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https://github.com/PixarAnimationStudios/OpenSubdiv
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243 lines
8.8 KiB
ReStructuredText
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Copyright 2015 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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OSD Tessellation shader Interface
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---------------------------------
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.. contents::
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:local:
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:backlinks: none
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Basic
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=====
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Starting with 3.0, **Osd** tessellation shaders can be used as a set of functions from
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client shader code. In order to tessellate **Osd** patches, client shader
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code should perform the following steps (regular B-spline patch case):
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* In a tessellation control shader
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1. fetch a PatchParam for the current patch
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2. call OsdComputePerPatchVertexBSpline() to compute OsdPerPatchVertexBezier.
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3. compute tessellation level. To prevent cracks on transition patches,
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two vec4 parameters (tessOuterHi, tessOuterLo) will be needed in addition to built-in gl_TessLevelInner/Outers.
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* In a tessellation evaluation shader
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1. call OsdGetTessParameterization() to remap gl_TessCoord to a patch parameter at which to evaluate.
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2. call OsdEvalPatchBezier()/OsdEvalPatchGregory() to evaluate the current patch.
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The following is a minimal example of GLSL code explaining how client shader code
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uses OpenSubdiv shader functions to tessellate patches of a patch table.
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Tessellation Control Shader Example (for BSpline patches)
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*********************************************************
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.. code:: glsl
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layout (vertices = 16) out;
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in vec3 position[];
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patch out vec4 tessOuterLo, tessOuterHi;
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out OsdPerPatchVertexBezier v;
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void main()
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{
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// Get a patch param from texture buffer.
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ivec3 patchParam = OsdGetPatchParam(gl_PrimitiveID);
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// Compute per-patch vertices.
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OsdComputePerPatchVertexBSpline(patchParam, gl_InvocationID, position, v);
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// Compute tessellation factors.
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if (gl_InvocationID == 0) {
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vec4 tessLevelOuter = vec4(0);
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vec2 tessLevelInner = vec2(0);
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OsdGetTessLevelsUniform(patchParam,
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tessLevelOuter, tessLevelInner,
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tessOuterLo, tessOuterHi);
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gl_TessLevelOuter[0] = tessLevelOuter[0];
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gl_TessLevelOuter[1] = tessLevelOuter[1];
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gl_TessLevelOuter[2] = tessLevelOuter[2];
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gl_TessLevelOuter[3] = tessLevelOuter[3];
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gl_TessLevelInner[0] = tessLevelInner[0];
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gl_TessLevelInner[1] = tessLevelInner[1];
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}
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}
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Tessellation Evaluation Shader Example (for BSpline patches)
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************************************************************
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.. code:: glsl
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layout(quads) in;
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patch in vec4 tessOuterLo, tessOuterHi;
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in OsdPerPatchVertexBezier v[];
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uniform mat4 mvpMatrix;
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void main()
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{
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// Compute tesscoord.
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vec2 UV = OsdGetTessParameterization(gl_TessCoord.xy, tessOuterLo, tessOuterHi);
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vec3 P = vec3(0), dPu = vec3(0), dPv = vec3(0);
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vec3 N = vec3(0), dNu = vec3(0), dNv = vec3(0);
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ivec3 patchParam = inpt[0].v.patchParam;
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// Evaluate patch at the tess coord UV
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OsdEvalPatchBezier(patchParam, UV, v, P, dPu, dPv, N, dNu, dNv);
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// Apply model-view-projection matrix.
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gl_Position = mvpMatrix * vec4(P, 1);
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}
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Basis Conversion
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================
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B-spline Patch
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**************
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The following diagram shows how the **Osd** shaders process b-spline patches.
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.. image:: images/osd_shader_bspline.png
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While regular patches are expressed as b-spline patches in Far::PatchTable,
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the **Osd** shader converts them into Bezier basis patches for simplicity and efficiency.
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This conversion is performed in the tessellation control stage. The boundary edge evaluation
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and single crease matrix evaluation are also resolved during this conversion.
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OsdComputePerPatchVertexBSpline() can be used for this process.
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The resulting Bezier control vertices are stored in OsdPerPatchVertexBezier struct.
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.. code:: glsl
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void OsdComputePerPatchVertexBSpline(
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ivec3 patchParam, int ID, vec3 cv[16], out OsdPerPatchVertexBezier result);
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The tessellation evaluation shader takes an array of OsdPerPatchVertexBezier struct,
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and then evaluates the patch using the OsdEvalPatchBezier() function.
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.. code:: glsl
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void OsdEvalPatchBezier(ivec3 patchParam, vec2 UV,
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OsdPerPatchVertexBezier cv[16],
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out vec3 P, out vec3 dPu, out vec3 dPv,
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out vec3 N, out vec3 dNu, out vec3 dNv)
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Gregory Basis Patch
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*******************
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In a similar way, Gregory basis patches are processed as follows:
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.. image:: images/osd_shader_gregory.png
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OsdComputePerPatchVertexGregoryBasis() can be used for the Gregory patches
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(although no basis conversion involved for the Gregory patches) and the resulting vertices
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are stored in a OsdPerPatchVertexGreogryBasis struct.
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.. code:: glsl
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void OsdComputePerPatchVertexGregoryBasis(
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ivec3 patchParam, int ID, vec3 cv, out OsdPerPatchVertexGregoryBasis result)
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The tessellation evaluation shader takes an array of OsdPerPatchVertexGregoryBasis struct,
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and then evaluates the patch using the OsdEvalPatchGregory() function.
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.. code:: glsl
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void
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OsdEvalPatchGregory(ivec3 patchParam, vec2 UV, vec3 cv[20],
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out vec3 P, out vec3 dPu, out vec3 dPv,
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out vec3 N, out vec3 dNu, out vec3 dNv)
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Legacy Gregory Patch (2.x compatibility)
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****************************************
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OpenSubdiv 3.0 also supports 2.x style Gregory patch evaluation (see far_overview).
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In order to evaluate a legacy Gregory patch, client needs to bind extra buffers and
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to perform extra steps in the vertex shader as shown in the following diagram:
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.. image:: images/osd_shader_legacy_gregory.png
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Tessellation levels
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===================
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**Osd** provides both uniform and screen-space adaptive tessellation level computation.
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Uniform tessellation
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OsdGetTessLevelsUniform()
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Screen-space adaptive tessellation
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OsdGetTessLevelsAdaptiveLimitPoints()
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Because of the nature of `feature adaptive subdivision <far_overview.html>`__,
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we need to pay extra attention for a patch's outer tessellation level for the screen-space
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adaptive case so that cracks don't appear.
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An edge of the patch marked as a transition edge is split into two segments (Hi and Lo).
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.. image:: images/osd_shader_patch.png
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The **Osd** shaders uses these two segments to ensure the same tessellation along the
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edge between different levels of subdivision. In the following example, suppose the left hand side
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patch has determined the tessellation level of its right edge to be 5. gl_TessLevelOuter is set to
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5 for the edge, and at the same time we also pass 2 and 3 to the tessellation evaluation shader
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as separate levels for the two segments of the edge split at the middle.
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.. image:: images/osd_shader_transition.png
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Then the tessellation evaluation shader takes gl_TessCoord and those two values, and remaps
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gl_TessCoord using OsdGetTessParameterization() to ensure the parameters are consistent
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across adjacent patches.
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.. image:: images/osd_shader_param_remap.png
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.. code:: glsl
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vec2 OsdGetTessParameterization(vec2 uv, vec4 tessOuterLo, vec4 tessOuterHi)
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These tessellation levels can be computed by OsdGetTessLevelsAdaptiveLimitPoints()
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in the tessellation control shader. Note that this function requires all 16 bezier control
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points, you need to call barrier() to ensure the conversion is done for all invocations.
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See osd/glslPatchBSpline.glsl for more details.
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.. code:: glsl
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void OsdGetTessLevelsAdaptiveLimitPoints(OsdPerPatchVertexBezier cpBezier[16],
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ivec3 patchParam,
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out vec4 tessLevelOuter, out vec2 tessLevelInner,
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out vec4 tessOuterLo, out vec4 tessOuterHi)
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.. container:: notebox
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**Release Notes (3.0.0)**
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* Currently OsdGetTessParameterization doesn't support fraction spacing.
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It will be fixed in a future release.
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