the shadow pass_class and assign the payload with the ID a_shader, otherewise the default shader payload will be injected, since the second rule will match all objects.

For a more complex example, let's consider the current behavior of RMS in relation to subsurface scattering. For SSS there are two passes that the renderer executes that we need to be concerned about in the context of shader binding.

1. SS_Render
2. Final

In the SS_Render pass we have the following attributes to the renderpass node:

"string pass_id" ["perspShape_SSRender"]
"string pass_phase" ["/Job/Frames/Maps/Subsurface"]
"string pass_class" ["SSRender"]
"string pass_flavor" [""]
"string pass_crew" ["blinn1SG"]
"string pass_camera_name" ["perspShape"]
"string pass_camera_flavor" [""]
"int pass_features_trace" [0]

Final pass has the following values:

"string pass_id" ["perspShape_Final"]
"string pass_phase" ["/Job/Frames/Images"]
"string pass_class" ["Final"]
"string pass_flavor" [""]
"string pass_crew" [""]
"string pass_camera_name" ["perspShape"]
"string pass_camera_flavor" [""]
"int pass_features_trace" [0]

These are the relevant parts of the shader declaration for each pass, using the previous RMS behavior.

SS_Render:
Surface "blinn1_rfm"  "string blinn1_rman__SSOutputFile" ["SSRender_blinn1SG.0001.ptc"]
                      "string blinn1_rman__SSMap" ["SSDiffuse_blinn1SG.0001.ptc"]

Final:
Surface "blinn1_rfm"  "string blinn1_rman__SSOutputFile" [""]
                  "string blinn1_rman__SSMap" ["SSDiffuse_blinn1SG.0001.ptc"]

Using XPath and dynamic edit rules we can achieve the same behavior.

First we will attach the blinn1 payload (the payload definitions are below this example):

Here for all passes except shadow, we attach the blinn1_shader payload to objects in the set "skin".

If we were using crews for the passes we could inject a visibility payload for all of the objects that aren't in the correct crew, which are represented as a set on the objects:

Alternatively you could reference the value of the pass_crew value directly:

/renderpass[@class = "SSRender"]//*[not(contains(@sets,/renderpass/@crew))]

Ri Request Edit Rules

XPath attributes are also used to refer to parameters of Ri requests. To match an attribute in XPath, use the @ character before the name of the attribute:

This rule will select the parameter of shaders that aren't in the SSRender pass class that have names that contain SSOutputFile and then will apply the payload final_SS_payload, which will replace the original value with an empty string. Since the shader uses the existence of the output file to inform its behavior, we have now achieved the same behavior that was accomplished with unique shader values for each pass.

These are the payloads that were used in the previous example:

Appendix

For more information on XPath Expressions:

• A quick reference with examples: http://en.wikipedia.org/wiki/XPath_1.0
• A technical language reference: http://www.w3.org/TR/xpath/