path: root/src/gallium/docs/source/context.rst

.. _context:

Context
=======

The context object represents the purest, most directly accessible, abilities
of the device's 3D rendering pipeline.

Methods
-------

CSO State
^^^^^^^^^

All CSO state is created, bound, and destroyed, with triplets of methods that
all follow a specific naming scheme. For example, ``create_blend_state``,
``bind_blend_state``, and ``destroy_blend_state``.

CSO objects handled by the context object:

* :ref:`Blend`: ``*_blend_state``
* :ref:`Sampler`: These are special; they can be bound to either vertex or
  fragment samplers, and they are bound in groups.
  ``bind_fragment_sampler_states``, ``bind_vertex_sampler_states``
* :ref:`Rasterizer`: ``*_rasterizer_state``
* :ref:`Depth, Stencil, & Alpha`: ``*_depth_stencil_alpha_state``
* :ref:`Shader`: These have two sets of methods. ``*_fs_state`` is for
  fragment shaders, and ``*_vs_state`` is for vertex shaders.
* :ref:`Vertex Elements`: ``*_vertex_elements_state``


Resource Binding State
^^^^^^^^^^^^^^^^^^^^^^

This state describes how resources in various flavours (textures,
buffers, surfaces) are bound to the driver.


* ``set_constant_buffer`` sets 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).

* ``set_framebuffer_state``

* ``set_vertex_buffers``

* ``set_index_buffer``

Non-CSO State
^^^^^^^^^^^^^

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_blend_color``.

* ``set_stencil_ref`` sets the stencil front and back reference values
  which are used as comparison values in stencil test.
* ``set_blend_color``
* ``set_sample_mask``
* ``set_clip_state``
* ``set_polygon_stipple``
* ``set_scissor_state`` sets the bounds for the scissor test, which culls
  pixels before blending to render targets. If the :ref:`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].
* ``set_viewport_state``


Sampler Views
^^^^^^^^^^^^^

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.

The ``first_level`` and ``last_level`` fields of sampler view template specify
the LOD range the texture is going to be constrained to.

* ``set_fragment_sampler_views`` binds 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.

* ``set_vertex_sampler_views`` binds 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_view`` creates a new sampler view. ``texture`` is 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_destroy`` destroys a sampler view and releases its reference
  to associated texture.


Clearing
^^^^^^^^

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 the driver
supports PIPE_CAP_DEPTHSTENCIL_CLEAR_SEPARATE.
If a surface includes several layers/slices (XXX: not yet...) 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.


Drawing
^^^^^^^

``draw_vbo`` draws a specified primitive.  The primitive mode and other
properties are described by ``pipe_draw_info``.

The ``mode``, ``start``, and ``count`` fields of ``pipe_draw_info`` specify the
the mode of the primitive and the vertices to be fetched, in the range between
``start`` to ``start``+``count``-1, inclusive.

Every instance with instanceID in the range between ``start_instance`` and
``start_instance``+``instance_count``-1, inclusive, will be drawn.

All vertex indices must fall inside the range given by ``min_index`` and
``max_index``.  In case non-indexed draw, ``min_index`` should be set to
``start`` and ``max_index`` should be set to ``start``+``count``-1.

``index_bias`` is a value added to every vertex index before fetching vertex
attributes.  It does not affect ``min_index`` and ``max_index``.

If there is an index buffer bound, and ``indexed`` field is true, all vertex
indices will be looked up in the index buffer.  ``min_index``, ``max_index``,
and ``index_bias`` apply after index lookup.

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.

  attribAddr = ``stride`` * index + ``src_offset``

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 ``instance_divisor``-th instance.

  attribAddr = ``stride`` * instanceID / ``instance_divisor`` + ``src_offset``

In the above formulas, ``src_offset`` is taken from the given vertex element
and ``stride`` is taken from a vertex buffer associated with the given
vertex element.

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
^^^^^^^

Queries gather some statistic from the 3D pipeline over one or more
draws.  Queries may be nested, though no state tracker currently
exercises this.  

Queries can be created with ``create_query`` and deleted with
``destroy_query``. To start a query, use ``begin_query``, and when finished,
use ``end_query`` to end the query.

``get_query_result`` is used to retrieve the results of a query.  If
the ``wait`` parameter is TRUE, then the ``get_query_result`` call
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 most common type of query is the occlusion query,
``PIPE_QUERY_OCCLUSION_COUNTER``, which counts the number of fragments which
are written to the framebuffer without being culled by
:ref:`Depth, Stencil, & Alpha` testing or shader KILL instructions.

Another type of query, ``PIPE_QUERY_TIME_ELAPSED``, returns the amount of
time, in nanoseconds, the context takes to perform operations.

Gallium does not guarantee the availability of any query types; one must
always check the capabilities of the :ref:`Screen` first.


Conditional Rendering
^^^^^^^^^^^^^^^^^^^^^

A drawing command can be skipped depending on the outcome of a query
(typically an occlusion query).  The ``render_condition`` function specifies
the query which should be checked prior to rendering anything.

If ``render_condition`` is called with ``query`` = NULL, conditional
rendering is disabled and drawing takes place normally.

If ``render_condition`` is called with a non-null ``query`` subsequent
drawing commands will be predicated on the outcome of the query.  If
the query result is zero subsequent drawing commands will be skipped.

If ``mode`` is PIPE_RENDER_COND_WAIT the driver will wait for the
query to complete before deciding whether to render.

If ``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.

If ``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 occulusion 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.


Flushing
^^^^^^^^

``flush``


Resource Busy Queries
^^^^^^^^^^^^^^^^^^^^^

``is_resource_referenced``



Blitting
^^^^^^^^

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 subresource of a resource to a
region of another subresource of a 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.

``resource_resolve`` resolves a multisampled resource into a non-multisampled
one. Formats and dimensions must match. This function must be present if a driver
supports multisampling.

The interfaces to these calls are likely to change to make it easier
for a driver to batch multiple blits with the same source and
destination.


Stream Output
^^^^^^^^^^^^^

Stream output, also known as transform feedback allows writing the results of the
vertex pipeline (after the geometry shader or vertex shader if no geometry shader
is present) to be written to a buffer created with a ``PIPE_BIND_STREAM_OUTPUT``
flag.

First a stream output state needs to be created with the
``create_stream_output_state`` call. It specific the details of what's being written,
to which buffer and with what kind of a writemask.

Then target buffers needs to be set with the call to ``set_stream_output_buffers``
which sets the buffers and the offsets from the start of those buffer to where
the data will be written to.


Transfers
^^^^^^^^^

These methods are used to get data to/from a resource.

``get_transfer`` creates a transfer object.

``transfer_destroy`` destroys the transfer object. May cause
data to be written to the resource at this point.

``transfer_map`` creates a memory mapping for the transfer object.
The returned map points to the start of the mapped range according to
the box region, not the beginning of the resource.

``transfer_unmap`` remove the memory mapping for the transfer object.
Any pointers into the map should be considered invalid and discarded.

``transfer_inline_write`` performs a simplified transfer for simple writes.
Basically get_transfer, transfer_map, data write, transfer_unmap, and
transfer_destroy all in one.

.. _transfer_flush_region:

transfer_flush_region
%%%%%%%%%%%%%%%%%%%%%

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.

.. _pipe_transfer:

PIPE_TRANSFER
^^^^^^^^^^^^^

These flags control the behavior of a transfer object.

* ``READ``: resource contents are read at transfer create time.
* ``WRITE``: resource contents will be written back at transfer destroy time.
* ``MAP_DIRECTLY``: a transfer should directly map the resource. May return
  NULL if not supported.
* ``DISCARD``: The memory within the mapped region is discarded.
  Cannot be used with ``READ``.
* ``DONTBLOCK``: Fail if the resource cannot be mapped immediately.
* ``UNSYNCHRONIZED``: 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
  ``READ``.
* ``FLUSH_EXPLICIT``: Written ranges will be notified later with
  :ref:`transfer_flush_region`. Cannot be used with ``READ``.