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https://github.com/PixarAnimationStudios/OpenSubdiv
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7f8fc77cfc
This is a good start, but more could be done to make it even more useful for developers. * Updated nav_template to include contributing * Removed contributing from intro * Fixed RST compiler error in hedits * Renamed "OpenSubdiv" header in nav to "OpenSubdiv User Guide"
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182 lines
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Copyright 2013 Pixar
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Licensed under the Apache License, Version 2.0 (the "Apache License")
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with the following modification; you may not use this file except in
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compliance with the Apache License and the following modification to it:
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Section 6. Trademarks. is deleted and replaced with:
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6. Trademarks. This License does not grant permission to use the trade
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names, trademarks, service marks, or product names of the Licensor
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and its affiliates, except as required to comply with Section 4(c) of
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the License and to reproduce the content of the NOTICE file.
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You may obtain a copy of the Apache License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the Apache License with the above modification is
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distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the Apache License for the specific
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language governing permissions and limitations under the Apache License.
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Hierarchical Edits
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------------------
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.. contents::
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:local:
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:backlinks: none
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----
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Hierarchical Edits
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==================
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To understand the hierarchical aspect of subdivision, we realize that
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subdivision itself leads to a natural hierarchy: after the first level of
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subdivision, each face in a subdivision mesh subdivides to four quads (in the
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Catmull-Clark scheme), or four triangles (in the Loop scheme). This creates a
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parent and child relationship between the original face and the resulting four
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subdivided faces, which in turn leads to a hierarchy of subdivision as each
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child in turn subdivides. A hierarchical edit is an edit made to any one of the
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faces, edges, or vertices that arise anywhere during subdivision. Normally
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these subdivision components inherit values from their parents based on a set
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of subdivision rules that depend on the subdivision scheme.
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A hierarchical edit overrides these values. This allows for a compact
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specification of localized detail on a subdivision surface, without having to
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express information about the rest of the subdivision surface at the same level
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of detail.
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.. image:: images/hedit_example1.png
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:align: center
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:height: 300
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:target: images/hedit_example1.png
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----
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.. container:: notebox
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**Release Notes (3.0.0)**
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Hierarchical Edits have been marked as "extended specification" and support for
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hierarchical features has been removed from the 3.0 release. This decision
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allows for great simplifications of many areas of the subdivision algorithms.
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If we can identify legitimate use-cases for hierarchical tags, we will consider
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re-implementing them in future releases, as time and resources allow.
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----
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Hierarchical Edits Paths
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************************
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In order to perform a hierarchical edit, we need to be able to name the
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subdivision component we are interested in, no matter where it may occur in the
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subdivision hierarchy. This leads us to a hierarchical path specification for
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faces, since once we have a face we can navigate to an incident edge or vertex
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by association. We note that in a subdivision mesh, a face always has incident
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vertices, which are labelled (in relation to the face) with an integer index
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starting at zero and in consecutive order according to the usual winding rules
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for subdivision surfaces. Faces also have incident edges, and these are
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labelled according to the origin vertex of the edge.
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.. image:: images/face_winding.png
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:align: center
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:target: images/face_winding.png
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.. role:: red
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.. role:: green
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.. role:: blue
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In this diagram, the indices of the vertices of the base face are marked in
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:red:`red`; so on the left we have an extraordinary Catmull-Clark face with
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five vertices (labeled :red:`0-4`) and on the right we have a regular
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Catmull-Clark face with four vertices (labelled :red:`0-3`). The indices of the
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child faces are :blue:`blue`; note that in both the extraordinary and regular
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cases, the child faces are indexed the same way, i.e. the sub-face labeled
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:blue:`n` has one incident vertex that is the result of the subdivision of the
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parent vertex also labeled :red:`n` in the parent face. Specifically, we note
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that the sub-face :blue:`1` in both the regular and extraordinary face is
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nearest to the vertex labelled :red:`1` in the parent.
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The indices of the vertices of the child faces are labeled :green:`green`, and
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this is where the difference lies between the extraordinary and regular case;
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in the extraordinary case, vertex to vertex subdivision always results in a
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vertex labeled :green:`0`, while in the regular case, vertex to vertex
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subdivision assigns the same index to the child vertex. Again, specifically, we
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note that the parent vertex indexed :red:`1` in the extraordinary case has a
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child vertex :green:`0`, while in the regular case the parent vertex indexed
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:red:`1` actually has a child vertex that is indexed :green:`1`. Note that this
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indexing scheme was chosen to maintain the property that the vertex labeled 0
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always has the lowest u/v parametric value on the face.
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.. image:: images/hedit_path.gif
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:align: center
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:target: images/hedit_path.gif
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By appending a vertex index to a face index, we can create a vertex path
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specification. For example, (:blue:`655` :green:`2` :red:`3` 0) specifies the
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1st. vertex of the :red:`3` rd. child face of the :green:`2` nd. child face of
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the of the :blue:`655` th. face of the subdivision mesh.
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----
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Vertex Edits
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************
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Vertex hierarchical edits can modify the value or the sharpness of primitive
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variables for vertices and sub-vertices anywhere in the subdivision hierarchy.
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.. image:: images/hedit_example1.png
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:align: center
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:height: 300
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:target: images/hedit_example1.png
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The edits are performed using either an "add" or a "set" operator. "set"
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indicates the primitive variable value or sharpness is to be set directly to
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the values specified. "add" adds a value to the normal result computed via
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standard subdivision rules. In other words, this operation allows value offsets
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to be applied to the mesh at any level of the hierarchy.
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.. image:: images/hedit_example2.png
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:align: center
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:height: 300
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:target: images/hedit_example2.png
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----
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Edge Edits
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**********
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Edge hierarchical edits can only modify the sharpness of primitive variables for edges
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and sub-edges anywhere in the subdivision hierarchy.
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.. image:: images/hedit_example4.png
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:align: center
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:height: 300
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:target: images/hedit_example4.png
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----
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Face Edits
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**********
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Face hierarchical edits can modify several properties of faces and sub-faces
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anywhere in the subdivision hierarchy.
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Modifiable properties include:
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* The "set" or "add" operators modify the value of primitive variables
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associated with faces.
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* The "hole" operation introduces holes (missing faces) into the subdivision
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mesh at any level in the subdivision hierarchy. The faces will be deleted,
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and none of their children will appear (you cannot "unhole" a face if any
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ancestor is a "hole"). This operation takes no float or string arguments.
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.. image:: images/hedit_example5.png
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:align: center
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:height: 300
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:target: images/hedit_example5.png
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