19. Drawing Commands
Drawing commands (commands with 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 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 pInputAssemblyState
member
of the VkGraphicsPipelineCreateInfo
structure, which is of type
VkPipelineInputAssemblyStateCreateInfo
:
typedef struct VkPipelineInputAssemblyStateCreateInfo {
VkStructureType sType;
const void* pNext;
VkPipelineInputAssemblyStateCreateFlags flags;
VkPrimitiveTopology topology;
VkBool32 primitiveRestartEnable;
} VkPipelineInputAssemblyStateCreateInfo;

sType
is the type of this structure. 
pNext
isNULL
or a pointer to an extensionspecific structure. 
flags
is reserved for future use. 
topology
is a VkPrimitiveTopology defining the primitive topology, as described below. 
primitiveRestartEnable
controls whether a special vertex index value is treated as restarting the assembly of primitives. This enable only applies to indexed draws (vkCmdDrawIndexed and vkCmdDrawIndexedIndirect), and the special index value is either 0xFFFFFFFF when theindexType
parameter ofvkCmdBindIndexBuffer
is equal toVK_INDEX_TYPE_UINT32
, or 0xFFFF whenindexType
is equal toVK_INDEX_TYPE_UINT16
. Primitive restart is not allowed for “list” topologies.
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
vertexOffset
value to the index value.
typedef VkFlags VkPipelineInputAssemblyStateCreateFlags;
VkPipelineInputAssemblyStateCreateFlags
is a bitmask type for setting
a mask, but is currently reserved for future use.
19.1. 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 VkPrimitiveTopology and include:
typedef enum VkPrimitiveTopology {
VK_PRIMITIVE_TOPOLOGY_POINT_LIST = 0,
VK_PRIMITIVE_TOPOLOGY_LINE_LIST = 1,
VK_PRIMITIVE_TOPOLOGY_LINE_STRIP = 2,
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST = 3,
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP = 4,
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN = 5,
VK_PRIMITIVE_TOPOLOGY_LINE_LIST_WITH_ADJACENCY = 6,
VK_PRIMITIVE_TOPOLOGY_LINE_STRIP_WITH_ADJACENCY = 7,
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST_WITH_ADJACENCY = 8,
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP_WITH_ADJACENCY = 9,
VK_PRIMITIVE_TOPOLOGY_PATCH_LIST = 10,
} VkPrimitiveTopology;
Each primitive topology, and its construction from a list of vertices, is summarized below with a supporting diagram. In each diagram, the numbered points show the sequencing of vertices in order within the vertex arrays; however the positions chosen are arbitrary and for illustration only. Vertices connected with solid lines belong to the main primitives. In the primitive types with adjacency, the vertices connected by dashed lines are the adjacent vertices that are accessible in a geometry shader.
Note
The terminology “vertex i” means “the vertex with index i in the ordered list of vertices defining this primitive”. 
Note
Depending on the polygon mode, a polygon
primitive generated from a drawing command with 
19.1.1. Point Lists
A series of individual points are specified with topology
VK_PRIMITIVE_TOPOLOGY_POINT_LIST
.
Each vertex defines a separate point.
19.1.2. Line Lists
Lists of line segments, with each segment defined by a pair of vertices, are
specified with topology
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.
19.1.3. Line Strips
A series of one or more connected line segments are specified with
topology
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, vertex i (for i > 0) specifies the beginning of the
ith segment and the end of the previous segment.
The last vertex specifies the end of the last segment.
If only one vertex is specified, then no primitive is generated.
19.1.4. Triangle Lists
Lists of separate triangles are specified with topology
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST
.
In this case, vertices 3 i, 3 i + 1, and 3 i + 2
(in that order) determine a triangle for each i = 0, 1, …, n1,
where there are 3 n + k vertices drawn.
k is either 0, 1, or 2; if k is not zero, the final k
vertices are ignored.
19.1.5. Triangle Strips
A triangle strip is a series of triangles connected along shared edges, and
is specified with topology
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.
If fewer than three vertices are specified, no primitive is produced.
The order of vertices in successive triangles changes as shown in the figure
below, so that all triangle faces have the same orientation.
19.1.6. Triangle Fans
A triangle fan is specified with topology
VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN
.
It is similar to a triangle strip, but changes the vertex replaced from the
previous triangle so that all triangles in the fan share a common vertex.
19.1.7. Line Lists With Adjacency
Lines with adjacency are specified with topology
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 shader.
If a geometry shader is not active, the adjacent vertices are ignored.
A line segment is drawn from vertex 4 i + 1 to vertex 4 i + 2 for each i = 0, 1, …, n1, where there are 4 n + k vertices. k is either 0, 1, 2, or 3; if k is not zero, the final k vertices are ignored. For line segment i, vertices 4 i and 4 i + 3 vertices are considered adjacent to vertices 4 i + 1 and 4 i + 2, respectively.
19.1.8. Line Strips With Adjacency
Line strips with adjacency are specified with topology
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 vertex i + 1 vertex to vertex i + 2 for each i = 0, 1, …, n1, where there are n + 3 vertices. If there are fewer than four vertices, all vertices are ignored. For line segment i, vertices i and i + 3 are considered adjacent to vertices i + 1 and i + 2, respectively.
19.1.9. Triangle Lists With Adjacency
Triangles with adjacency are specified with topology
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.
Vertices 6 i, 6 i + 2, and 6 i + 4 (in that order) determine a triangle for each i = 0, 1, …, n1, where there are 6 n+k vertices. k is either 0, 1, 2, 3, 4, or 5; if k is nonzero, the final k vertices are ignored. For triangle i, vertices 6 i + 1, 6 i + 3, and 6 i + 5 vertices are considered adjacent to edges from vertex 6 i to 6 i + 2, from 6 i + 2 to 6 i + 4, and from 6 i + 4 to 6 i vertices, respectively.
19.1.10. Triangle Strips With Adjacency
Triangle strips with adjacency are specified with topology
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, n triangles are drawn where there are 2 (n + 2) + k vertices. k is either 0 or 1; if k is 1, the final vertex is ignored. If there are fewer than 6 vertices, the entire primitive is ignored.
The table below illustrates the vertices and order used to draw each triangle, and which vertices are considered adjacent to each edge of those triangles. 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 (i = 0, n = 1), the first triangle of several (i = 0, n > 0), odd middle triangles (i = 1, 3, 5 …), even middle triangles (i = 2, 4, 6, …), and special cases for the last triangle, when i is either even or odd. For the purposes of this table, both the first vertex and first triangle are numbered 0.
Primitive Vertices  Adjacent Vertices  

Primitive 
1st 
2nd 
3rd 
1/2 
2/3 
3/1 
only (i = 0, n = 1) 
0 
2 
4 
1 
5 
3 
first (i = 0) 
0 
2 
4 
1 
6 
3 
middle (i odd) 
2 i + 2 
2 i 
2 i + 4 
2 i2 
2 i + 3 
2 i + 6 
middle (i even) 
2 i 
2 i + 2 
2 i + 4 
2 i2 
2 i + 6 
2 i + 3 
last (i=n1, i odd) 
2 i + 2 
2 i 
2 i + 4 
2 i2 
2 i + 3 
2 i + 5 
last (i=n1, i even) 
2 i 
2 i + 2 
2 i + 4 
2 i2 
2 i + 5 
2 i + 3 
19.1.11. Separate Patches
Separate patches are specified with topology
VK_PRIMITIVE_TOPOLOGY_PATCH_LIST
.
A patch is an ordered collection of vertices used for
primitive tessellation.
The vertices comprising a patch have no implied geometric ordering, and are
used by tessellation shaders and the fixedfunction tessellator to generate
new point, line, or triangle primitives.
Each patch in the series has a fixed number of vertices, specified by the
patchControlPoints
member of the
VkPipelineTessellationStateCreateInfo structure passed to
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 v, vertices v × i through v × i + v  1 (in that order) determine a patch for each i = 0, 1, …, n1, where there are v × n + k vertices. k is in the range [0, v  1]; if k is not zero, the final k vertices are ignored.
19.2. Primitive Order
Primitives generated by drawing commands progress through the stages of the graphics pipeline in primitive order. Primitive order is initially determined in the following way:

Submission order determines the initial ordering

For indirect draw commands, the order in which accessed instances of the VkDrawIndirectCommand are stored in
buffer
, from lower indirect buffer addresses to higher addresses. 
If a draw command includes multiple instances, the order in which instances are executed, from lower numbered instances to higher.

The order in which primitives are specified by a draw command:

For nonindexed draws, from vertices with a lower numbered
vertexIndex
to a higher numberedvertexIndex
. 
For indexed draws, vertices sourced from a lower index buffer addresses to higher addresses.

Within this order implementations further sort primitives:

If tessellation shading is active, by an implementationdependent order of new primitives generated by tessellation.

If geometry shading is active, by the order new primitives are generated by geometry shading.

If the polygon mode is not
VK_POLYGON_MODE_FILL
, by an implementationdependent ordering of the new primitives generated within the original primitive.
Primitive order is later used to define rasterization order, which determines the order in which fragments output results to a framebuffer.
19.3. 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 implementationdependent 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 pervertex inputs from the primitive assembly
stage  the vertexIndex
and the instanceIndex
.
How these values are generated is defined below, with each command.
Drawing commands fall roughly into two categories:

Nonindexed drawing commands present a sequential
vertexIndex
to the vertex shader. The sequential index is generated automatically by the device (see FixedFunction Vertex Processing for details on both specifying the vertex attributes indexed byvertexIndex
, as well as binding vertex buffers containing those attributes to a command buffer). These commands are: 
Indexed drawing commands read index values from an index buffer and use this to compute the
vertexIndex
value for the vertex shader. These commands are:
To bind an index buffer to a command buffer, call:
void vkCmdBindIndexBuffer(
VkCommandBuffer commandBuffer,
VkBuffer buffer,
VkDeviceSize offset,
VkIndexType indexType);

commandBuffer
is the command buffer into which the command is recorded. 
buffer
is the buffer being bound. 
offset
is the starting offset in bytes withinbuffer
used in index buffer address calculations. 
indexType
is a VkIndexType value specifying whether indices are treated as 16 bits or 32 bits.
Possible values of vkCmdBindIndexBuffer::indexType
, specifying
the size of indices, are:
typedef enum VkIndexType {
VK_INDEX_TYPE_UINT16 = 0,
VK_INDEX_TYPE_UINT32 = 1,
} VkIndexType;

VK_INDEX_TYPE_UINT16
specifies that indices are 16bit unsigned integer values. 
VK_INDEX_TYPE_UINT32
specifies that indices are 32bit unsigned integer values.
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 Robust Buffer Access feature.
To record a nonindexed draw, call:
void vkCmdDraw(
VkCommandBuffer commandBuffer,
uint32_t vertexCount,
uint32_t instanceCount,
uint32_t firstVertex,
uint32_t firstInstance);

commandBuffer
is the command buffer into which the command is recorded. 
vertexCount
is the number of vertices to draw. 
instanceCount
is the number of instances to draw. 
firstVertex
is the index of the first vertex to draw. 
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 vertexCount
consecutive vertex indices with the
first vertexIndex
value equal to firstVertex
.
The primitives are drawn instanceCount
times with instanceIndex
starting with firstInstance
and increasing sequentially for each
instance.
The assembled primitives execute the bound graphics pipeline.
To record an indexed draw, call:
void vkCmdDrawIndexed(
VkCommandBuffer commandBuffer,
uint32_t indexCount,
uint32_t instanceCount,
uint32_t firstIndex,
int32_t vertexOffset,
uint32_t firstInstance);

commandBuffer
is the command buffer into which the command is recorded. 
indexCount
is the number of vertices to draw. 
instanceCount
is the number of instances to draw. 
firstIndex
is the base index within the index buffer. 
vertexOffset
is the value added to the vertex index before indexing into the vertex buffer. 
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 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 vkCmdBindIndexBuffer::indexType
parameter
with which the buffer was bound.
The first vertex index is at an offset of firstIndex
* indexSize
+ offset
within the bound index buffer, where offset
is the
offset specified by vkCmdBindIndexBuffer
and indexSize
is the
byte size of the type specified by 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 indexType
is VK_INDEX_TYPE_UINT16
)
and have vertexOffset
added to them, before being supplied as the
vertexIndex
value.
The primitives are drawn instanceCount
times with instanceIndex
starting with firstInstance
and increasing sequentially for each
instance.
The assembled primitives execute the bound graphics pipeline.
To record a nonindexed indirect draw, call:
void vkCmdDrawIndirect(
VkCommandBuffer commandBuffer,
VkBuffer buffer,
VkDeviceSize offset,
uint32_t drawCount,
uint32_t stride);

commandBuffer
is the command buffer into which the command is recorded. 
buffer
is the buffer containing draw parameters. 
offset
is the byte offset intobuffer
where parameters begin. 
drawCount
is the number of draws to execute, and can be zero. 
stride
is the byte stride between successive sets of draw parameters.
vkCmdDrawIndirect
behaves similarly to vkCmdDraw except that the
parameters are read by the device from a buffer during execution.
drawCount
draws are executed by the command, with parameters taken
from buffer
starting at offset
and increasing by stride
bytes for each successive draw.
The parameters of each draw are encoded in an array of
VkDrawIndirectCommand structures.
If drawCount
is less than or equal to one, stride
is ignored.
The VkDrawIndirectCommand
structure is defined as:
typedef struct VkDrawIndirectCommand {
uint32_t vertexCount;
uint32_t instanceCount;
uint32_t firstVertex;
uint32_t firstInstance;
} VkDrawIndirectCommand;

vertexCount
is the number of vertices to draw. 
instanceCount
is the number of instances to draw. 
firstVertex
is the index of the first vertex to draw. 
firstInstance
is the instance ID of the first instance to draw.
The members of VkDrawIndirectCommand
have the same meaning as the
similarly named parameters of vkCmdDraw.
To record a nonindexed draw call with a draw call count sourced from a buffer, call:
void vkCmdDrawIndirectCountKHR(
VkCommandBuffer commandBuffer,
VkBuffer buffer,
VkDeviceSize offset,
VkBuffer countBuffer,
VkDeviceSize countBufferOffset,
uint32_t maxDrawCount,
uint32_t stride);

commandBuffer
is the command buffer into which the command is recorded. 
buffer
is the buffer containing draw parameters. 
offset
is the byte offset intobuffer
where parameters begin. 
countBuffer
is the buffer containing the draw count. 
countBufferOffset
is the byte offset intocountBuffer
where the draw count begins. 
maxDrawCount
specifies the maximum number of draws that will be executed. The actual number of executed draw calls is the minimum of the count specified incountBuffer
andmaxDrawCount
. 
stride
is the byte stride between successive sets of draw parameters.
vkCmdDrawIndirectCountKHR
behaves similarly to vkCmdDrawIndirect
except that the draw count is read by the device from a buffer during
execution.
The command will read an unsigned 32bit integer from countBuffer
located at countBufferOffset
and use this as the draw count.
To record an indexed indirect draw, call:
void vkCmdDrawIndexedIndirect(
VkCommandBuffer commandBuffer,
VkBuffer buffer,
VkDeviceSize offset,
uint32_t drawCount,
uint32_t stride);

commandBuffer
is the command buffer into which the command is recorded. 
buffer
is the buffer containing draw parameters. 
offset
is the byte offset intobuffer
where parameters begin. 
drawCount
is the number of draws to execute, and can be zero. 
stride
is the byte stride between successive sets of draw parameters.
vkCmdDrawIndexedIndirect
behaves similarly to vkCmdDrawIndexed
except that the parameters are read by the device from a buffer during
execution.
drawCount
draws are executed by the command, with parameters taken
from buffer
starting at offset
and increasing by stride
bytes for each successive draw.
The parameters of each draw are encoded in an array of
VkDrawIndexedIndirectCommand structures.
If drawCount
is less than or equal to one, stride
is ignored.
The VkDrawIndexedIndirectCommand
structure is defined as:
typedef struct VkDrawIndexedIndirectCommand {
uint32_t indexCount;
uint32_t instanceCount;
uint32_t firstIndex;
int32_t vertexOffset;
uint32_t firstInstance;
} VkDrawIndexedIndirectCommand;

indexCount
is the number of vertices to draw. 
instanceCount
is the number of instances to draw. 
firstIndex
is the base index within the index buffer. 
vertexOffset
is the value added to the vertex index before indexing into the vertex buffer. 
firstInstance
is the instance ID of the first instance to draw.
The members of VkDrawIndexedIndirectCommand
have the same meaning as
the similarly named parameters of vkCmdDrawIndexed.
To record an indexed draw call with a draw call count sourced from a buffer, call:
void vkCmdDrawIndexedIndirectCountKHR(
VkCommandBuffer commandBuffer,
VkBuffer buffer,
VkDeviceSize offset,
VkBuffer countBuffer,
VkDeviceSize countBufferOffset,
uint32_t maxDrawCount,
uint32_t stride);

commandBuffer
is the command buffer into which the command is recorded. 
buffer
is the buffer containing draw parameters. 
offset
is the byte offset intobuffer
where parameters begin. 
countBuffer
is the buffer containing the draw count. 
countBufferOffset
is the byte offset intocountBuffer
where the draw count begins. 
maxDrawCount
specifies the maximum number of draws that will be executed. The actual number of executed draw calls is the minimum of the count specified incountBuffer
andmaxDrawCount
. 
stride
is the byte stride between successive sets of draw parameters.
vkCmdDrawIndexedIndirectCountKHR
behaves similarly to
vkCmdDrawIndexedIndirect except that the draw count is read by the
device from a buffer during execution.
The command will read an unsigned 32bit integer from countBuffer
located at countBufferOffset
and use this as the draw count.