1// Copyright 2015-2023 The Khronos Group Inc. 2// 3// SPDX-License-Identifier: CC-BY-4.0 4 5[[tessellation]] 6= Tessellation 7 8Tessellation involves three pipeline stages. 9First, a <<shaders-tessellation-control,tessellation control shader>> 10transforms control points of a patch and can: produce per-patch data. 11Second, a fixed-function tessellator generates multiple primitives 12corresponding to a tessellation of the patch in (u,v) or (u,v,w) parameter 13space. 14Third, a <<shaders-tessellation-evaluation,tessellation evaluation shader>> 15transforms the vertices of the tessellated patch, for example to compute 16their positions and attributes as part of the tessellated surface. 17The tessellator is enabled when the pipeline contains both a tessellation 18control shader and a tessellation evaluation shader. 19 20 21== Tessellator 22 23If a pipeline includes both tessellation shaders (control and evaluation), 24the tessellator consumes each input patch (after vertex shading) and 25produces a new set of independent primitives (points, lines, or triangles). 26These primitives are logically produced by subdividing a geometric primitive 27(rectangle or triangle) according to the per-patch outer and inner 28tessellation levels written by the tessellation control shader. 29These levels are specified using the <<interfaces-builtin-variables,built-in 30variables>> code:TessLevelOuter and code:TessLevelInner, respectively. 31This subdivision is performed in an implementation-dependent manner. 32If no tessellation shaders are present in the pipeline, the tessellator is 33disabled and incoming primitives are passed through without modification. 34 35The type of subdivision performed by the tessellator is specified by an 36code:OpExecutionMode instruction using one of the code:Triangles, 37code:Quads, or code:IsoLines execution modes. 38ifdef::VK_EXT_shader_object[] 39When using <<shaders-objects, shader objects>>, this instruction must: be 40specified in the tessellation evaluation shader, and may: also be specified 41in the tessellation control shader. 42When using pipelines, this 43endif::VK_EXT_shader_object[] 44ifndef::VK_EXT_shader_object[This] 45instruction may: be specified in either the tessellation evaluation or 46tessellation control shader. 47ifdef::VK_EXT_shader_object[] 48When using shader objects, tessellation-related modes that are required: 49must: be specified in the tessellation evaluation shader, and may: also be 50specified in the tessellation control shader. 51Other tessellation-related modes may: be specified in the tessellation 52evaluation shader. 53When using pipelines, other 54endif::VK_EXT_shader_object[] 55ifndef::VK_EXT_shader_object[Other] 56tessellation-related execution modes can: also be specified in either the 57tessellation control or tessellation evaluation shaders. 58 59Any tessellation-related modes specified in both the tessellation control 60and tessellation evaluation shaders must: be the same. 61 62Tessellation execution modes include: 63 64 * code:Triangles, code:Quads, and code:IsoLines. 65 These control the type of subdivision and topology of the output 66 primitives. 67ifdef::VK_EXT_shader_object[] 68 When using <<shaders-objects, shader objects>>, one mode must: be set in 69 at least the tessellation evaluation stage. 70 When using pipelines, one 71endif::VK_EXT_shader_object[] 72ifndef::VK_EXT_shader_object[One] 73 mode must: be set in at least one of the tessellation shader stages. 74ifdef::VK_KHR_portability_subset[] 75 If the `apiext:VK_KHR_portability_subset` extension is enabled, and 76 slink:VkPhysicalDevicePortabilitySubsetFeaturesKHR::pname:tessellationIsolines 77 is ename:VK_FALSE, then isoline tessellation is not supported by the 78 implementation, and code:IsoLines must: not be used in either 79 tessellation shader stage. 80endif::VK_KHR_portability_subset[] 81 * code:VertexOrderCw and code:VertexOrderCcw. 82 These control the orientation of triangles generated by the tessellator. 83ifdef::VK_EXT_shader_object[] 84 When using <<shaders-objects, shader objects>>, one mode must: be set in 85 at least the tessellation evaluation stage. 86 When using pipelines, one 87endif::VK_EXT_shader_object[] 88ifndef::VK_EXT_shader_object[One] 89 mode must: be set in at least one of the tessellation shader stages. 90 * code:PointMode. 91 Controls generation of points rather than triangles or lines. 92 This functionality defaults to disabled, and is enabled if either shader 93 stage includes the execution mode. 94ifdef::VK_EXT_shader_object[] 95 When using <<shaders-objects, shader objects>>, if code:PointMode is set 96 in the tessellation control stage, it must: be identically set in the 97 tessellation evaluation stage. 98endif::VK_EXT_shader_object[] 99ifdef::VK_KHR_portability_subset[] 100 If the `apiext:VK_KHR_portability_subset` extension is enabled, and 101 slink:VkPhysicalDevicePortabilitySubsetFeaturesKHR::pname:tessellationPointMode 102 is ename:VK_FALSE, then point mode tessellation is not supported by the 103 implementation, and code:PointMode must: not be used in either 104 tessellation shader stage. 105endif::VK_KHR_portability_subset[] 106 * code:SpacingEqual, code:SpacingFractionalEven, and 107 code:SpacingFractionalOdd. 108 Controls the spacing of segments on the edges of tessellated primitives. 109ifdef::VK_EXT_shader_object[] 110 When using <<shaders-objects, shader objects>>, one mode must: be set in 111 at least the tessellation evaluation stage. 112 When using pipelines, one 113endif::VK_EXT_shader_object[] 114ifndef::VK_EXT_shader_object[One] 115 mode must: be set in at least one of the tessellation shader stages. 116 * code:OutputVertices. 117 Controls the size of the output patch of the tessellation control 118 shader. 119ifdef::VK_EXT_shader_object[] 120 When using <<shaders-objects, shader objects>>, one value must: be set 121 in at least the tessellation control stage. 122 When using pipelines, one 123endif::VK_EXT_shader_object[] 124ifndef::VK_EXT_shader_object[One] 125 value must: be set in at least one of the tessellation shader stages. 126 127For triangles, the tessellator subdivides a triangle primitive into smaller 128triangles. 129For quads, the tessellator subdivides a rectangle primitive into smaller 130triangles. 131For isolines, the tessellator subdivides a rectangle primitive into a 132collection of line segments arranged in strips stretching across the 133rectangle in the [eq]#u# dimension (i.e. the coordinates in code:TessCoord 134are of the form [eq]#(0,x)# through [eq]#(1,x)# for all tessellation 135evaluation shader invocations that share a line). 136 137Each vertex produced by the tessellator has an associated (u,v,w) or (u,v) 138position in a normalized parameter space, with parameter values in the range 139[eq]#[0,1]#, as illustrated 140ifdef::VK_VERSION_1_1,VK_KHR_maintenance2[] 141in figures <<img-tessellation-topology-ul>> and 142<<img-tessellation-topology-ll>>. 143The domain space can: have either an upper-left or lower-left origin, 144selected by the pname:domainOrigin member of 145slink:VkPipelineTessellationDomainOriginStateCreateInfo. 146endif::VK_VERSION_1_1,VK_KHR_maintenance2[] 147ifndef::VK_VERSION_1_1,VK_KHR_maintenance2[] 148in figure <<img-tessellation-topology-ul>>. 149The domain space has an upper-left origin. 150endif::VK_VERSION_1_1,VK_KHR_maintenance2[] 151 152[[img-tessellation-topology-ul]] 153image::{images}/tessparamUL.svg[align="center",title="Domain parameterization for tessellation primitive modes (upper-left origin)",opts="{imageopts}"] 154 155ifdef::VK_VERSION_1_1,VK_KHR_maintenance2[] 156[[img-tessellation-topology-ll]] 157image::{images}/tessparam.svg[align="center",title="Domain parameterization for tessellation primitive modes (lower-left origin)",opts="{imageopts}"] 158endif::VK_VERSION_1_1,VK_KHR_maintenance2[] 159 160.Caption 161**** 162In the domain parameterization diagrams, the coordinates illustrate the 163value of code:TessCoord at the corners of the domain. 164The labels on the edges indicate the inner (IL0 and IL1) and outer (OL0 165through OL3) tessellation level values used to control the number of 166subdivisions along each edge of the domain. 167**** 168 169For triangles, the vertex's position is a barycentric coordinate 170[eq]#(u,v,w)#, where [eq]#u {plus} v {plus} w = 1.0#, and indicates the 171relative influence of the three vertices of the triangle on the position of 172the vertex. 173For quads and isolines, the position is a [eq]#(u,v)# coordinate indicating 174the relative horizontal and vertical position of the vertex relative to the 175subdivided rectangle. 176The subdivision process is explained in more detail in subsequent sections. 177 178 179== Tessellator Patch Discard 180 181A patch is discarded by the tessellator if any relevant outer tessellation 182level is less than or equal to zero. 183 184Patches will also be discarded if any relevant outer tessellation level 185corresponds to a floating-point [eq]#NaN# (not a number) in implementations 186supporting [eq]#NaN#. 187 188No new primitives are generated and the tessellation evaluation shader is 189not executed for patches that are discarded. 190For code:Quads, all four outer levels are relevant. 191For code:Triangles and code:IsoLines, only the first three or two outer 192levels, respectively, are relevant. 193Negative inner levels will not cause a patch to be discarded; they will be 194clamped as described below. 195 196 197[[tessellation-tessellator-spacing]] 198== Tessellator Spacing 199 200Each of the tessellation levels is used to determine the number and spacing 201of segments used to subdivide a corresponding edge. 202The method used to derive the number and spacing of segments is specified by 203an code:OpExecutionMode in the tessellation control or tessellation 204evaluation shader using one of the identifiers code:SpacingEqual, 205code:SpacingFractionalEven, or code:SpacingFractionalOdd. 206 207If code:SpacingEqual is used, the floating-point tessellation level is first 208clamped to [eq]#[1, pname:maxLevel]#, where [eq]#pname:maxLevel# is the 209implementation-dependent maximum tessellation level 210(sname:VkPhysicalDeviceLimits::pname:maxTessellationGenerationLevel). 211The result is rounded up to the nearest integer [eq]#n#, and the 212corresponding edge is divided into [eq]#n# segments of equal length in (u,v) 213space. 214 215If code:SpacingFractionalEven is used, the tessellation level is first 216clamped to [eq]#[2, pname:maxLevel]# and then rounded up to the nearest even 217integer [eq]#n#. 218If code:SpacingFractionalOdd is used, the tessellation level is clamped to 219[eq]#[1, pname:maxLevel - 1]# and then rounded up to the nearest odd integer 220[eq]#n#. 221If [eq]#n# is one, the edge will not be subdivided. 222Otherwise, the corresponding edge will be divided into [eq]#n - 2# segments 223of equal length, and two additional segments of equal length that are 224typically shorter than the other segments. 225The length of the two additional segments relative to the others will 226decrease monotonically with [eq]#n - f#, where [eq]#f# is the clamped 227floating-point tessellation level. 228When [eq]#n - f# is zero, the additional segments will have equal length to 229the other segments. 230As [eq]#n - f# approaches 2.0, the relative length of the additional 231segments approaches zero. 232The two additional segments must: be placed symmetrically on opposite sides 233of the subdivided edge. 234The relative location of these two segments is implementation-dependent, but 235must: be identical for any pair of subdivided edges with identical values of 236[eq]#f#. 237 238When tessellating triangles or quads using <<tessellation-point-mode, point 239mode>> with fractional odd spacing, the tessellator may: produce _interior 240vertices_ that are positioned on the edge of the patch if an inner 241tessellation level is less than or equal to one. 242Such vertices are considered distinct from vertices produced by subdividing 243the outer edge of the patch, even if there are pairs of vertices with 244identical coordinates. 245 246 247[[tessellation-primitive-order]] 248== Tessellation Primitive Ordering 249 250Few guarantees are provided for the relative ordering of primitives produced 251by tessellation, as they pertain to <<drawing-primitive-order, primitive 252order>>. 253 254 * The output primitives generated from each input primitive are passed to 255 subsequent pipeline stages in an implementation-dependent order. 256 * All output primitives generated from a given input primitive are passed 257 to subsequent pipeline stages before any output primitives generated 258 from subsequent input primitives. 259 260 261[[tessellation-vertex-winding-order]] 262== Tessellator Vertex Winding Order 263 264When the tessellator produces triangles (in the code:Triangles or code:Quads 265modes), the orientation of all triangles is specified with an 266code:OpExecutionMode of code:VertexOrderCw or code:VertexOrderCcw in the 267tessellation control or tessellation evaluation shaders. 268If the order is code:VertexOrderCw, the vertices of all generated triangles 269will have clockwise ordering in (u,v) or (u,v,w) space. 270If the order is code:VertexOrderCcw, the vertices will have 271counter-clockwise ordering in that space. 272 273If the tessellation domain has an upper-left origin, the vertices of a 274triangle have counter-clockwise ordering if 275 276 {empty}:: [eq]#a = u~0~ v~1~ - u~1~ v~0~ {plus} u~1~ v~2~ - u~2~ v~1~ 277 {plus} u~2~ v~0~ - u~0~ v~2~# 278 279is negative, and clockwise ordering if [eq]#a# is positive. 280[eq]#u~i~# and [eq]#v~i~# are the [eq]#u# and [eq]#v# coordinates in 281normalized parameter space of the [eq]##i##th vertex of the triangle. 282ifdef::VK_VERSION_1_1,VK_KHR_maintenance2[] 283If the tessellation domain has a lower-left origin, the vertices of a 284triangle have counter-clockwise ordering if [eq]#a# is positive, and 285clockwise ordering if [eq]#a# is negative. 286endif::VK_VERSION_1_1,VK_KHR_maintenance2[] 287 288[NOTE] 289.Note 290==== 291The value [eq]#a# is proportional (with a positive factor) to the signed 292area of the triangle. 293 294In code:Triangles mode, even though the vertex coordinates have a [eq]#w# 295value, it does not participate directly in the computation of [eq]#a#, being 296an affine combination of [eq]#u# and [eq]#v#. 297==== 298 299 300[[tessellation-triangle-tessellation]] 301== Triangle Tessellation 302 303If the tessellation primitive mode is code:Triangles, an equilateral 304triangle is subdivided into a collection of triangles covering the area of 305the original triangle. 306First, the original triangle is subdivided into a collection of concentric 307equilateral triangles. 308The edges of each of these triangles are subdivided, and the area between 309each triangle pair is filled by triangles produced by joining the vertices 310on the subdivided edges. 311The number of concentric triangles and the number of subdivisions along each 312triangle except the outermost is derived from the first inner tessellation 313level. 314The edges of the outermost triangle are subdivided independently, using the 315first, second, and third outer tessellation levels to control the number of 316subdivisions of the [eq]#u = 0# (left), [eq]#v = 0# (bottom), and [eq]#w = 3170# (right) edges, respectively. 318The second inner tessellation level and the fourth outer tessellation level 319have no effect in this mode. 320 321If the first inner tessellation level and all three outer tessellation 322levels are exactly one after clamping and rounding, only a single triangle 323with [eq]#(u,v,w)# coordinates of [eq]#(0,0,1)#, [eq]#(1,0,0)#, and 324[eq]#(0,1,0)# is generated. 325If the inner tessellation level is one and any of the outer tessellation 326levels is greater than one, the inner tessellation level is treated as 327though it were originally specified as [eq]#1 {plus} {epsilon}# and will 328result in a two- or three-segment subdivision depending on the tessellation 329spacing. 330When used with fractional odd spacing, the three-segment subdivision may: 331produce _inner vertices_ positioned on the edge of the triangle. 332 333If any tessellation level is greater than one, tessellation begins by 334producing a set of concentric inner triangles and subdividing their edges. 335First, the three outer edges are temporarily subdivided using the clamped 336and rounded first inner tessellation level and the specified tessellation 337spacing, generating [eq]#n# segments. 338For the outermost inner triangle, the inner triangle is degenerate -- a 339single point at the center of the triangle -- if [eq]#n# is two. 340Otherwise, for each corner of the outer triangle, an inner triangle corner 341is produced at the intersection of two lines extended perpendicular to the 342corner's two adjacent edges running through the vertex of the subdivided 343outer edge nearest that corner. 344If [eq]#n# is three, the edges of the inner triangle are not subdivided and 345it is the final triangle in the set of concentric triangles. 346Otherwise, each edge of the inner triangle is divided into [eq]#n - 2# 347segments, with the [eq]#n - 1# vertices of this subdivision produced by 348intersecting the inner edge with lines perpendicular to the edge running 349through the [eq]#n - 1# innermost vertices of the subdivision of the outer 350edge. 351Once the outermost inner triangle is subdivided, the previous subdivision 352process repeats itself, using the generated triangle as an outer triangle. 353This subdivision process is illustrated in <<img-innertri,Inner Triangle 354Tessellation>>. 355 356[[img-innertri]] 357image::{images}/innertri.svg[align="center",title="Inner Triangle Tessellation",opts="{imageopts}"] 358 359.Caption 360**** 361In the <<img-innertri,Inner Triangle Tessellation>> diagram, inner 362tessellation levels of (a) four and (b) five are shown (not to scale). 363Solid black circles depict vertices along the edges of the concentric 364triangles. 365The edges of inner triangles are subdivided by intersecting the edge with 366segments perpendicular to the edge passing through each inner vertex of the 367subdivided outer edge. 368Dotted lines depict edges connecting corresponding vertices on the inner and 369outer triangle edges. 370**** 371 372Once all the concentric triangles are produced and their edges are 373subdivided, the area between each pair of adjacent inner triangles is filled 374completely with a set of non-overlapping triangles. 375In this subdivision, two of the three vertices of each triangle are taken 376from adjacent vertices on a subdivided edge of one triangle; the third is 377one of the vertices on the corresponding edge of the other triangle. 378If the innermost triangle is degenerate (i.e., a point), the triangle 379containing it is subdivided into six triangles by connecting each of the six 380vertices on that triangle with the center point. 381If the innermost triangle is not degenerate, that triangle is added to the 382set of generated triangles as-is. 383 384After the area corresponding to any inner triangles is filled, the 385tessellator generates triangles to cover the area between the outermost 386triangle and the outermost inner triangle. 387To do this, the temporary subdivision of the outer triangle edge above is 388discarded. 389Instead, the [eq]#u = 0#, [eq]#v = 0#, and [eq]#w = 0# edges are subdivided 390according to the first, second, and third outer tessellation levels, 391respectively, and the tessellation spacing. 392The original subdivision of the first inner triangle is retained. 393The area between the outer and first inner triangles is completely filled by 394non-overlapping triangles as described above. 395If the first (and only) inner triangle is degenerate, a set of triangles is 396produced by connecting each vertex on the outer triangle edges with the 397center point. 398 399After all triangles are generated, each vertex in the subdivided triangle is 400assigned a barycentric (u,v,w) coordinate based on its location relative to 401the three vertices of the outer triangle. 402 403The algorithm used to subdivide the triangular domain in (u,v,w) space into 404individual triangles is implementation-dependent. 405However, the set of triangles produced will completely cover the domain, and 406no portion of the domain will be covered by multiple triangles. 407 408Output triangles are generated with a topology similar to 409<<drawing-triangle-lists, triangle lists>>, except that the order in which 410each triangle is generated, and the order in which the vertices are 411generated for each triangle, are implementation-dependent. 412However, the order of vertices in each triangle is consistent across the 413domain as described in <<tessellation-vertex-winding-order>>. 414 415 416[[tessellation-quad-tessellation]] 417== Quad Tessellation 418 419If the tessellation primitive mode is code:Quads, a rectangle is subdivided 420into a collection of triangles covering the area of the original rectangle. 421First, the original rectangle is subdivided into a regular mesh of 422rectangles, where the number of rectangles along the [eq]#u = 0# and [eq]#u 423= 1# (vertical) and [eq]#v = 0# and [eq]#v = 1# (horizontal) edges are 424derived from the first and second inner tessellation levels, respectively. 425All rectangles, except those adjacent to one of the outer rectangle edges, 426are decomposed into triangle pairs. 427The outermost rectangle edges are subdivided independently, using the first, 428second, third, and fourth outer tessellation levels to control the number of 429subdivisions of the [eq]#u = 0# (left), [eq]#v = 0# (bottom), [eq]#u = 1# 430(right), and [eq]#v = 1# (top) edges, respectively. 431The area between the inner rectangles of the mesh and the outer rectangle 432edges are filled by triangles produced by joining the vertices on the 433subdivided outer edges to the vertices on the edge of the inner rectangle 434mesh. 435 436If both clamped inner tessellation levels and all four clamped outer 437tessellation levels are exactly one, only a single triangle pair covering 438the outer rectangle is generated. 439Otherwise, if either clamped inner tessellation level is one, that 440tessellation level is treated as though it was originally specified as 441[eq]#1 {plus} {epsilon}# and will result in a two- or three-segment 442subdivision depending on the tessellation spacing. 443When used with fractional odd spacing, the three-segment subdivision may: 444produce _inner vertices_ positioned on the edge of the rectangle. 445 446If any tessellation level is greater than one, tessellation begins by 447subdividing the [eq]#u = 0# and [eq]#u = 1# edges of the outer rectangle 448into [eq]#m# segments using the clamped and rounded first inner tessellation 449level and the tessellation spacing. 450The [eq]#v = 0# and [eq]#v = 1# edges are subdivided into [eq]#n# segments 451using the second inner tessellation level. 452Each vertex on the [eq]#u = 0# and [eq]#v = 0# edges are joined with the 453corresponding vertex on the [eq]#u = 1# and [eq]#v = 1# edges to produce a 454set of vertical and horizontal lines that divide the rectangle into a grid 455of smaller rectangles. 456The primitive generator emits a pair of non-overlapping triangles covering 457each such rectangle not adjacent to an edge of the outer rectangle. 458The boundary of the region covered by these triangles forms an inner 459rectangle, the edges of which are subdivided by the grid vertices that lie 460on the edge. 461If either [eq]#m# or [eq]#n# is two, the inner rectangle is degenerate, and 462one or both of the rectangle's _edges_ consist of a single point. 463This subdivision is illustrated in Figure <<img-innerquad,Inner Quad 464Tessellation>>. 465 466[[img-innerquad]] 467image::{images}/innerquad.svg[align="center",title="Inner Quad Tessellation",opts="{imageopts}"] 468 469.Caption 470**** 471In the <<img-innerquad,Inner Quad Tessellation>> diagram, inner quad 472tessellation levels of (a) [eq]#(4,2)# and (b) [eq]#(7,4)# are shown. 473The regions highlighted in red in figure (b) depict the 10 inner rectangles, 474each of which will be subdivided into two triangles. 475Solid black circles depict vertices on the boundary of the outer and inner 476rectangles, where the inner rectangle of figure (a) is degenerate (a single 477line segment). 478Dotted lines depict the horizontal and vertical edges connecting 479corresponding vertices on the inner and outer rectangle edges. 480**** 481 482After the area corresponding to the inner rectangle is filled, the 483tessellator must: produce triangles to cover the area between the inner and 484outer rectangles. 485To do this, the subdivision of the outer rectangle edge above is discarded. 486Instead, the [eq]#u = 0#, [eq]#v = 0#, [eq]#u = 1#, and [eq]#v = 1# edges 487are subdivided according to the first, second, third, and fourth outer 488tessellation levels, respectively, and the tessellation spacing. 489The original subdivision of the inner rectangle is retained. 490The area between the outer and inner rectangles is completely filled by 491non-overlapping triangles. 492Two of the three vertices of each triangle are adjacent vertices on a 493subdivided edge of one rectangle; the third is one of the vertices on the 494corresponding edge of the other rectangle. 495If either edge of the innermost rectangle is degenerate, the area near the 496corresponding outer edges is filled by connecting each vertex on the outer 497edge with the single vertex making up the _inner edge_. 498 499The algorithm used to subdivide the rectangular domain in (u,v) space into 500individual triangles is implementation-dependent. 501However, the set of triangles produced will completely cover the domain, and 502no portion of the domain will be covered by multiple triangles. 503 504Output triangles are generated with a topology similar to 505<<drawing-triangle-lists, triangle lists>>, except that the order in which 506each triangle is generated, and the order in which the vertices are 507generated for each triangle, are implementation-dependent. 508However, the order of vertices in each triangle is consistent across the 509domain as described in <<tessellation-vertex-winding-order>>. 510 511 512[[tessellation-isoline-tessellation]] 513== Isoline Tessellation 514 515If the tessellation primitive mode is code:IsoLines, a set of independent 516horizontal line segments is drawn. 517The segments are arranged into connected strips called _isolines_, where the 518vertices of each isoline have a constant v coordinate and u coordinates 519covering the full range [eq]#[0,1]#. 520The number of isolines generated is derived from the first outer 521tessellation level; the number of segments in each isoline is derived from 522the second outer tessellation level. 523Both inner tessellation levels and the third and fourth outer tessellation 524levels have no effect in this mode. 525 526As with quad tessellation above, isoline tessellation begins with a 527rectangle. 528The [eq]#u = 0# and [eq]#u = 1# edges of the rectangle are subdivided 529according to the first outer tessellation level. 530For the purposes of this subdivision, the tessellation spacing mode is 531ignored and treated as equal_spacing. 532An isoline is drawn connecting each vertex on the [eq]#u = 0# rectangle edge 533to the corresponding vertex on the [eq]#u = 1# rectangle edge, except that 534no line is drawn between [eq]#(0,1)# and [eq]#(1,1)#. 535If the number of isolines on the subdivided [eq]#u = 0# and [eq]#u = 1# 536edges is [eq]#n#, this process will result in [eq]#n# equally spaced lines 537with constant v coordinates of 0, latexmath:[\frac{1}{n}, \frac{2}{n}, 538\ldots, \frac{n-1}{n}]. 539 540Each of the [eq]#n# isolines is then subdivided according to the second 541outer tessellation level and the tessellation spacing, resulting in [eq]#m# 542line segments. 543Each segment of each line is emitted by the tessellator. 544These line segments are generated with a topology similar to 545<<drawing-line-lists, line lists>>, except that the order in which each line 546is generated, and the order in which the vertices are generated for each 547line segment, are implementation-dependent. 548 549ifdef::VK_KHR_portability_subset[] 550[NOTE] 551.Note 552==== 553If the `apiext:VK_KHR_portability_subset` extension is enabled, and 554slink:VkPhysicalDevicePortabilitySubsetFeaturesKHR::pname:tessellationIsolines 555is ename:VK_FALSE, then isoline tessellation is not supported by the 556implementation. 557==== 558endif::VK_KHR_portability_subset[] 559 560 561[[tessellation-point-mode]] 562== Tessellation Point Mode 563 564For all primitive modes, the tessellator is capable of generating points 565instead of lines or triangles. 566If the tessellation control or tessellation evaluation shader specifies the 567code:OpExecutionMode code:PointMode, the primitive generator will generate 568one point for each distinct vertex produced by tessellation, rather than 569emitting triangles or lines. 570Otherwise, the tessellator will produce a collection of line segments or 571triangles according to the primitive mode. 572These points are generated with a topology similar to <<drawing-point-lists, 573point lists>>, except the order in which the points are generated for each 574input primitive is undefined:. 575 576ifdef::VK_KHR_portability_subset[] 577[NOTE] 578.Note 579==== 580If the `apiext:VK_KHR_portability_subset` extension is enabled, and 581slink:VkPhysicalDevicePortabilitySubsetFeaturesKHR::pname:tessellationPointMode 582is ename:VK_FALSE, then tessellation point mode is not supported by the 583implementation. 584==== 585endif::VK_KHR_portability_subset[] 586 587 588== Tessellation Pipeline State 589 590The pname:pTessellationState member of slink:VkGraphicsPipelineCreateInfo is 591a pointer to a sname:VkPipelineTessellationStateCreateInfo structure. 592 593[open,refpage='VkPipelineTessellationStateCreateInfo',desc='Structure specifying parameters of a newly created pipeline tessellation state',type='structs'] 594-- 595The sname:VkPipelineTessellationStateCreateInfo structure is defined as: 596 597include::{generated}/api/structs/VkPipelineTessellationStateCreateInfo.adoc[] 598 599 * pname:sType is a elink:VkStructureType value identifying this structure. 600 * pname:pNext is `NULL` or a pointer to a structure extending this 601 structure. 602 * pname:flags is reserved for future use. 603 * pname:patchControlPoints is the number of control points per patch. 604 605.Valid Usage 606**** 607 * [[VUID-VkPipelineTessellationStateCreateInfo-patchControlPoints-01214]] 608 pname:patchControlPoints must: be greater than zero and less than or 609 equal to sname:VkPhysicalDeviceLimits::pname:maxTessellationPatchSize 610**** 611 612include::{generated}/validity/structs/VkPipelineTessellationStateCreateInfo.adoc[] 613-- 614 615[open,refpage='VkPipelineTessellationStateCreateFlags',desc='Reserved for future use',type='flags'] 616-- 617include::{generated}/api/flags/VkPipelineTessellationStateCreateFlags.adoc[] 618 619tname:VkPipelineTessellationStateCreateFlags is a bitmask type for setting a 620mask, but is currently reserved for future use. 621-- 622 623ifdef::VK_VERSION_1_1,VK_KHR_maintenance2[] 624[open,refpage='VkPipelineTessellationDomainOriginStateCreateInfo',desc='Structure specifying the orientation of the tessellation domain',type='structs'] 625-- 626The sname:VkPipelineTessellationDomainOriginStateCreateInfo structure is 627defined as: 628 629include::{generated}/api/structs/VkPipelineTessellationDomainOriginStateCreateInfo.adoc[] 630 631ifdef::VK_KHR_maintenance2[] 632or the equivalent 633 634include::{generated}/api/structs/VkPipelineTessellationDomainOriginStateCreateInfoKHR.adoc[] 635endif::VK_KHR_maintenance2[] 636 637 * pname:sType is a elink:VkStructureType value identifying this structure. 638 * pname:pNext is `NULL` or a pointer to a structure extending this 639 structure. 640 * pname:domainOrigin is a elink:VkTessellationDomainOrigin value 641 controlling the origin of the tessellation domain space. 642 643If the sname:VkPipelineTessellationDomainOriginStateCreateInfo structure is 644included in the pname:pNext chain of 645slink:VkPipelineTessellationStateCreateInfo, it controls the origin of the 646tessellation domain. 647If this structure is not present, it is as if pname:domainOrigin was 648ename:VK_TESSELLATION_DOMAIN_ORIGIN_UPPER_LEFT. 649 650include::{generated}/validity/structs/VkPipelineTessellationDomainOriginStateCreateInfo.adoc[] 651-- 652 653[open,refpage='VkTessellationDomainOrigin',desc='Enum describing tessellation domain origin',type='enums'] 654-- 655The possible tessellation domain origins are specified by the 656elink:VkTessellationDomainOrigin enumeration: 657 658include::{generated}/api/enums/VkTessellationDomainOrigin.adoc[] 659 660ifdef::VK_KHR_maintenance2[] 661or the equivalent 662 663include::{generated}/api/enums/VkTessellationDomainOriginKHR.adoc[] 664endif::VK_KHR_maintenance2[] 665 666 * ename:VK_TESSELLATION_DOMAIN_ORIGIN_UPPER_LEFT specifies that the origin 667 of the domain space is in the upper left corner, as shown in figure 668 <<img-tessellation-topology-ul>>. 669 * ename:VK_TESSELLATION_DOMAIN_ORIGIN_LOWER_LEFT specifies that the origin 670 of the domain space is in the lower left corner, as shown in figure 671 <<img-tessellation-topology-ll>>. 672 673This enum affects how the code:VertexOrderCw and code:VertexOrderCcw 674tessellation execution modes are interpreted, since the winding is defined 675relative to the orientation of the domain. 676-- 677endif::VK_VERSION_1_1,VK_KHR_maintenance2[] 678 679ifdef::VK_EXT_extended_dynamic_state3,VK_EXT_shader_object[] 680 681[open,refpage='vkCmdSetTessellationDomainOriginEXT',desc='Specify the origin of the tessellation domain space dynamically for a command buffer',type='protos'] 682-- 683To <<pipelines-dynamic-state, dynamically set>> the origin of the 684tessellation domain space, call: 685 686include::{generated}/api/protos/vkCmdSetTessellationDomainOriginEXT.adoc[] 687 688 * pname:commandBuffer is the command buffer into which the command will be 689 recorded. 690 * pname:domainOrigin specifies the origin of the tessellation domain 691 space. 692 693This command sets the origin of the tessellation domain space for subsequent 694drawing commands 695ifdef::VK_EXT_shader_object[] 696ifdef::VK_EXT_extended_dynamic_state3[when drawing using <<shaders-objects, shader objects>>, or] 697ifndef::VK_EXT_extended_dynamic_state3[when drawing using <<shaders-objects, shader objects>>.] 698endif::VK_EXT_shader_object[] 699ifdef::VK_EXT_extended_dynamic_state3[] 700when the graphics pipeline is created with 701ename:VK_DYNAMIC_STATE_TESSELLATION_DOMAIN_ORIGIN_EXT set in 702slink:VkPipelineDynamicStateCreateInfo::pname:pDynamicStates. 703endif::VK_EXT_extended_dynamic_state3[] 704Otherwise, this state is specified by the 705slink:VkPipelineTessellationDomainOriginStateCreateInfo::pname:domainOrigin 706value used to create the currently active pipeline. 707 708:refpage: vkCmdSetTessellationDomainOriginEXT 709:requiredfeature: extendedDynamicState3TessellationDomainOrigin 710 711.Valid Usage 712**** 713include::{chapters}/commonvalidity/dynamic_state3_feature_common.adoc[] 714**** 715 716include::{generated}/validity/protos/vkCmdSetTessellationDomainOriginEXT.adoc[] 717-- 718 719endif::VK_EXT_extended_dynamic_state3,VK_EXT_shader_object[] 720
