# 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:

Operation | Equation
--- | ---
`CLEAR` | \(0\)
`NOR` | \(\lnot(s \lor d)\)
`AND_INVERTED` | \(\lnot s \land d\)
`COPY_INVERTED` | \(\lnot s\)
`AND_REVERSE` | \(s \land \lnot d\)
`INVERT` | \(\lnot d\)
`XOR` | \(s \oplus d\)
`NAND` | \(\lnot(s \land d)\)
`AND` | \(s \land d\)
`EQUIV` | \(\lnot(s \oplus d)\)
`NOOP` | \(d\)
`OR_INVERTED` | \(\lnot s \lor d\)
`COPY` | \(s\)
`OR_REVERSE` | \(s \lor \lnot d\)
`OR` | \(s \lor d\)
`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.

alpha_to_coverage

  If enabled, the fragment’s alpha value is used to override the fragment’s coverage mask. The coverage mask will be all zeros if the alpha value is zero. The coverage mask will be all ones if the alpha value is one. Otherwise, the number of bits set in the coverage mask will be proportional to the alpha value. Note that this step happens regardless of whether multisample is enabled or the destination buffer is multisampled.

alpha_to_one

  If enabled, the fragment’s alpha value will be set to one. As with alpha_to_coverage, this step happens regardless of whether multisample is enabled or the destination buffer is multisampled.

max_rt

  The index of the max render target (irrespective of whether independent blend is enabled), i.e. the number of MRTs minus one. This is provided so that the driver can avoid the overhead of programming unused MRTs.

## 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.