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.. _blend:

Blend

=====

This state controls blending of the final fragments into the target rendering

buffers.

Blend Factors

-------------

The blend factors largely follow the same pattern as their counterparts

in other modern and legacy drawing APIs.

Dual source blend factors are supported for up to 1 MRT, although

you can advertise > 1 MRT, the stack cannot handle them for a few reasons.

There is no definition on how the 1D array of shader outputs should be mapped

to something that would be a 2D array (location, index). No current hardware

exposes > 1 MRT, and we should revisit this issue if anyone ever does.

Logical Operations

------------------

Logical operations, also known as logicops, lops, or rops, are supported.

Only two-operand logicops are available. When logicops are enabled, all other

blend state is ignored, including per-render-target state, so logicops are

performed on all render targets.

.. warning::

   The blend_enable flag is ignored for all render targets when logical

   operations are enabled.

For a source component `s` and destination component `d`, the logical

operations are defined as taking the bits of each channel of each component,

and performing one of the following operations per-channel:

* ``CLEAR``: 0

* ``NOR``: :math:`\lnot(s \lor d)`

* ``AND_INVERTED``: :math:`\lnot s \land d`

* ``COPY_INVERTED``: :math:`\lnot s`

* ``AND_REVERSE``: :math:`s \land \lnot d`

* ``INVERT``: :math:`\lnot d`

* ``XOR``: :math:`s \oplus d`

* ``NAND``: :math:`\lnot(s \land d)`

* ``AND``: :math:`s \land d`

* ``EQUIV``: :math:`\lnot(s \oplus d)`

* ``NOOP``: :math:`d`

* ``OR_INVERTED``: :math:`\lnot s \lor d`

* ``COPY``: :math:`s`

* ``OR_REVERSE``: :math:`s \lor \lnot d`

* ``OR``: :math:`s \lor d`

* ``SET``: 1

.. note::

   The logical operation names and definitions match those of the OpenGL API,

   and are similar to the ROP2 and ROP3 definitions of GDI. This is

   intentional, to ease transitions to Gallium.

Members

-------

These members affect all render targets.

dither

%%%%%%

Whether dithering is enabled.

.. note::

   Dithering is completely implementation-dependent. It may be ignored by

   drivers for any reason, and some render targets may always or never be

   dithered depending on their format or usage flags.

logicop_enable

%%%%%%%%%%%%%%

Whether the blender should perform a logicop instead of blending.

logicop_func

%%%%%%%%%%%%

The logicop to use. One of ``PIPE_LOGICOP``.

independent_blend_enable

   If enabled, blend state is different for each render target, and

   for each render target set in the respective member of the rt array.

   If disabled, blend state is the same for all render targets, and only

   the first member of the rt array contains valid data.

rt

   Contains the per-rendertarget blend state.

Per-rendertarget Members

------------------------

blend_enable

   If blending is enabled, perform a blend calculation according to blend

   functions and source/destination factors. Otherwise, the incoming fragment

   color gets passed unmodified (but colormask still applies).

rgb_func

   The blend function to use for rgb channels. One of PIPE_BLEND.

rgb_src_factor

   The blend source factor to use for rgb channels. One of PIPE_BLENDFACTOR.

rgb_dst_factor

   The blend destination factor to use for rgb channels. One of PIPE_BLENDFACTOR.

alpha_func

   The blend function to use for the alpha channel. One of PIPE_BLEND.

alpha_src_factor

   The blend source factor to use for the alpha channel. One of PIPE_BLENDFACTOR.

alpha_dst_factor

   The blend destination factor to use for alpha channel. One of PIPE_BLENDFACTOR.

colormask

   Bitmask of which channels to write. Combination of PIPE_MASK bits.