A Gallium rendering context encapsulates the state which effects 3D rendering such as blend state, depth/stencil state, texture samplers, etc.
Note that resource/texture allocation is not per-context but per-screen.
All Constant State Object (CSO) state is created, bound, and destroyed,
with triplets of methods that all follow a specific naming scheme.
CSO objects handled by the context object:
- Sampler: Texture sampler states are bound separately for fragment,
vertex and geometry samplers. Note that sampler states are set en masse.
If M is the max number of sampler units supported by the driver and N
samplers are bound with
bind_fragment_sampler_statesthen sampler units N..M-1 are considered disabled/NULL.
- Depth, Stencil, & Alpha:
- Shader: These are create, bind and destroy methods for vertex, fragment and geometry shaders.
- Vertex Elements:
Resource Binding State¶
This state describes how resources in various flavours (textures, buffers, surfaces) are bound to the driver.
set_constant_buffersets a constant buffer to be used for a given shader type. index is used to indicate which buffer to set (some apis may allow multiple ones to be set, and binding a specific one later, though drivers are mostly restricted to the first one right now).
These pieces of state are too small, variable, and/or trivial to have CSO
objects. They all follow simple, one-method binding calls, e.g.
set_stencil_refsets the stencil front and back reference values which are used as comparison values in stencil test.
set_scissor_statessets the bounds for the scissor test, which culls pixels before blending to render targets. If the Rasterizer does not have the scissor test enabled, then the scissor bounds never need to be set since they will not be used. Note that scissor xmin and ymin are inclusive, but xmax and ymax are exclusive. The inclusive ranges in x and y would be [xmin..xmax-1] and [ymin..ymax-1]. The number of scissors should be the same as the number of set viewports and can be up to PIPE_MAX_VIEWPORTS.
These are the means to bind textures to shader stages. To create one, specify its format, swizzle and LOD range in sampler view template.
If texture format is different than template format, it is said the texture is being cast to another format. Casting can be done only between compatible formats, that is formats that have matching component order and sizes.
Swizzle fields specify they way in which fetched texel components are placed
in the result register. For example,
swizzle_r specifies what is going to be
placed in first component of result register.
last_level fields of sampler view template specify
the LOD range the texture is going to be constrained to. Note that these
values are in addition to the respective min_lod, max_lod values in the
pipe_sampler_state (that is if min_lod is 2.0, and first_level 3, the first mip
level used for sampling from the resource is effectively the fifth).
last_layer fields specify the layer range the
texture is going to be constrained to. Similar to the LOD range, this is added
to the array index which is used for sampling.
set_fragment_sampler_viewsbinds an array of sampler views to fragment shader stage. Every binding point acquires a reference to a respective sampler view and releases a reference to the previous sampler view. If M is the maximum number of sampler units and N units is passed to set_fragment_sampler_views, the driver should unbind the sampler views for units N..M-1.
set_vertex_sampler_viewsbinds an array of sampler views to vertex shader stage. Every binding point acquires a reference to a respective sampler view and releases a reference to the previous sampler view.
create_sampler_viewcreates a new sampler view.
textureis associated with the sampler view which results in sampler view holding a reference to the texture. Format specified in template must be compatible with texture format.
sampler_view_destroydestroys a sampler view and releases its reference to associated texture.
Shader resources are textures or buffers that may be read or written from a shader without an associated sampler. This means that they have no support for floating point coordinates, address wrap modes or filtering.
Shader resources are specified for all the shader stages at once using
set_shader_resources method. When binding texture resources,
last_layer pipe_surface fields
specify the mipmap level and the range of layers the texture will be
constrained to. In the case of buffers,
last_element specify the range within the buffer that will be used
by the shader resource. Writes to a shader resource are only allowed
writable flag is set.
These are the means to use resources as color render targets or depthstencil attachments. To create one, specify the mip level, the range of layers, and the bind flags (either PIPE_BIND_DEPTH_STENCIL or PIPE_BIND_RENDER_TARGET). Note that layer values are in addition to what is indicated by the geometry shader output variable XXX_FIXME (that is if first_layer is 3 and geometry shader indicates index 2, the 5th layer of the resource will be used). These first_layer and last_layer parameters will only be used for 1d array, 2d array, cube, and 3d textures otherwise they are 0.
create_surfacecreates a new surface.
surface_destroydestroys a surface and releases its reference to the associated resource.
Stream output targets¶
Stream output, also known as transform feedback, allows writing the primitives produced by the vertex pipeline to buffers. This is done after the geometry shader or vertex shader if no geometry shader is present.
The stream output targets are views into buffer resources which can be bound as stream outputs and specify a memory range where it’s valid to write primitives. The pipe driver must implement memory protection such that any primitives written outside of the specified memory range are discarded.
Two stream output targets can use the same resource at the same time, but with a disjoint memory range.
Additionally, the stream output target internally maintains the offset into the buffer which is incremented everytime something is written to it. The internal offset is equal to how much data has already been written. It can be stored in device memory and the CPU actually doesn’t have to query it.
The stream output target can be used in a draw command to provide the vertex count. The vertex count is derived from the internal offset discussed above.
create_stream_output_targetcreate a new target.
stream_output_target_destroydestroys a target. Users of this should use pipe_so_target_reference instead.
set_stream_output_targetsbinds stream output targets. The parameter append_bitmask is a bitmask, where the i-th bit specifies whether new primitives should be appended to the i-th buffer (writing starts at the internal offset), or whether writing should start at the beginning (the internal offset is effectively set to 0).
NOTE: The currently-bound vertex or geometry shader must be compiled with the properly-filled-in structure pipe_stream_output_info describing which outputs should be written to buffers and how. The structure is part of pipe_shader_state.
Clear is one of the most difficult concepts to nail down to a single interface (due to both different requirements from APIs and also driver/hw specific differences).
clear initializes some or all of the surfaces currently bound to
the framebuffer to particular RGBA, depth, or stencil values.
Currently, this does not take into account color or stencil write masks (as
used by GL), and always clears the whole surfaces (no scissoring as used by
GL clear or explicit rectangles like d3d9 uses). It can, however, also clear
only depth or stencil in a combined depth/stencil surface.
If a surface includes several layers then all layers will be cleared.
clear_render_target clears a single color rendertarget with the specified
color value. While it is only possible to clear one surface at a time (which can
include several layers), this surface need not be bound to the framebuffer.
clear_depth_stencil clears a single depth, stencil or depth/stencil surface
with the specified depth and stencil values (for combined depth/stencil buffers,
is is also possible to only clear one or the other part). While it is only
possible to clear one surface at a time (which can include several layers),
this surface need not be bound to the framebuffer.
draw_vbo draws a specified primitive. The primitive mode and other
properties are described by
count fields of
pipe_draw_info specify the
the mode of the primitive and the vertices to be fetched, in the range between
Every instance with instanceID in the range between
start_instance``+``instance_count-1, inclusive, will be drawn.
If there is an index buffer bound, and
indexed field is true, all vertex
indices will be looked up in the index buffer.
In indexed draw,
max_index respectively provide a lower
and upper bound of the indices contained in the index buffer inside the range
start``+``count-1. This allows the driver to
determine which subset of vertices will be referenced during te draw call
without having to scan the index buffer. Providing a over-estimation of the
the true bounds, for example, a
max_index of 0 and
0xffffffff respectively, must give exactly the same rendering, albeit with less
performance due to unreferenced vertex buffers being unnecessarily DMA’ed or
processed. Providing a underestimation of the true bounds will result in
undefined behavior, but should not result in program or system failure.
In case of non-indexed draw,
min_index should be set to
max_index should be set to
index_bias is a value added to every vertex index after lookup and before
fetching vertex attributes.
When drawing indexed primitives, the primitive restart index can be
used to draw disjoint primitive strips. For example, several separate
line strips can be drawn by designating a special index value as the
restart index. The
primitive_restart flag enables/disables this
restart_index field specifies the restart index value.
When primitive restart is in use, array indexes are compared to the restart index before adding the index_bias offset.
If a given vertex element has
instance_divisor set to 0, it is said
it contains per-vertex data and effective vertex attribute address needs
to be recalculated for every index.
stride* index +
If a given vertex element has
instance_divisor set to non-zero,
it is said it contains per-instance data and effective vertex attribute
address needs to recalculated for every
stride* instanceID /
In the above formulas,
src_offset is taken from the given vertex element
stride is taken from a vertex buffer associated with the given
The calculated attribAddr is used as an offset into the vertex buffer to fetch the attribute data.
The value of
instanceID can be read in a vertex shader through a system
value register declared with INSTANCEID semantic name.
Queries gather some statistic from the 3D pipeline over one or more draws. Queries may be nested, though not all state trackers exercise this.
Queries can be created with
create_query and deleted with
destroy_query. To start a query, use
begin_query, and when finished,
end_query to end the query.
get_query_result is used to retrieve the results of a query. If
wait parameter is TRUE, then the
will block until the results of the query are ready (and TRUE will be
returned). Otherwise, if the
wait parameter is FALSE, the call
will not block and the return value will be TRUE if the query has
completed or FALSE otherwise.
The interface currently includes the following types of queries:
PIPE_QUERY_OCCLUSION_COUNTER counts the number of fragments which
are written to the framebuffer without being culled by
Depth, Stencil, & Alpha testing or shader KILL instructions.
The result is an unsigned 64-bit integer.
This query can be used with
In cases where a boolean result of an occlusion query is enough,
PIPE_QUERY_OCCLUSION_PREDICATE should be used. It is just like
PIPE_QUERY_OCCLUSION_COUNTER except that the result is a boolean
value of FALSE for cases where COUNTER would result in 0 and TRUE
for all other cases.
This query can be used with
PIPE_QUERY_TIME_ELAPSED returns the amount of time, in nanoseconds,
the context takes to perform operations.
The result is an unsigned 64-bit integer.
PIPE_QUERY_TIMESTAMP returns a device/driver internal timestamp,
scaled to nanoseconds, recorded after all commands issued prior to
end_query have been processed.
This query does not require a call to
The result is an unsigned 64-bit integer.
PIPE_QUERY_TIMESTAMP_DISJOINT can be used to check the
internal timer resolution and whether the timestamp counter has become
unreliable due to things like throttling etc. - only if this is FALSE
a timestamp query (within the timestamp_disjoint query) should be trusted.
The result is a 64-bit integer specifying the timer resolution in Hz,
followed by a boolean value indicating whether the timestamp counter
is discontinuous or disjoint.
PIPE_QUERY_PRIMITIVES_GENERATED returns a 64-bit integer indicating
the number of primitives processed by the pipeline (regardless of whether
stream output is active or not).
PIPE_QUERY_PRIMITIVES_EMITTED returns a 64-bit integer indicating
the number of primitives written to stream output buffers.
PIPE_QUERY_SO_STATISTICS returns 2 64-bit integers corresponding to
the result of
the number of primitives that would have been written to stream output buffers
if they had infinite space available (primitives_storage_needed), in this order.
PIPE_QUERY_SO_OVERFLOW_PREDICATE returns a boolean value indicating
whether the stream output targets have overflowed as a result of the
commands issued between
This query can be used with
PIPE_QUERY_GPU_FINISHED returns a boolean value indicating whether
all commands issued before
end_query have completed. However, this
does not imply serialization.
This query does not require a call to
PIPE_QUERY_PIPELINE_STATISTICS returns an array of the following
Number of vertices read from vertex buffers.
Number of primitives read from vertex buffers.
Number of vertex shader threads launched.
Number of geometry shader threads launched.
Number of primitives generated by geometry shaders.
Number of primitives forwarded to the rasterizer.
Number of primitives rasterized.
Number of fragment shader threads launched.
Number of tessellation control shader threads launched.
Number of tessellation evaluation shader threads launched.
If a shader type is not supported by the device/driver,
the corresponding values should be set to 0.
Gallium does not guarantee the availability of any query types; one must always check the capabilities of the Screen first.
A drawing command can be skipped depending on the outcome of a query
(typically an occlusion query, or streamout overflow predicate).
render_condition function specifies the query which should be checked
prior to rendering anything. Functions honoring render_condition include
(and are limited to) draw_vbo, clear, clear_render_target, clear_depth_stencil.
render_condition is called with
query = NULL, conditional
rendering is disabled and drawing takes place normally.
render_condition is called with a non-null
drawing commands will be predicated on the outcome of the query.
Commands will be skipped if
condition is equal to the predicate result
(for non-boolean queries such as OCCLUSION_QUERY, zero counts as FALSE,
non-zero as TRUE).
mode is PIPE_RENDER_COND_WAIT the driver will wait for the
query to complete before deciding whether to render.
mode is PIPE_RENDER_COND_NO_WAIT and the query has not yet
completed, the drawing command will be executed normally. If the query
has completed, drawing will be predicated on the outcome of the query.
mode is PIPE_RENDER_COND_BY_REGION_WAIT or
PIPE_RENDER_COND_BY_REGION_NO_WAIT rendering will be predicated as above
for the non-REGION modes but in the case that an occlusion query returns
a non-zero result, regions which were occluded may be ommitted by subsequent
drawing commands. This can result in better performance with some GPUs.
Normally, if the occlusion query returned a non-zero result subsequent
drawing happens normally so fragments may be generated, shaded and
processed even where they’re known to be obscured.
Resource Busy Queries¶
These methods emulate classic blitter controls.
These methods operate directly on
pipe_resource objects, and stand
apart from any 3D state in the context. Blitting functionality may be
moved to a separate abstraction at some point in the future.
resource_copy_region blits a region of a resource to a region of another
resource, provided that both resources have the same format, or compatible
formats, i.e., formats for which copying the bytes from the source resource
unmodified to the destination resource will achieve the same effect of a
textured quad blitter.. The source and destination may be the same resource,
but overlapping blits are not permitted.
This can be considered the equivalent of a CPU memcpy.
blit blits a region of a resource to a region of another resource, including
scaling, format conversion, and up-/downsampling, as well as
a destination clip rectangle (scissors).
As opposed to manually drawing a textured quad, this lets the pipe driver choose
the optimal method for blitting (like using a special 2D engine), and usually
offers, for example, accelerated stencil-only copies even where
PIPE_CAP_SHADER_STENCIL_EXPORT is not available.
These methods are used to get data to/from a resource.
transfer_map creates a memory mapping and the transfer object
associated with it.
The returned pointer points to the start of the mapped range according to
the box region, not the beginning of the resource. If transfer_map fails,
the returned pointer to the buffer memory is NULL, and the pointer
to the transfer object remains unchanged (i.e. it can be non-NULL).
transfer_unmap remove the memory mapping for and destroy
the transfer object. The pointer into the resource should be considered
invalid and discarded.
transfer_inline_write performs a simplified transfer for simple writes.
Basically transfer_map, data write, and transfer_unmap all in one.
The box parameter to some of these functions defines a 1D, 2D or 3D region of pixels. This is self-explanatory for 1D, 2D and 3D texture targets.
For PIPE_TEXTURE_1D_ARRAY and PIPE_TEXTURE_2D_ARRAY, the box::z and box::depth fields refer to the array dimension of the texture.
For PIPE_TEXTURE_CUBE, the box:z and box::depth fields refer to the faces of the cube map (z + depth <= 6).
For PIPE_TEXTURE_CUBE_ARRAY, the box:z and box::depth fields refer to both the face and array dimension of the texture (face = z % 6, array = z / 6).
If a transfer was created with
FLUSH_EXPLICIT, it will not automatically
be flushed on write or unmap. Flushes must be requested with
transfer_flush_region. Flush ranges are relative to the mapped range, not
the beginning of the resource.
This function flushes all pending writes to the currently-set surfaces and invalidates all read caches of the currently-set samplers.
These flags control the behavior of a transfer object.
- Resource contents read back (or accessed directly) at transfer create time.
- Resource contents will be written back at transfer_unmap time (or modified as a result of being accessed directly).
- a transfer should directly map the resource. May return NULL if not supported.
- The memory within the mapped region is discarded. Cannot be used with
- Discards all memory backing the resource. It should not be used with
- Fail if the resource cannot be mapped immediately.
- Do not synchronize pending operations on the resource when mapping. The
interaction of any writes to the map and any operations pending on the
resource are undefined. Cannot be used with
- Written ranges will be notified later with transfer_flush_region.
Cannot be used with
Compute kernel execution¶
A compute program can be defined, bound or destroyed using
Any of the subroutines contained within the compute program can be
executed on the device using the
launch_grid method. This method
will execute as many instances of the program as elements in the
specified N-dimensional grid, hopefully in parallel.
The compute program has access to four special resources:
GLOBALrepresents a memory space shared among all the threads running on the device. An arbitrary buffer created with the
PIPE_BIND_GLOBALflag can be mapped into it using the
LOCALrepresents a memory space shared among all the threads running in the same working group. The initial contents of this resource are undefined.
PRIVATErepresents a memory space local to a single thread. The initial contents of this resource are undefined.
INPUTrepresents a read-only memory space that can be initialized at
These resources use a byte-based addressing scheme, and they can be
accessed from the compute program by means of the LOAD/STORE TGSI
opcodes. Additional resources to be accessed using the same opcodes
may be specified by the user with the
In addition, normal texture sampling is allowed from the compute
bind_compute_sampler_states may be used to set up texture
samplers for the compute stage and
be used to bind a number of sampler views to it.