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')]
A Practical Scenario
XPath uses a matching syntax that allows the selection of
children based on parents and attributes. It also alows specific selection
based on path, index, and ancestry.
Th purpose of this section is to outline a few scenarios and how
the XPath expression would be written to select the desired item in order to
deliver the appropriate payload.
Say we have a scene with 10 robots, all based on the same model.
They are textured the same, and multiple Assemblies have been pre-baked and are
inserted into the Maya scene as lightweight Read Archives. All of the lookdev
has been done, and explicit connections were made in the Look File associated
with the model. Consider the following tasks:
·
I need to hide the "front_panel" so that it isn't
rendered on robot_3.
/robot_3//front_panel
·
The render is taking to long, so I need to hide all of the bolts
on all of the robots, except for the closest one: robot_2.
/assembly[name!="robot_2"]//shapes/name[contains(text(),'bolt')]text()
·
Robot_4 is generally too shiny, so the director has
asked for the shinyness (Ks) to be
taken down by 1/2./assembly[name="robot_4"]//shape/name[containts(text(),'bolt')]/surface/roughness
·
I need to hide all of the objects tagged in the set expendable.
//@expendable
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:
|
Injectable Rule Set
|
||
|
Continue Matching
|
Rule
|
Payload
|
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:
|
Injectable Rule Set
|
||
|
Continue Matching
|
Rule
|
Payload
|
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):
|
Injectable Rule Set
|
||
|
Continue Matching
|
Rule
|
Payload
|
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:
|
Injectable Rule Set
|
||
|
Continue Matching
|
Rule
|
Payload
|
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:
|
Edit Rule Set
|
||
|
Continue Matching
|
Rule
|
Payload
|
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:
|
Injectable Payloads
|
|
|
blinn1_shader:
|
|
|
Edit Payloads
|
|
|
final_SS_payload:
|
|
No comments:
Post a Comment