\nThis chapter introduces the process of surface subdivision in the rendering pipeline.<\/p>\n<\/blockquote>\n\n\n\n
The tessellation shader is located after the vertex shader, and the tessellation is divided into three steps: Hull, Tesselllator and Domain, among which Tessellator is not programmable.<\/p>\n\n\n\nThe first step of tessellation is the tessellation control shader (also known as Tessellation Control Shader, TCS), which will output control points and tessellation factors. This stage mainly consists of two parallel functions: Hull Function and Patch Constant Function.<\/p>\n\n\n\nBoth functions receive patches, which are a set of vertex indices. For example, a triangle uses three numbers to represent the vertex indices. One patch can form a fragment, for example, a triangle fragment is composed of three vertex indices.<\/p>\n\n\n\n
Moreover, the Hull Function is executed once for each vertex, and the Path Constant Function is executed once for each Patch. The former outputs the modified control point data (usually including vertex position, possible normals, texture coordinates and other attributes), while the latter outputs the constant data related to the entire fragment, that is, the subdivision factor. The subdivision factor tells the next stage (the tessellator) how to subdivide each fragment.<\/p>\n\n\n\n
In general, the Hull Function modifies each control point, while the Patch Constant Function determines the level of subdivision based on the distance from the camera.<\/p>\n\n\n\nNext comes the non-programmable stage, the tessellator. It receives the patch and the subdivision factor just obtained. The tessellator generates a barycentric coordinate for each vertex data.<\/p>\n\n\n\nNext comes the last step, the Domain Stage (also known as Tessellation Evaluation Shader, TES), which is programmable. This part consists of domain functions, which are executed once per vertex. It receives the barycentric coordinates and the results generated by the two functions in the Patch and Hull Stage. Most of the logic is written here. The most important thing is that you can reposition the vertices in this stage, which is the most important part of tessellation.<\/p>\n\n\n\nIf there is a geometry shader, it will be executed after the Domain Stage. But if not, it will come to the rasterization stage.<\/p>\n\n\n\nIn summary, the first thing is the vertex shader. The Hull stage accepts vertex data and decides how to subdivide the mesh. Then the tessellator stage processes the subdivided mesh, and finally the Domain stage outputs vertices for the fragment shader.<\/p>\n\n\n\n
2. Surface subdivision analysis<\/h2>\n\n\n\n\nThis chapter contains code analysis of Unity's surface subdivision, practical example effects display and an overview of the underlying principles.<\/p>\n<\/blockquote>\n\n\n\n
2.1 Key code analysis<\/h3>\n\n\n\n2.1.1 Basic settings of Unity tessellation<\/h3>\n\n\n\n First of all, the tessellation shader needs to use shader target 5.0.<\/p>\n\n\n\n
<\/circle><\/circle><\/circle><\/g><\/svg><\/span><\/path><\/path><\/svg><\/span>HLSLPROGRAM<\/span><\/span>\n#<\/span>Pragmas<\/span> <\/span>target<\/span> <\/span>5.0<\/span> <\/span>\/\/ 5.0 required for tessellation<\/span><\/span>\n<\/span>\n#<\/span>Pragmas<\/span> <\/span>vertex<\/span> <\/span>Vertex<\/span><\/span>\n#<\/span>Pragmas<\/span> <\/span>hull<\/span> <\/span>Hull<\/span><\/span>\n#<\/span>Pragmas<\/span> <\/span>domain<\/span> <\/span>Domain<\/span><\/span>\n#<\/span>Pragmas<\/span> <\/span>fragment<\/span> <\/span>Fragment<\/span><\/span>\n<\/span>\nENDHLSL<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n2.1.2 Hull Stage Code 1 \u2013 Hull Function<\/h3>\n\n\n\n In the classic process, the vertex shader converts the position and normal information into world space. Then the output result is passed to the Hull Stage. It should be noted that, unlike the vertex shader, the vertices of the Hull shader are represented by INTERNALTESSPOS semantics instead of POSITION semantics. The reason is that Hull does not need to output these vertex positions to the next rendering process, but for its own internal tessellation algorithm, so it will convert these vertices to a coordinate system that is more suitable for tessellation. In addition, developers can also distinguish more clearly.<\/p>\n\n\n\n
<\/circle><\/circle><\/circle><\/g><\/svg><\/span><\/path><\/path><\/svg><\/span>struct<\/span> <\/span>Attributes<\/span> <\/span>{<\/span><\/span>\n <\/span>float3<\/span> positionOS <\/span>:<\/span> <\/span>POSITION<\/span>;<\/span><\/span>\n <\/span>float3<\/span> normalOS <\/span>:<\/span> <\/span>NORMAL<\/span>;<\/span><\/span>\n <\/span>UNITY_VERTEX_INPUT_INSTANCE_ID<\/span><\/span>\n}<\/span>;<\/span><\/span>\n<\/span>\nstruct<\/span> <\/span>TessellationControlPoint<\/span> <\/span>{<\/span><\/span>\n <\/span>float3<\/span> positionWS <\/span>:<\/span> <\/span>INTERNAL LTESS POS<\/span>;<\/span><\/span>\n <\/span>float3<\/span> normalWS <\/span>:<\/span> <\/span>NORMAL<\/span>;<\/span><\/span>\n <\/span>UNITY_VERTEX_INPUT_INSTANCE_ID<\/span><\/span>\n}<\/span>;<\/span><\/span>\n<\/span>\nTessellationControlPoint<\/span> <\/span>Vertex<\/span>(<\/span>Attributes<\/span> <\/span>input<\/span>) <\/span>{<\/span><\/span>\n <\/span>TessellationControlPoint<\/span> <\/span>output<\/span>;<\/span><\/span>\n<\/span>\n <\/span>UNITY_SETUP_INSTANCE_ID<\/span>(<\/span>input<\/span>)<\/span>;<\/span><\/span>\n <\/span>UNITY_TRANSFER_INSTANCE_ID<\/span>(<\/span>input<\/span>,<\/span> <\/span>output<\/span>)<\/span>;<\/span><\/span>\n<\/span>\n <\/span>VertexPositionInputs<\/span> <\/span>posnInputs<\/span> <\/span>=<\/span> <\/span>GetVertexPositionInputs<\/span>(<\/span>input<\/span>.<\/span>positionOS<\/span>)<\/span>;<\/span><\/span>\n <\/span>VertexNormalInputs<\/span> <\/span>normalInputs<\/span> <\/span>=<\/span> <\/span>GetVertexNormalInputs<\/span>(<\/span>input<\/span>.<\/span>normalOS<\/span>)<\/span>;<\/span><\/span>\n<\/span>\n <\/span>output<\/span>.<\/span>positionWS<\/span> <\/span>=<\/span> <\/span>posnInputs<\/span>.<\/span>positionWS<\/span>;<\/span><\/span>\n <\/span>output<\/span>.<\/span>normalWS<\/span> <\/span>=<\/span> <\/span>normalInputs<\/span>.<\/span>normalWS<\/span>;<\/span><\/span>\n <\/span>return<\/span> <\/span>output<\/span>;<\/span><\/span>\n}<\/span><\/span><\/code><\/pre><\/div>\n\n\n\nBelow are some setting parameters for the Hull Shader.<\/p>\n\n\n\n
The first line, domain, defines the domain type of the tessellation shader, which means that both the input and output are triangle primitives. You can choose tri (triangle), quad (quadrilateral), etc.<\/p>\n\n\n\n
The second line outputcontrolpoints indicates the number of output control points, 3 corresponds to the three vertices of the triangle.<\/p>\n\n\n\n
The third line outputtopology indicates the topological structure of the primitive after subdivision. triangle_cw means that the vertices of the output triangle are sorted clockwise. The correct order can ensure that the surface faces outward. triangle_cw (clockwise around the triangle), triangle_ccw (counterclockwise around the triangle), line (line segment)<\/p>\n\n\n\n
The fourth line patchconstantfunc is another function of the Hull Stage, which outputs constant data such as subdivision factors. A patch is executed only once.<\/p>\n\n\n\n
The fifth line, partitioning, specifies how to distribute additional vertices to the edges of the original Path primitive. This step can make the subdivision process smoother and more uniform. integer, fractional_even, fractional_odd.<\/p>\n\n\n\n
The maxtessfactor in the sixth line represents the maximum subdivision factor. Limiting the maximum subdivision can control the rendering burden.<\/p>\n\n\n\n
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