Vulkan-Docs/doc/specs/vulkan/chapters/drawing.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[drawing]]
= Drawing Commands

_Drawing commands_ (command with ftext:Draw in the name) provoke work in a
graphics pipeline. Drawing commands are recorded into a command buffer and
when executed by a queue, will produce work which executes according to the
currently bound graphics pipeline. A graphics pipeline must: be bound to a
command buffer before any drawing commands are recorded in that command
buffer.

Each draw is made up of zero or more vertices and zero or more instances,
which are processed by the device and result in the assembly of
primitives. Primitives are assembled according to the
pname:pInputAssemblyState member of the sname:VkGraphicsPipelineCreateInfo
structure, which is of type sname:VkPipelineInputAssemblyStateCreateInfo:

include::../structs/VkPipelineInputAssemblyStateCreateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.
  * pname:primitiveRestartEnable controls whether a special vertex index
    value is treated as restarting the assembly of primitives. This enable
    only applies to indexed draws (flink:vkCmdDrawIndexed and
    flink:vkCmdDrawIndexedIndirect), and the special index value is either
    0xFFFFFFFF when the pname:indexType parameter of
    fname:vkCmdBindIndexBuffer is equal to ename:VK_INDEX_TYPE_UINT32, or
    0xFFFF when pname:indexType is equal to ename:VK_INDEX_TYPE_UINT16.
    Primitive restart is not allowed for ``list'' topologies.
  * pname:topology is a elink:VkPrimitiveTopology defining the primitive
    topology, as described below.

include::../validity/structs/VkPipelineInputAssemblyStateCreateInfo.txt[]

Restarting the assembly of primitives discards the most recent index values
if those elements formed an incomplete primitive, and restarts the primitive
assembly using the subsequent indices, but only assembling the immediately
following element through the end of the originally specified elements. The
primitive restart index value comparison is performed before adding the
pname:vertexOffset value to the index value.


[[drawing-primitive-topologies]]
== Primitive Topologies

_Primitive topology_ determines how consecutive vertices are organized into
primitives, and determines the type of primitive that is used at the
beginning of the graphics pipeline. The effective topology for later stages
of the pipeline is altered by tessellation or geometry shading (if either is
in use) and depends on the execution modes of those shaders. Supported
topologies are defined by elink:VkPrimitiveTopology and include:

include::../enums/VkPrimitiveTopology.txt[]

Each primitive topology, and its construction from a list of vertices, is
summarized below.


=== Points

A series of individual points are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_POINT_LIST. Each vertex defines a separate
point.


=== Separate Lines

Individual line segments, each defined by a pair of vertices, are specified
with pname:topology ename:VK_PRIMITIVE_TOPOLOGY_LINE_LIST. The first two
vertices define the first segment, with subsequent pairs of vertices each
defining one more segment. If the number of vertices is odd, then the last
vertex is ignored.


=== Line Strips

A series of one or more connected line segments are specified with
pname:topology ename:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP. In this case, the
first vertex specifies the first segment's start point while the second
vertex specifies the first segment's endpoint and the second segment's start
point. In general, the latexmath:[$i$]th vertex (for latexmath:[$i > 0$])
specifies the beginning of the latexmath:[$i$]th segment and the end of the
latexmath:[$i-1$]st. The last vertex specifies the end of the last segment.
If only one vertex is specified, then no primitive is generated.


=== Triangle Strips

A triangle strip is a series of triangles connected along shared edges, and
is specified with pname:topology ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP.
In this case, the first three vertices define the first triangle, and their
order is significant. Each subsequent vertex defines a new triangle using
that point along with the last two vertices from the previous triangle, as
shown in figure <<fig-triangles>>. If fewer than three vertices are
specified, no primitive is produced. The order of vertices in successive
triangles changes as shown in the figure, so that all triangle faces have
the same orientation.

[[fig-triangles]]
.Triangle strips, fans, and lists
image:images/triangles.{svgpdf}["Triangle strips, fans, and lists",{fullimagewidth}]

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: Add full caption:

* (a) A triangle strip.
* (b) A triangle fan.
* (c) Independent triangles.

The numbers give the sequencing of the vertices in order within the vertex
arrays. Note that in (a) and (b) triangle edge ordering is determined by the
first triangle, while in (c) the order of each triangle's edges is
independent of the other triangles.
====
endif::editing-notes[]


=== Triangle Fans

A triangle fan is specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN. It is similar to a triangle strip,
but changes the vertex replaced from the previous triangle as shown in
figure <<fig-triangles>>, so that all triangles in the fan share a common
vertex.


=== Separate Triangles

Separate triangles are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST, as shown in figure
<<fig-triangles>>. In this case, vertices latexmath:[$3i$],
latexmath:[$3i+1$], and latexmath:[$3i+2$] vertices (in that order)
determine a triangle for each latexmath:[$i=0,1,\ldots,n-1$], where there
are latexmath:[$3n+k$] vertices drawn. latexmath:[$k$] is either 0, 1, or 2;
if latexmath:[$k$] is not zero, the final latexmath:[$k$] vertices are
ignored.


=== Lines With Adjacency

Lines with adjacency are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY, and are independent
line segments where each endpoint has a corresponding _adjacent_ vertex that
is accessible in a <<geometry,geometry shader>>.
If a geometry shader is not active, the adjacent vertices are ignored.

A line segment is drawn from the latexmath:[$4 i+1$]st vertex to the
latexmath:[$4 i+2$]nd vertex for each latexmath:[$i=0,1,\ldots, n-1$], where
there are latexmath:[$4 n+k$] vertices. latexmath:[$k$] is either 0,
1, 2, or 3; if latexmath:[$k$] is not zero, the final latexmath:[$k$]
vertices are ignored. For line segment latexmath:[$i$], the latexmath:[$4
i$]th and latexmath:[$4 i+3$]rd vertices are considered adjacent to the
latexmath:[$4 i+1$]st and latexmath:[$4 i+2$]nd vertices, respectively, as
shown in figure <<fig-lineadj>>.

[[fig-lineadj]]
.Lines with adjacency
image:images/lineadj.{svgpdf}["Lines with adjacency",{fullimagewidth}]

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: Add full caption:

Lines with adjacency (a) and line strips with adjacency (b). The vertices
connected with solid lines belong to the main primitives; the vertices
connected by dashed lines are the adjacent vertices that are accessible in a
geometry shader.
====
endif::editing-notes[]


=== Line Strips With Adjacency

Line strips with adjacency are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY and are similar to
line strips, except that each line segment has a pair of adjacent vertices
that are accessible in a geometry shader. If a geometry shader is not
active, the adjacent vertices are ignored.

A line segment is drawn from the latexmath:[$i+1$]st vertex to the
latexmath:[$i+2$]nd vertex for each latexmath:[$i=0,1,\ldots, n-1$], where
there are latexmath:[$n+3$] vertices. If there are fewer than four
vertices, all vertices are ignored. For line segment latexmath:[$i$], the
latexmath:[$i$]th and latexmath:[$i+3$]rd vertex are considered adjacent
to the latexmath:[$i+1$]st and latexmath:[$i+2$]nd vertices, respectively,
as shown in figure <<fig-lineadj>>.


=== Triangle List With Adjacency

Triangles with adjacency are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY, and are similar to
separate triangles except that each triangle edge has an adjacent vertex
that is accessible in a geometry shader. If a geometry shader is not
active, the adjacent vertices are ignored.

The latexmath:[$6i$]th, latexmath:[$6i+2$]nd, and latexmath:[$6i+4$]th
vertices (in that order) determine a triangle for each
latexmath:[$i=0,1, \ldots, n-1$], where there are latexmath:[$6 n+k$]
vertices. latexmath:[$k$] is either 0, 1, 2, 3, 4, or 5; if
latexmath:[$k$] is non-zero, the final latexmath:[$k$] vertices are ignored.
For triangle latexmath:[$i$], the latexmath:[$6 i+1$]st, latexmath:[$6
i+3$]rd, and latexmath:[$6 i+5$]th vertices are considered adjacent to edges
from the latexmath:[$6 i$]th to the latexmath:[$6 i+2$]nd, from the
latexmath:[$6 i+2$]nd to the latexmath:[$6 i+4$]th, and from the
latexmath:[$6 i+4$]th to the latexmath:[$6 i$]th vertices, respectively, as
shown in figure <<fig-triadj>>.

[[fig-triadj]]
.Triangles with adjacency
image:images/triadj.{svgpdf}["Triangles with adjacency",{fullimagewidth}]

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: Add full caption:

Triangles with adjacency. The vertices connected with solid lines belong
to the main primitive; the vertices connected by dashed lines are the
adjacent vertices that are accessible in a geometry shader.}
====
endif::editing-notes[]


=== Triangle Strips With Adjacency

Triangle strips with adjacency are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY, and are similar
to triangle strips except that each triangle edge has an adjacent vertex
that is accessible in a geometry shader. If a geometry shader is not
active, the adjacent vertices are ignored.

In triangle strips with adjacency, latexmath:[$n$] triangles are drawn where
there are latexmath:[$2(n+2)+k$] vertices. latexmath:[$k$] is
either 0 or 1; if latexmath:[$k$] is 1, the final vertex is ignored. If
there are fewer than 6 vertices, the entire primitive is ignored. Table
<<trigenadj>> describes the vertices and order used to draw each triangle,
and which vertices are considered adjacent to each edge of the triangle, as
shown in figure <<fig-tstripadj>>.


[[trigenadj]]
.Triangles generated by triangle strips with adjacency.
[options="header", cols="2,1,1,1,1,1,1"]
|====
|                                               3+| Primitive Vertices                      3+| Adjacent Vertices
| Primitive                                       | 1st                 | 2nd                 | 3rd                 | 1/2                 | 2/3                   | 3/1
| only (latexmath:[$i=0$], latexmath:[$n=1$])     | 0                   | 2                   | 4                   | 1                   | 5                     | 3
| first (latexmath:[$i=0$])                       | 0                   | 2                   | 4                   | 1                   | 6                     | 3
| middle (latexmath:[$i$] odd)                    | latexmath:[$2 i+2$] | latexmath:[$2 i  $] | latexmath:[$2 i+4$] | latexmath:[$2 i-2$] | latexmath:[$2 i+3$] | latexmath:[$2 i+6$]
| middle (latexmath:[$i$] even)                   | latexmath:[$2 i  $] | latexmath:[$2 i+2$] | latexmath:[$2 i+4$] | latexmath:[$2 i-2$] | latexmath:[$2 i+6$] | latexmath:[$2 i+3$]
| last (latexmath:[$i=n-1$], latexmath:[$i$] odd) | latexmath:[$2 i+2$] | latexmath:[$2 i  $] | latexmath:[$2 i+4$] | latexmath:[$2 i-2$] | latexmath:[$2 i+3$] | latexmath:[$2 i+5$]
| last (latexmath:[$i=n-1$], latexmath:[$i$] even)| latexmath:[$2 i  $] | latexmath:[$2 i+2$] | latexmath:[$2 i+4$] | latexmath:[$2 i-2$] | latexmath:[$2 i+5$] | latexmath:[$2 i+3$]
|====

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: Add full caption:

Triangles generated by triangle strips with adjacency. Each triangle is
drawn using the vertices whose numbers are in the *1st*, *2nd*, and *3rd*
columns under *primitive vertices*, in that order. The vertices in the 1/2,
2/3, and 3/1 columns under *adjacent vertices* are considered adjacent to
the edges from the first to the second, from the second to the third, and
from the third to the first vertex of the triangle, respectively. The six
rows correspond to six cases: the first and only triangle latexmath:[$(i=0,
n=1)$], the first triangle of several latexmath:[$(i=0, n>0)$], ``odd''
middle triangles latexmath:[$(i=1,3,5 \ldots)$], ``even'' middle triangles
latexmath:[$(i=2,4,6,\ldots)$], and special cases for the last triangle,
when latexmath:[$i$] is either even or odd. For the purposes of this table,
both the first vertex and first triangle are numbered latexmath:[$0$].
====
endif::editing-notes[]

[[fig-tstripadj]]
.Triangle strips with adjacency
image:images/tstripadj.{svgpdf}["Triangle strips with adjacency",width="{svgpdf@pdf:400:800}"]

ifdef::editing-notes[]
[NOTE]
.editing-note
====
TODO: Add full caption:

Triangle strips with adjacency. The vertices connected with solid lines
belong to the main primitives; the vertices connected by dashed lines are
the adjacent vertices that are accessible in a geometry shader.
====
endif::editing-notes[]


[[drawing-primitive-topologies-patches]]
=== Separate Patches

Separate patches are specified with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_PATCH_LIST. A patch is an ordered collection of
vertices used for <<tessellation,primitive tessellation>>. The
vertices comprising a patch have no implied geometric ordering, and are used
by tessellation shaders and the fixed-function tessellator to generate new
point, line, or triangle primitives.

Each patch in the series has a fixed number of vertices, specified by the
pname:patchControlPoints member of the
slink:VkPipelineTessellationStateCreateInfo structure passed to
flink:vkCreateGraphicsPipelines. Once assembled and vertex shaded, these
patches are provided as input to the tessellation control shader stage.

If the number of vertices in a patch is given by latexmath:[$v$], the
latexmath:[$v i$]th through latexmath:[$v i+v-1$]st vertices (in that
order) determine a patch for each
    latexmath:[$i=0,1,\dots n-1$],
where there are latexmath:[$v n+k$] vertices. latexmath:[$k$] is in the
range latexmath:[$[0,v-1\]$]; if latexmath:[$k$] is not zero, the final
latexmath:[$k$] vertices are ignored.


=== General Considerations For Polygon Primitives

Depending on the <<primsrast-polygonmode,polygon mode>>, a _polygon
primitive_ generated from a drawing command with pname:topology
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN,
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP,
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST,
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY, or
ename:VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY is rendered in
one of several ways, such as outlining its border or filling its interior.
The order of vertices in such a primitive is significant during
<<primsrast-polygons-basic,polygon rasterization>> and <<ref-tbd,fragment
shading>>.


=== Programmable Primitive Shading

Once primitives are assembled, they proceed to the vertex shading stage of
the pipeline. If the draw includes multiple instances, then the set of
primitives is sent to the vertex shading stage multiple times, once for each
instance.

It is undefined whether vertex shading occurs on vertices that are discarded
as part of incomplete primitives, but if it does occur then it operates as
if they were vertices in complete primitives and such invocations can: have
side effects.

Vertex shading receives two per-vertex inputs from the primitive assembly
stage - the code:vertexIndex and the code:instanceIndex. How these values
are generated is defined below, with each command.

Drawing commands fall roughly into two categories:

  * Non-indexed drawing (fname:vkCmdDraw and fname:vkCmdDrawIndirect)
    commands present a sequential code:vertexIndex to the vertex shader. The
    sequential index is generated automatically by the device.
  * Indexed drawing commands (fname:vkCmdDrawIndexed and
    fname:vkCmdDrawIndexed) read index values from an _index buffer_ and use
    this to compute the code:vertexIndex value for the vertex shader.

An index buffer is bound to a command buffer by calling:

include::../protos/vkCmdBindIndexBuffer.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:buffer is the buffer being bound.
  * pname:offset is the starting offset in bytes within pname:buffer used in
    index buffer address calculations.
  * pname:indexType selects whether indices are treated as 16 bits or 32
    bits. Possible values include:
+
include::../enums/VkIndexType.txt[]

include::../validity/protos/vkCmdBindIndexBuffer.txt[]

The parameters for each drawing command are specified directly in the
command or read from buffer memory, depending on the command. Drawing
commands that source their parameters from buffer memory are known as
_indirect_ drawing commands.

All drawing commands interact with the
<<features-features-robustBufferAccess, Robust Buffer Access>> feature.

[[drawing-primitive-assembly-apiorder]]
Primitives assembled by draw commands are considered to have an
<<fundamentals-queueoperation-apiorder,API order>>, which defines the order
their fragments affect the framebuffer. When a draw command includes
multiple instances, the lower numbered instances are earlier in API order.
For non-indexed draws, primitives with lower numbered code:vertexIndex
values are earlier in API order. For indexed draws, primitives assembled
from lower index buffer addresses are earlier in API order.

To record a non-indexed draw, call:

include::../protos/vkCmdDraw.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:vertexCount is the number of vertices to draw.
  * pname:instanceCount is the number of instances to draw.
  * pname:firstVertex is the index of the first vertex to draw.
  * pname:firstInstance is the instance ID of the first instance to draw.

When the command is executed, primitives are assembled using the current
primitive topology and pname:vertexCount consecutive vertex indices with the
first code:vertexIndex value equal to pname:firstVertex. The primitives are
drawn pname:instanceCount times with code:instanceIndex starting with
pname:firstInstance and increasing sequentially for each instance. The
assembled primitives execute the currently bound graphics pipeline.

include::../validity/protos/vkCmdDraw.txt[]

To record an indexed draw, call:

include::../protos/vkCmdDrawIndexed.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:indexCount is the number of vertices to draw.
  * pname:instanceCount is the number of instances to draw.
  * pname:firstIndex is the base index within the index buffer.
  * pname:vertexOffset is the base offset within the index buffer.
  * pname:firstInstance is the instance ID of the first instance to draw.

When the command is executed, primitives are assembled using the current
primitive topology and pname:indexCount vertices whose indices are retrieved
from the index buffer. The index buffer is treated as an array of tightly
packed unsigned integers of size defined by the
flink:vkCmdBindIndexBuffer::pname:indexType parameter with which the buffer
was bound.

The first vertex index is at an offset of pname:firstIndex * code:indexSize
+ pname:offset within the currently bound index buffer, where pname:offset
is the offset specified by fname:vkCmdBindIndexBuffer and code:indexSize is
the byte size of the type specified by pname:indexType. Subsequent index
values are retrieved from consecutive locations in the index buffer. Indices
are first compared to the primitive restart value, then zero extended to 32
bits (if the code:indexType is ename:VK_INDEX_TYPE_UINT16) and have
pname:vertexOffset added to them, before being supplied as the
code:vertexIndex value.

The primitives are drawn pname:instanceCount times with code:instanceIndex
starting with pname:firstInstance and increasing sequentially for each
instance. The assembled primitives execute the currently bound graphics
pipeline.

include::../validity/protos/vkCmdDrawIndexed.txt[]

A non-indexed indirect draw is recorded by calling:

include::../protos/vkCmdDrawIndirect.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:buffer is the buffer containing draw parameters.
  * pname:offset is the byte offset into pname:buffer where parameters
    begin.
  * pname:drawCount is the number of draws to execute, and can: be zero.
  * pname:stride is the byte stride between successive sets of draw
    parameters.

fname:vkCmdDrawIndirect behaves similarly to flink:vkCmdDraw except that the
parameters are read by the device from a buffer during execution.
pname:drawCount draws are executed by the command, with parameters taken
from pname:buffer starting at pname:offset and increasing by pname:stride
bytes for each successive draw. The parameters of each draw are encoded in
an array of slink:VkDrawIndirectCommand structures. If pname:drawCount is
less than or equal to one, pname:stride is ignored.

include::../validity/protos/vkCmdDrawIndirect.txt[]

The definition of sname:VkDrawIndirectCommand is:

include::../structs/VkDrawIndirectCommand.txt[]

The members of sname:VkDrawIndirectCommand have the same meaning as the
similarly named parameters of flink:vkCmdDraw.

include::../validity/structs/VkDrawIndirectCommand.txt[]

An indexed indirect draw is recorded by calling:

include::../protos/vkCmdDrawIndexedIndirect.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:buffer is the buffer containing draw parameters.
  * pname:offset is the byte offset into pname:buffer where parameters
    begin.
  * pname:drawCount is the number of draws to execute, and can: be zero.
  * pname:stride is the byte stride between successive sets of draw
    parameters.

fname:vkCmdDrawIndexedIndirect behaves similarly to flink:vkCmdDrawIndirect
except that the parameters are read by the device from a buffer during
execution. pname:drawCount draws are executed by the command, with
parameters taken from pname:buffer starting at pname:offset and increasing
by pname:stride bytes for each successive draw. The parameters of each draw
are encoded in an array of slink:VkDrawIndexedIndirectCommand structures. If
pname:drawCount is less than or equal to one, pname:stride is ignored.

include::../validity/protos/vkCmdDrawIndexedIndirect.txt[]

The definition of sname:VkDrawIndexedIndirectCommand is

include::../structs/VkDrawIndexedIndirectCommand.txt[]

The members of sname:VkDrawIndexedIndirectCommand have the same meaning as
the similarly named parameters of flink:vkCmdDrawIndexed.

include::../validity/structs/VkDrawIndexedIndirectCommand.txt[]