27. The Framebuffer
27.1. Blending
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 color sample covered by the fragment, 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 basic blend operations are performed with a precision and dynamic range no lower than that used to represent destination components.
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Note
Blending is only defined for floating-point, UNORM, SNORM, and sRGB formats.
Within those formats, the implementation may only support blending on some
subset of them.
Which formats support blending is indicated by
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The pipeline blend state is included in the
VkPipelineColorBlendStateCreateInfo structure during graphics pipeline
creation:
The VkPipelineColorBlendStateCreateInfo structure is defined as:
// Provided by VK_VERSION_1_0
typedef struct VkPipelineColorBlendStateCreateInfo {
VkStructureType sType;
const void* pNext;
VkPipelineColorBlendStateCreateFlags flags;
VkBool32 logicOpEnable;
VkLogicOp logicOp;
uint32_t attachmentCount;
const VkPipelineColorBlendAttachmentState* pAttachments;
float blendConstants[4];
} VkPipelineColorBlendStateCreateInfo;-
sTypeis a VkStructureType value identifying this structure. -
pNextisNULLor a pointer to a structure extending this structure. -
flagsis reserved for future use. -
logicOpEnablecontrols whether to apply Logical Operations. -
logicOpselects which logical operation to apply. -
attachmentCountis the number of VkPipelineColorBlendAttachmentState elements inpAttachments. It is ignored if the pipeline is created withVK_DYNAMIC_STATE_COLOR_BLEND_ENABLE_EXT,VK_DYNAMIC_STATE_COLOR_BLEND_EQUATION_EXT, andVK_DYNAMIC_STATE_COLOR_WRITE_MASK_EXTdynamic states set, and eitherVK_DYNAMIC_STATE_COLOR_BLEND_ADVANCED_EXTset or advancedBlendCoherentOperations is not enabled on the device. -
pAttachmentsis a pointer to an array of VkPipelineColorBlendAttachmentState structures defining blend state for each color attachment. It is ignored if the pipeline is created withVK_DYNAMIC_STATE_COLOR_BLEND_ENABLE_EXT,VK_DYNAMIC_STATE_COLOR_BLEND_EQUATION_EXT, andVK_DYNAMIC_STATE_COLOR_WRITE_MASK_EXTdynamic states set, and eitherVK_DYNAMIC_STATE_COLOR_BLEND_ADVANCED_EXTset or advancedBlendCoherentOperations is not enabled on the device. -
blendConstantsis a pointer to 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.
// Provided by VK_VERSION_1_0
typedef VkFlags VkPipelineColorBlendStateCreateFlags;VkPipelineColorBlendStateCreateFlags is a bitmask type for setting a
mask, but is currently reserved for future use.
The VkPipelineColorBlendAttachmentState structure is defined as:
// Provided by VK_VERSION_1_0
typedef struct VkPipelineColorBlendAttachmentState {
VkBool32 blendEnable;
VkBlendFactor srcColorBlendFactor;
VkBlendFactor dstColorBlendFactor;
VkBlendOp colorBlendOp;
VkBlendFactor srcAlphaBlendFactor;
VkBlendFactor dstAlphaBlendFactor;
VkBlendOp alphaBlendOp;
VkColorComponentFlags colorWriteMask;
} VkPipelineColorBlendAttachmentState;-
blendEnablecontrols 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. -
srcColorBlendFactorselects which blend factor is used to determine the source factors (Sr,Sg,Sb). -
dstColorBlendFactorselects which blend factor is used to determine the destination factors (Dr,Dg,Db). -
colorBlendOpselects which blend operation is used to calculate the RGB values to write to the color attachment. -
srcAlphaBlendFactorselects which blend factor is used to determine the source factor Sa. -
dstAlphaBlendFactorselects which blend factor is used to determine the destination factor Da. -
alphaBlendOpselects which blend operation is used to calculate the alpha values to write to the color attachment. -
colorWriteMaskis a bitmask of VkColorComponentFlagBits specifying which of the R, G, B, and/or A components are enabled for writing, as described for the Color Write Mask.
To dynamically set blendEnable, call:
// Provided by VK_EXT_extended_dynamic_state3, VK_EXT_shader_object
void vkCmdSetColorBlendEnableEXT(
VkCommandBuffer commandBuffer,
uint32_t firstAttachment,
uint32_t attachmentCount,
const VkBool32* pColorBlendEnables);-
commandBufferis the command buffer into which the command will be recorded. -
firstAttachmentthe first color attachment the color blending enable applies. -
attachmentCountthe number of color blending enables in thepColorBlendEnablesarray. -
pColorBlendEnablesan array of booleans to indicate whether color blending is enabled for the corresponding attachment.
This command sets the color blending enable of the specified color
attachments for subsequent drawing commands
when drawing using shader objects, or
when the graphics pipeline is created with
VK_DYNAMIC_STATE_COLOR_BLEND_ENABLE_EXT set in
VkPipelineDynamicStateCreateInfo::pDynamicStates.
Otherwise, this state is specified by the
VkPipelineColorBlendAttachmentState::blendEnable values used to
create the currently active pipeline.
To dynamically set color blend factors and operations, call:
// Provided by VK_EXT_extended_dynamic_state3, VK_EXT_shader_object
void vkCmdSetColorBlendEquationEXT(
VkCommandBuffer commandBuffer,
uint32_t firstAttachment,
uint32_t attachmentCount,
const VkColorBlendEquationEXT* pColorBlendEquations);-
commandBufferis the command buffer into which the command will be recorded. -
firstAttachmentthe first color attachment the color blend factors and operations apply to. -
attachmentCountthe number of VkColorBlendEquationEXT elements in thepColorBlendEquationsarray. -
pColorBlendEquationsan array of VkColorBlendEquationEXT structs that specify the color blend factors and operations for the corresponding attachments.
This command sets the color blending factors and operations of the specified
attachments for subsequent drawing commands
when drawing using shader objects, or
when the graphics pipeline is created with
VK_DYNAMIC_STATE_COLOR_BLEND_EQUATION_EXT set in
VkPipelineDynamicStateCreateInfo::pDynamicStates.
Otherwise, this state is specified by the
VkPipelineColorBlendAttachmentState::srcColorBlendFactor,
VkPipelineColorBlendAttachmentState::dstColorBlendFactor,
VkPipelineColorBlendAttachmentState::colorBlendOp,
VkPipelineColorBlendAttachmentState::srcAlphaBlendFactor,
VkPipelineColorBlendAttachmentState::dstAlphaBlendFactor, and
VkPipelineColorBlendAttachmentState::alphaBlendOp values used to
create the currently active pipeline.
The VkColorBlendEquationEXT structure is defined as:
// Provided by VK_EXT_extended_dynamic_state3, VK_EXT_shader_object
typedef struct VkColorBlendEquationEXT {
VkBlendFactor srcColorBlendFactor;
VkBlendFactor dstColorBlendFactor;
VkBlendOp colorBlendOp;
VkBlendFactor srcAlphaBlendFactor;
VkBlendFactor dstAlphaBlendFactor;
VkBlendOp alphaBlendOp;
} VkColorBlendEquationEXT;-
srcColorBlendFactorselects which blend factor is used to determine the source factors (Sr,Sg,Sb). -
dstColorBlendFactorselects which blend factor is used to determine the destination factors (Dr,Dg,Db). -
colorBlendOpselects which blend operation is used to calculate the RGB values to write to the color attachment. -
srcAlphaBlendFactorselects which blend factor is used to determine the source factor Sa. -
dstAlphaBlendFactorselects which blend factor is used to determine the destination factor Da. -
alphaBlendOpselects which blend operation is use to calculate the alpha values to write to the color attachment.
To dynamically set the color write masks, call:
// Provided by VK_EXT_extended_dynamic_state3, VK_EXT_shader_object
void vkCmdSetColorWriteMaskEXT(
VkCommandBuffer commandBuffer,
uint32_t firstAttachment,
uint32_t attachmentCount,
const VkColorComponentFlags* pColorWriteMasks);-
commandBufferis the command buffer into which the command will be recorded. -
firstAttachmentthe first color attachment the color write masks apply to. -
attachmentCountthe number of VkColorComponentFlags values in thepColorWriteMasksarray. -
pColorWriteMasksan array of VkColorComponentFlags values that specify the color write masks of the corresponding attachments.
This command sets the color write masks of the specified attachments for
subsequent drawing commands
when drawing using shader objects, or
when the graphics pipeline is created with
VK_DYNAMIC_STATE_COLOR_WRITE_MASK_EXT set in
VkPipelineDynamicStateCreateInfo::pDynamicStates.
Otherwise, this state is specified by the
VkPipelineColorBlendAttachmentState::colorWriteMask values used
to create the currently active pipeline.
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Note
Formats with bits that are shared between components specified by
VkColorComponentFlagBits, such as
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27.1.1. Blend Factors
The source and destination color and alpha blending factors are selected from the enum:
// Provided by VK_VERSION_1_0
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 the enum values are described in the table below:
| 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:
-
Rs0,Gs0,Bs0 and As0 represent the first source color R, G, B, and A components, respectively, for the fragment output location corresponding to the color attachment being blended.
-
Rs1,Gs1,Bs1 and As1 represent the second source color R, G, B, and A components, respectively, used in dual source blending modes, for the fragment output location corresponding to the color attachment being blended.
-
Rd,Gd,Bd and Ad represent the R, G, B, and A components of the destination color. That is, the color currently in the corresponding color attachment for this fragment/sample.
-
Rc,Gc,Bc and Ac represent the blend constant R, G, B, and A components, respectively.
To dynamically set and change the blend constants, call:
// Provided by VK_VERSION_1_0
void vkCmdSetBlendConstants(
VkCommandBuffer commandBuffer,
const float blendConstants[4]);-
commandBufferis the command buffer into which the command will be recorded. -
blendConstantsis a pointer to an array of four values specifying the Rc, Gc, Bc, and Ac components of the blend constant color used in blending, depending on the blend factor.
This command sets blend constants for subsequent drawing commands when
when drawing using shader objects, or
the graphics pipeline is created with VK_DYNAMIC_STATE_BLEND_CONSTANTS
set in VkPipelineDynamicStateCreateInfo::pDynamicStates.
Otherwise, this state is specified by the
VkPipelineColorBlendStateCreateInfo::blendConstants values used
to create the currently active pipeline.
27.1.2. Dual-Source Blending
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 implementation
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.
Color outputs can be 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 value of the second
input is undefined.
27.1.3. Blend Operations
Once the source and destination blend factors have been selected, they along with the source and destination components are passed to the blending operations. RGB and alpha components can use different operations. Possible values of VkBlendOp, specifying the operations, are:
// Provided by VK_VERSION_1_0
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 the basic blend operations are described in the table below:
| VkBlendOp | RGB Components | Alpha Component |
|---|---|---|
|
R = Rs0 × Sr + Rd × Dr |
A = As0 × Sa + Ad × Da |
|
R = Rs0 × Sr - Rd × Dr |
A = As0 × Sa - Ad × Da |
|
R = Rd × Dr - Rs0 × Sr |
A = Ad × Da - As0 × Sa |
|
R = min(Rs0,Rd) |
A = min(As0,Ad) |
|
R = max(Rs0,Rd) |
A = max(As0,Ad) |
In this table, the following conventions are used:
-
Rs0, Gs0, Bs0 and As0 represent the first source color R, G, B, and A components, respectively.
-
Rd, Gd, Bd and Ad represent the R, G, B, and A components of the destination color. That is, the color currently in the corresponding color attachment for this fragment/sample.
-
Sr, Sg, Sb and Sa represent the source blend factor R, G, B, and A components, respectively.
-
Dr, Dg, Db and Da represent the destination blend factor R, G, B, and A components, respectively.
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.
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 “sRGB EOTF” 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 “sRGB EOTF -1” section of the Khronos Data Format Specification.
If the numeric format of a framebuffer color attachment 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.
Writes are not performed to framebuffer color attachments greater than or
equal to the VkSubpassDescription::colorAttachmentCount
or VkSubpassDescription2::colorAttachmentCount
value.
27.2. Logical Operations
The application can enable a logical operation between the fragment’s color values and the existing value in the framebuffer attachment. This logical operation is applied prior to updating the framebuffer attachment. Logical operations are applied only for signed and unsigned integer and normalized integer framebuffers. Logical operations are not applied to floating-point or sRGB format color attachments.
Logical operations are controlled by the logicOpEnable and
logicOp members of VkPipelineColorBlendStateCreateInfo.
The logicOpEnable state can also be controlled by
vkCmdSetLogicOpEnableEXT if graphics pipeline is created with
VK_DYNAMIC_STATE_LOGIC_OP_ENABLE_EXT set in
VkPipelineDynamicStateCreateInfo::pDynamicStates.
If logicOpEnable is VK_TRUE, then a logical operation selected
by logicOp is applied between each color attachment and the fragment’s
corresponding output value, and blending of all attachments is treated as if
it were disabled.
Any attachments using color formats for which logical operations are not
supported simply pass through the color values unmodified.
The logical operation is applied independently for each of the red, green,
blue, and alpha components.
The logicOp is selected from the following operations:
// Provided by VK_VERSION_1_0
typedef enum VkLogicOp {
VK_LOGIC_OP_CLEAR = 0,
VK_LOGIC_OP_AND = 1,
VK_LOGIC_OP_AND_REVERSE = 2,
VK_LOGIC_OP_COPY = 3,
VK_LOGIC_OP_AND_INVERTED = 4,
VK_LOGIC_OP_NO_OP = 5,
VK_LOGIC_OP_XOR = 6,
VK_LOGIC_OP_OR = 7,
VK_LOGIC_OP_NOR = 8,
VK_LOGIC_OP_EQUIVALENT = 9,
VK_LOGIC_OP_INVERT = 10,
VK_LOGIC_OP_OR_REVERSE = 11,
VK_LOGIC_OP_COPY_INVERTED = 12,
VK_LOGIC_OP_OR_INVERTED = 13,
VK_LOGIC_OP_NAND = 14,
VK_LOGIC_OP_SET = 15,
} VkLogicOp;The logical operations supported by Vulkan are summarized in the following table in which
-
¬ is bitwise invert,
-
∧ is bitwise and,
-
∨ is bitwise or,
-
⊕ is bitwise exclusive or,
-
s is the fragment’s Rs0, Gs0, Bs0 or As0 component value for the fragment output corresponding to the color attachment being updated, and
-
d is the color attachment’s R, G, B or A component value:
| Mode | Operation |
|---|---|
|
0 |
|
s ∧ d |
|
s ∧ ¬ d |
|
s |
|
¬ s ∧ d |
|
d |
|
s ⊕ d |
|
s ∨ d |
|
¬ (s ∨ d) |
|
¬ (s ⊕ d) |
|
¬ d |
|
s ∨ ¬ d |
|
¬ s |
|
¬ s ∨ d |
|
¬ (s ∧ d) |
|
all 1s |
The result of the logical operation is then written to the color attachment as controlled by the component write mask, described in Blend Operations.
To dynamically set whether logical operations are enabled, call:
// Provided by VK_EXT_extended_dynamic_state3, VK_EXT_shader_object
void vkCmdSetLogicOpEnableEXT(
VkCommandBuffer commandBuffer,
VkBool32 logicOpEnable);-
commandBufferis the command buffer into which the command will be recorded. -
logicOpEnablespecifies whether logical operations are enabled.
This command sets whether logical operations are enabled for subsequent
drawing commands
when drawing using shader objects, or
when the graphics pipeline is created with
VK_DYNAMIC_STATE_LOGIC_OP_ENABLE_EXT set in
VkPipelineDynamicStateCreateInfo::pDynamicStates.
Otherwise, this state is specified by the
VkPipelineColorBlendStateCreateInfo::logicOpEnable value used to
create the currently active pipeline.
To dynamically set the logical operation to apply for blend state, call:
// Provided by VK_EXT_shader_object
void vkCmdSetLogicOpEXT(
VkCommandBuffer commandBuffer,
VkLogicOp logicOp);-
commandBufferis the command buffer into which the command will be recorded. -
logicOpspecifies the logical operation to apply for blend state.
This command sets the logical operation for blend state for subsequent
drawing commands
when drawing using shader objects.
Otherwise, this state is specified by the
VkPipelineColorBlendStateCreateInfo::logicOp value used to
create the currently active pipeline.
27.3. Color Write Mask
Bits which can be set in
VkPipelineColorBlendAttachmentState::colorWriteMask, determining
whether the final color values R, G, B and A are written to the
framebuffer attachment, are:
// Provided by VK_VERSION_1_0
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;-
VK_COLOR_COMPONENT_R_BITspecifies that the R value is written to the color attachment for the appropriate sample. Otherwise, the value in memory is unmodified. -
VK_COLOR_COMPONENT_G_BITspecifies that the G value is written to the color attachment for the appropriate sample. Otherwise, the value in memory is unmodified. -
VK_COLOR_COMPONENT_B_BITspecifies that the B value is written to the color attachment for the appropriate sample. Otherwise, the value in memory is unmodified. -
VK_COLOR_COMPONENT_A_BITspecifies that the A value is written to the color attachment for the appropriate sample. Otherwise, the value in memory is unmodified.
The color write mask operation is applied regardless of whether blending is enabled.
// Provided by VK_VERSION_1_0
typedef VkFlags VkColorComponentFlags;VkColorComponentFlags is a bitmask type for setting a mask of zero or
more VkColorComponentFlagBits.