Blending combines the incoming source fragment’s R, G, B, and A values with the destination R, G, B, and A values of each sample stored in the framebuffer at the fragment’s (xf,yf) location. Blending is performed for each pixel sample, rather than just once for each fragment.
Source and destination values are combined according to the blend operation, quadruplets of source and destination weighting factors determined by the blend factors, and a blend constant, to obtain a new set of R, G, B, and A values, as described below.
Blending is computed and applied separately to each color attachment used by the subpass, with separate controls for each attachment.
Prior to performing the blend operation, signed and unsigned normalized fixed-point color components undergo an implied conversion to floating-point as specified by Conversion from Normalized Fixed-Point to Floating-Point. Blending computations are treated as if carried out in floating-point, and will be performed with a precision and dynamic range no lower than that used to represent destination components.
Blending applies only to fixed-point and floating-point color attachments. If the color attachment has an integer format, blending is not applied.
The pipeline blend state is included in the
VkPipelineColorBlendStateCreateInfo
structure during graphics pipeline
creation:
The VkPipelineColorBlendStateCreateInfo
structure is defined as:
typedef struct VkPipelineColorBlendStateCreateInfo { VkStructureType sType; const void* pNext; VkPipelineColorBlendStateCreateFlags flags; VkBool32 logicOpEnable; VkLogicOp logicOp; uint32_t attachmentCount; const VkPipelineColorBlendAttachmentState* pAttachments; float blendConstants[4]; } VkPipelineColorBlendStateCreateInfo;
sType
is the type of this structure.
pNext
is NULL
or a pointer to an extension-specific structure.
flags
is reserved for future use.
logicOpEnable
controls whether to apply Logical Operations.
logicOp
selects which logical operation to apply.
attachmentCount
is the number of
VkPipelineColorBlendAttachmentState
elements in
pAttachments
.
This value must equal the colorAttachmentCount
for the subpass in
which this pipeline is used.
pAttachments
: is a pointer to array of per target attachment
states.
blendConstants
is an array of four values used as the R, G, B, and
A components of the blend constant that are used in blending, depending
on the blend factor.
Each element of the pAttachments
array is a
VkPipelineColorBlendAttachmentState
structure specifying per-target
blending state for each individual color attachment.
If the independent blending feature
is not enabled on the device, all VkPipelineColorBlendAttachmentState
elements in the pAttachments
array must be identical.
The VkPipelineColorBlendAttachmentState
structure is defined as:
typedef struct VkPipelineColorBlendAttachmentState { VkBool32 blendEnable; VkBlendFactor srcColorBlendFactor; VkBlendFactor dstColorBlendFactor; VkBlendOp colorBlendOp; VkBlendFactor srcAlphaBlendFactor; VkBlendFactor dstAlphaBlendFactor; VkBlendOp alphaBlendOp; VkColorComponentFlags colorWriteMask; } VkPipelineColorBlendAttachmentState;
blendEnable
controls whether blending is enabled for the
corresponding color attachment.
If blending is not enabled, the source fragment’s color for that
attachment is passed through unmodified.
srcColorBlendFactor
selects which blend factor is used to
determine the source factors (Sr,Sg,Sb).
dstColorBlendFactor
selects which blend factor is used to
determine the destination factors (Dr,Dg,Db).
colorBlendOp
selects which blend operation is used to calculate
the RGB values to write to the color attachment.
srcAlphaBlendFactor
selects which blend factor is used to
determine the source factor Sa.
dstAlphaBlendFactor
selects which blend factor is used to
determine the destination factor Da.
alphaBlendOp
selects which blend operation is use to calculate the
alpha values to write to the color attachment.
colorWriteMask
is a bitmask selecting which of the R, G, B, and/or
A components are enabled for writing, as described later in this
chapter.
The source and destination color and alpha blending factors are selected from the enum:
typedef enum VkBlendFactor { VK_BLEND_FACTOR_ZERO = 0, VK_BLEND_FACTOR_ONE = 1, VK_BLEND_FACTOR_SRC_COLOR = 2, VK_BLEND_FACTOR_ONE_MINUS_SRC_COLOR = 3, VK_BLEND_FACTOR_DST_COLOR = 4, VK_BLEND_FACTOR_ONE_MINUS_DST_COLOR = 5, VK_BLEND_FACTOR_SRC_ALPHA = 6, VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA = 7, VK_BLEND_FACTOR_DST_ALPHA = 8, VK_BLEND_FACTOR_ONE_MINUS_DST_ALPHA = 9, VK_BLEND_FACTOR_CONSTANT_COLOR = 10, VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_COLOR = 11, VK_BLEND_FACTOR_CONSTANT_ALPHA = 12, VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_ALPHA = 13, VK_BLEND_FACTOR_SRC_ALPHA_SATURATE = 14, VK_BLEND_FACTOR_SRC1_COLOR = 15, VK_BLEND_FACTOR_ONE_MINUS_SRC1_COLOR = 16, VK_BLEND_FACTOR_SRC1_ALPHA = 17, VK_BLEND_FACTOR_ONE_MINUS_SRC1_ALPHA = 18, } VkBlendFactor;
The semantics of each enum value is described in the table below:
Table 26.1. Blend Factors
VkBlendFactor | RGB Blend Factors (Sr,Sg,Sb) or (Dr,Dg,Db) | Alpha Blend Factor (Sa or Da) |
---|---|---|
| (0,0,0) | 0 |
| (1,1,1) | 1 |
| (Rs0,Gs0,Bs0) | As0 |
| (1-Rs0,1-Gs0,1-Bs0) | 1-As0 |
| (Rd,Gd,Bd) | Ad |
| (1-Rd,1-Gd,1-Bd) | 1-Ad |
| (As0,As0,As0) | As0 |
| (1-As0,1-As0,1-As0) | 1-As0 |
| (Ad,Ad,Ad) | Ad |
| (1-Ad,1-Ad,1-Ad) | 1-Ad |
| (Rc,Gc,Bc) | Ac |
| (1-Rc,1-Gc,1-Bc) | 1-Ac |
| (Ac,Ac,Ac) | Ac |
| (1-Ac,1-Ac,1-Ac) | 1-Ac |
| (f,f,f); f = min(As0,1-Ad) | 1 |
| (Rs1,Gs1,Bs1) | As1 |
| (1-Rs1,1-Gs1,1-Bs1) | 1-As1 |
| (As1,As1,As1) | As1 |
| (1-As1,1-As1,1-As1) | 1-As1 |
In this table, the following conventions are used:
If the pipeline state object is created without the
VK_DYNAMIC_STATE_BLEND_CONSTANTS
dynamic state enabled then the blend
constant (Rc,Gc,Bc,Ac) is specified via the
blendConstants
member of VkPipelineColorBlendStateCreateInfo
.
Otherwise, to dynamically set and change the blend constant, call:
void vkCmdSetBlendConstants( VkCommandBuffer commandBuffer, const float blendConstants[4]);
commandBuffer
is the command buffer into which the command will be
recorded.
blendConstants
is an array of four values specifying the R, G, B,
and A components of the blend constant color used in blending, depending
on the blend factor.
Blend factors that use the secondary color input
(Rs1,Gs1,Bs1,As1) (VK_BLEND_FACTOR_SRC1_COLOR
,
VK_BLEND_FACTOR_ONE_MINUS_SRC1_COLOR
,
VK_BLEND_FACTOR_SRC1_ALPHA
, and
VK_BLEND_FACTOR_ONE_MINUS_SRC1_ALPHA
) may consume hardware resources
that could otherwise be used for rendering to multiple color attachments.
Therefore, the number of color attachments that can be used in a
framebuffer may be lower when using dual-source blending.
Dual-source blending is only supported if the
dualSrcBlend
feature is enabled.
The maximum number of color attachments that can be used in a subpass when
using dual-source blending functions is implementation-dependent and is
reported as the maxFragmentDualSrcAttachments
member of
VkPhysicalDeviceLimits
.
When using a fragment shader with dual-source blending functions, the color
outputs are bound to the first and second inputs of the blender using the
Index
decoration, as described in Fragment Output Interface.
If the second color input to the blender is not written in the shader, or if
no output is bound to the second input of a blender, the result of the
blending operation is not defined.
Once the source and destination blend factors have been selected, they along with the source and destination components are passed to the blending operation. The blending operations are selected from the following enum, with RGB and alpha components potentially using different blend operations:
typedef enum VkBlendOp { VK_BLEND_OP_ADD = 0, VK_BLEND_OP_SUBTRACT = 1, VK_BLEND_OP_REVERSE_SUBTRACT = 2, VK_BLEND_OP_MIN = 3, VK_BLEND_OP_MAX = 4, } VkBlendOp;
The semantics of each enum value is described in the table below:
Table 26.2. Blend Operations
VkBlendOp | RGB Components | Alpha Component |
---|---|---|
| R = Rs0 × Sr + Rd × Dr G = Gs0 × Sg + Gd × Dg B = Bs0 × Sb + Bd × Db | A = As0 × Sa + Ad × Da |
| R = Rs0 × Sr - Rd × Dr G = Gs0 × Sg - Gd × Dg B = Bs0 × Sb - Bd × Db | A = As0 × Sa - Ad × Da |
| R = Rd × Dr - Rs0 × Sr G = Gd × Dg - Gs0 × Sg B = Bd × Db - Bs0 × Sb | A = Ad × Da - As0 × Sa |
| R = min(Rs0,Rd) G = min(Gs0,Gd) B = min(Bs0,Bd) | A = min(As0,Ad) |
| R = max(Rs0,Rd) G = max(Gs0,Gd) B = max(Bs0,Bd) | A = max(As0,Ad) |
In this table, the following conventions are used:
The blending operation produces a new set of values R, G, B and A, which are written to the framebuffer attachment. If blending is not enabled for this attachment, then R, G, B and A are assigned Rs0, Gs0, Bs0 and As0, respectively.
If the color attachment is fixed-point, the components of the source and destination values and blend factors are each clamped to [0,1] or [-1,1] respectively for an unsigned normalized or signed normalized color attachment prior to evaluating the blend operations. If the color attachment is floating-point, no clamping occurs.
The colorWriteMask
member of VkPipelineColorBlendAttachmentState
determines whether the final color values R, G, B and A are
written to the framebuffer attachment.
colorWriteMask
is any combination of the following bits:
typedef enum VkColorComponentFlagBits { VK_COLOR_COMPONENT_R_BIT = 0x00000001, VK_COLOR_COMPONENT_G_BIT = 0x00000002, VK_COLOR_COMPONENT_B_BIT = 0x00000004, VK_COLOR_COMPONENT_A_BIT = 0x00000008, } VkColorComponentFlagBits;
If VK_COLOR_COMPONENT_R_BIT
is set, then the R value is written
to color attachment for the appropriate sample, otherwise the value in
memory is unmodified.
The VK_COLOR_COMPONENT_G_BIT
, VK_COLOR_COMPONENT_B_BIT
, and
VK_COLOR_COMPONENT_A_BIT
bits similarly control writing of the G,
B, and A values.
The colorWriteMask
is applied regardless of whether blending is
enabled.
If the numeric format of a framebuffer attachment uses sRGB encoding, the R, G, and B destination color values (after conversion from fixed-point to floating-point) are considered to be encoded for the sRGB color space and hence are linearized prior to their use in blending. Each R, G, and B component is converted from nonlinear to linear as described in the “KHR_DF_TRANSFER_SRGB” section of the Khronos Data Format Specification. If the format is not sRGB, no linearization is performed.
If the numeric format of a framebuffer attachment uses sRGB encoding, then the final R, G and B values are converted into the nonlinear sRGB representation before being written to the framebuffer attachment as described in the “KHR_DF_TRANSFER_SRGB” section of the Khronos Data Format Specification.
If the framebuffer color attachment numeric format is not sRGB encoded then the resulting cs values for R, G and B are unmodified. The value of A is never sRGB encoded. That is, the alpha component is always stored in memory as linear.
If the framebuffer color attachment is VK_ATTACHMENT_UNUSED
, no writes
are performed through that attachment.
Framebuffer color attachments greater than or equal to
VkSubpassDescription
::colorAttachmentCount
perform no writes.