mesa/src/gallium/docs/source/context.rst

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.


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_fragment_sampler_textures``
* ``set_vertex_sampler_textures``
* ``set_vertex_buffers``


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_edgeflags``.

* ``set_blend_color``
* ``set_clip_state``
* ``set_polygon_stipple``
* ``set_scissor_state``
* ``set_viewport_state``
* ``set_vertex_elements``


Clearing
^^^^^^^^

``clear`` initializes some or all of the surfaces currently bound to
the framebuffer to particular RGBA, depth, or stencil values.

Clear is one of the most difficult concepts to nail down to a single
interface and it seems likely that we will want to add additional
clear paths, for instance clearing surfaces not bound to the
framebuffer, or read-modify-write clears such as depth-only or
stencil-only clears of packed depth-stencil buffers.  


Drawing
^^^^^^^

``draw_arrays`` draws a specified primitive.

This command is equivalent to calling ``draw_arrays_instanced``
with ``startInstance`` set to 0 and ``instanceCount`` set to 1.

``draw_elements`` draws a specified primitive using an optional
index buffer.

This command is equivalent to calling ``draw_elements_instanced``
with ``startInstance`` set to 0 and ``instanceCount`` set to 1.

``draw_range_elements``

XXX: this is (probably) a temporary entrypoint, as the range
information should be available from the vertex_buffer state.
Using this to quickly evaluate a specialized path in the draw
module.

``draw_arrays_instanced`` draws multiple instances of the same primitive.

This command is equivalent to calling ``draw_elements_instanced``
with ``indexBuffer`` set to NULL and ``indexSize`` set to 0.

``draw_elements_instanced`` draws multiple instances of the same primitive
using an optional index buffer.

For instanceID in the range between ``startInstance``
and ``startInstance``+``instanceCount``-1, inclusive, draw a primitive
specified by ``mode`` and sequential numbers in the range between ``start``
and ``start``+``count``-1, inclusive.

If ``indexBuffer`` is not NULL, it specifies an index buffer with index
byte size of ``indexSize``. The sequential numbers are used to lookup
the index buffer and the resulting indices in turn are used to fetch
vertex attributes.

If ``indexBuffer`` is NULL, the sequential numbers are used directly
as indices to fetch vertex attributes.

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 enable a query, use ``begin_query``, and when finished,
use ``end_query`` to stop the query. Finally, ``get_query_result`` is used
to retrieve the results.

Flushing
^^^^^^^^

``flush``


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

``is_texture_referenced``

``is_buffer_referenced``



Blitting
^^^^^^^^

These methods emulate classic blitter controls. They are not guaranteed to be
available; if they are set to NULL, then they are not present.

These methods operate directly on ``pipe_surface`` 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.

``surface_fill`` performs a fill operation on a section of a surface.

``surface_copy`` blits a region of a surface to a region of another surface,
provided that both surfaces are the same format. The source and destination
may be the same surface, and overlapping blits are permitted.

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.