366 lines
18 KiB
Plaintext
366 lines
18 KiB
Plaintext
// Copyright (c) 2015-2016 The Khronos Group Inc.
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// Copyright notice at https://www.khronos.org/registry/speccopyright.html
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[[framebuffer]]
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= The Framebuffer
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[[framebuffer-blending]]
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== Blending
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Blending combines the incoming ``source'' fragment's R, G, B, and A values
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with the ``destination'' R, G, B, and A values of each sample stored in the
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framebuffer at the fragment's latexmath:[$(x_f,y_f)$] location. Blending is
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performed for each pixel sample, rather than just once for each fragment.
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Source and destination values are combined according to the
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<<framebuffer-blendoperations,blend operation>>, quadruplets of source and
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destination weighting factors determined by the <<framebuffer-blendfactors,
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blend factors>>, and a <<framebuffer-blendconstants,blend constant>>,
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to obtain a new set of R, G, B, and A values, as described below.
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Blending is computed and applied separately to each color attachment used by
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the subpass, with separate controls for each attachment.
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Prior to performing the blend operation, signed and unsigned normalized
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fixed-point color components undergo an implied conversion to floating-point
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as specified by <<fundamentals-fixedfpconv,Conversion
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from Normalized Fixed-Point to Floating-Point>>. Blending
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computations are treated as if carried out in floating-point, and will be
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performed with a precision and dynamic range no lower than that used to
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represent destination components.
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Blending applies only to fixed-point and floating-point color attachments.
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If the color attachment has an integer format, blending is not applied.
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The pipeline blend state is included in the
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sname:VkPipelineColorBlendStateCreateInfo struct during graphics pipeline
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creation:
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include::../structs/VkPipelineColorBlendStateCreateInfo.txt[]
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The members of the sname:VkPipelineColorBlendStateCreateInfo structure are
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as follows:
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* pname:sType is the type of this structure.
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* pname:pNext is `NULL` or a pointer to an extension-specific structure.
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* pname:flags is reserved for future use.
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* pname:logicOpEnable controls whether to apply <<framebuffer-logicop,
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Logical Operations>>.
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* pname:logicOp selects which logical operation to apply.
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* pname:attachmentCount is the number of
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sname:VkPipelineColorBlendAttachmentState elements in
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pname:pAttachments. This value must: equal the
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pname:colorAttachmentCount for the subpass in which this pipeline is
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used.
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* pname:pAttachments: pointer to array of per target attachment states.
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* pname:blendConstants is an array of four values used as the R, G, B, and
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A components of the blend constant that are used in blending, depending
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on the <<framebuffer-blendfactors,blend factor>>.
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include::../validity/structs/VkPipelineColorBlendStateCreateInfo.txt[]
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The elements of the pname:pAttachments array specify per-target blending
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state, and are of type:
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include::../structs/VkPipelineColorBlendAttachmentState.txt[]
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Blending of each individual color attachment is controlled by the
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corresponding element of the pname:pAttachments array. If the
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<<features-features-independent-blending,independent blending>> feature is
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not enabled on the device, all sname:VkPipelineColorBlendAttachmentState
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elements in the pname:pAttachments array must: be identical. The members of
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the sname:VkPipelineColorBlendAttachmentState struct have the following
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meanings:
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* pname:blendEnable controls whether blending is enabled for the
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corresponding color attachment. If blending is not enabled, the source
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fragment's color for that attachment is passed through unmodified.
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* pname:srcColorBlendFactor selects which blend factor is used to
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determine the source factors latexmath:[$S_r,S_g,S_b$].
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* pname:dstColorBlendFactor selects which blend factor is used to
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determine the destination factors latexmath:[$D_r,D_g,D_b$].
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* pname:colorBlendOp selects which blend operation is used to calculate
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the RGB values to write to the color attachment.
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* pname:srcAlphaBlendFactor selects which blend factor is used to
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determine the source factor latexmath:[$S_a$].
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* pname:dstAlphaBlendFactor selects which blend factor is used to
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determine the destination factor latexmath:[$D_a$].
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* pname:alphaBlendOp selects which blend operation is use to calculate the
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alpha values to write to the color attachment.
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* pname:colorWriteMask is a bitmask selecting which of the R, G, B,
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and/or A components are enabled for writing, as described later in this
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chapter.
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include::../validity/structs/VkPipelineColorBlendAttachmentState.txt[]
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[[framebuffer-blendfactors]]
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=== Blend Factors
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The source and destination color and alpha blending factors are selected
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from the enum:
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include::../enums/VkBlendFactor.txt[]
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The semantics of each enum value is described in the table below:
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.Blend Factors
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[width="100%",options="header",align="center",cols="59%,28%,13%"]
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|========================================
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|VkBlendFactor | RGB Blend Factors (latexmath:[$S_r,S_g,S_b$]) or (latexmath:[$D_r,D_g,D_b$]) | Alpha Blend Factor (latexmath:[$S_a$] or latexmath:[$D_a$])
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|ename:VK_BLEND_FACTOR_ZERO | latexmath:[$(0,0,0)$] | latexmath:[$0$]
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|ename:VK_BLEND_FACTOR_ONE | latexmath:[$(1,1,1)$] | latexmath:[$1$]
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|ename:VK_BLEND_FACTOR_SRC_COLOR | latexmath:[$(R_{s0},G_{s0},B_{s0})$] | latexmath:[$A_{s0}$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_SRC_COLOR | latexmath:[$(1-R_{s0},1-G_{s0},1-B_{s0})$] | latexmath:[$1-A_{s0}$]
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|ename:VK_BLEND_FACTOR_DST_COLOR | latexmath:[$(R_d,G_d,B_d)$] | latexmath:[$A_d$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_DST_COLOR | latexmath:[$(1-R_d,1-G_d,1-B_d)$] | latexmath:[$1-A_d$]
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|ename:VK_BLEND_FACTOR_SRC_ALPHA | latexmath:[$(A_{s0},A_{s0},A_{s0})$] | latexmath:[$A_{s0}$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_SRC_ALPHA | latexmath:[$(1-A_{s0},1-A_{s0},1-A_{s0})$] | latexmath:[$1-A_{s0}$]
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|ename:VK_BLEND_FACTOR_DST_ALPHA | latexmath:[$(A_d,A_d,A_d)$] | latexmath:[$A_d$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_DST_ALPHA | latexmath:[$(1-A_d,1-A_d,1-A_d)$] | latexmath:[$1-A_d$]
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|ename:VK_BLEND_FACTOR_CONSTANT_COLOR | latexmath:[$(R_c,G_c,B_c)$] | latexmath:[$A_c$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_COLOR | latexmath:[$(1-R_c,1-G_c,1-B_c)$] | latexmath:[$1-A_c$]
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|ename:VK_BLEND_FACTOR_CONSTANT_ALPHA | latexmath:[$(A_c,A_c,A_c)$] | latexmath:[$A_c$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_CONSTANT_ALPHA | latexmath:[$(1-A_c,1-A_c,1-A_c)$] | latexmath:[$1-A_c$]
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|ename:VK_BLEND_FACTOR_SRC_ALPHA_SATURATE | latexmath:[$(f,f,f); f=\min(A_{s0},1-A_d)$] | latexmath:[$1$]
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|ename:VK_BLEND_FACTOR_SRC1_COLOR | latexmath:[$(R_{s1},G_{s1},B_{s1})$] | latexmath:[$A_{s1}$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_SRC1_COLOR | latexmath:[$(1-R_{s1},1-G_{s1},1-B_{s1})$] | latexmath:[$1-A_{s1}$]
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|ename:VK_BLEND_FACTOR_SRC1_ALPHA | latexmath:[$(A_{s1},A_{s1},A_{s1})$] | latexmath:[$A_{s1}$]
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|ename:VK_BLEND_FACTOR_ONE_MINUS_SRC1_ALPHA | latexmath:[$(1-A_{s1},1-A_{s1},1-A_{s1})$] | latexmath:[$1-A_{s1}$]
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|========================================
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In this table, the following conventions are used:
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- latexmath:[$R_{s0},G_{s0},B_{s0}$] and latexmath:[$A_{s0}$] represent
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the first source color R, G, B, and A components, respectively, for the
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fragment output location corresponding to the color attachment being
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blended.
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- latexmath:[$R_{s1},G_{s1},B_{s1}$] and latexmath:[$A_{s1}$] represent
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the second source color R, G, B, and A components, respectively, used in
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dual source blending modes, for the fragment output location
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corresponding to the color attachment being blended.
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- latexmath:[$R_d,G_d,B_d$] and latexmath:[$A_d$] represent the R, G, B,
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and A components of the destination color. That is, the color currently
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in the corresponding color attachment for this fragment/sample.
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- latexmath:[$R_c,G_c,B_c$] and latexmath:[$A_c$] represent the blend
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constant R, G, B, and A components, respectively.
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[[framebuffer-blendconstants]]
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If the pipeline state object is created without the
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ename:VK_DYNAMIC_STATE_BLEND_CONSTANTS dynamic state enabled then the
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``blend constant'' latexmath:[$(R_c,G_c,B_c,A_c)$] is specified via the
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pname:blendConstants member of slink:VkPipelineColorBlendStateCreateInfo.
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Otherwise the blend constant is dynamically set and changed by calling the
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command:
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include::../protos/vkCmdSetBlendConstants.txt[]
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* pname:commandBuffer is the command buffer into which the command will be
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recorded.
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* pname:blendConstants is an array of four values specifying the R, G, B,
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and A components of the blend constant color used in blending, depending
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on the <<framebuffer-blendfactors,blend factor>>.
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include::../validity/protos/vkCmdSetBlendConstants.txt[]
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[[framebuffer-dsb]]
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=== Dual-Source Blending
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Blend factors that use the secondary color input
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latexmath:[$(R_{s1},G_{s1},B_{s1},A_{s1})$]
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(ename:VK_BLEND_FACTOR_SRC1_COLOR,
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ename:VK_BLEND_FACTOR_ONE_MINUS_SRC1_COLOR,
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ename:VK_BLEND_FACTOR_SRC1_ALPHA, and
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ename:VK_BLEND_FACTOR_ONE_MINUS_SRC1_ALPHA) may: consume hardware resources
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that could otherwise be used for rendering to multiple color attachments.
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Therefore, the number of color attachments that can: be used in a
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framebuffer may: be lower when using dual-source blending.
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Dual-source blending is only supported if the
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<<features-features-dualSrcBlend,pname:dualSrcBlend>> feature is enabled.
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The maximum number of color attachments that can: be used in a subpass when
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using dual-source blending functions is implementation-dependent and is
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reported as the pname:maxFragmentDualSrcAttachments member of
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sname:VkPhysicalDeviceLimits.
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When using a fragment shader with dual-source blending functions, the color
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outputs are bound to the first and second inputs of the blender using the
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code:Index decoration, as described in <<interfaces-fragmentoutput,Fragment
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Output Interface>>. If the second color input to the blender is not
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written in the shader, or if no output is bound to the second input of a
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blender, the result of the blending operation is not defined.
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[[framebuffer-blendoperations]]
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=== Blend Operations
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Once the source and destination blend factors have been selected, they along
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with the source and destination components are passed to the blending
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operation. The blending operations are selected from the following enum,
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with RGB and alpha components potentially using different blend operations:
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include::../enums/VkBlendOp.txt[]
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The semantics of each enum value is described in the table below:
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.Blend Operations
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[width="100%",cols="45%,30%,25%",options="header",style="verse",align="center"]
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|==========================================
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|VkBlendOp | RGB Components | Alpha Component
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|ename:VK_BLEND_OP_ADD
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| latexmath:[$R=R_{s0}\times S_r+R_d\times D_r$] +
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latexmath:[$G=G_{s0}\times S_g+G_d\times D_g$] +
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latexmath:[$B=B_{s0}\times S_b+B_d\times D_b$]
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| latexmath:[$A=A_{s0}\times S_a+A_d\times D_a$]
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|ename:VK_BLEND_OP_SUBTRACT
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| latexmath:[$R=R_{s0}\times S_r-R_d\times D_r$] +
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latexmath:[$G=G_{s0}\times S_g-G_d\times D_g$] +
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latexmath:[$B=B_{s0}\times S_b-B_d\times D_b$] +
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| latexmath:[$A=A_{s0}\times S_a-A_d\times D_a$]
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|ename:VK_BLEND_OP_REVERSE_SUBTRACT
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| latexmath:[$R=R_d\times D_r-R_{s0}\times S_r$] +
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latexmath:[$G=G_d\times D_g-G_{s0}\times S_g$] +
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latexmath:[$B=B_d\times D_b-B_{s0}\times S_b$] +
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| latexmath:[$A=A_d\times D_a-A_{s0}\times S_a$]
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|ename:VK_BLEND_OP_MIN
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| latexmath:[$R=\min(R_{s0},R_d)$] +
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latexmath:[$G=\min(G_{s0},G_d)$] +
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latexmath:[$B=\min(B_{s0},B_d)$]
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| latexmath:[$A=\min(A_{s0},A_d)$]
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|ename:VK_BLEND_OP_MAX
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| latexmath:[$R=\max(R_{s0},R_d)$] +
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latexmath:[$G=\max(G_{s0},G_d)$] +
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latexmath:[$B=\max(B_{s0},B_d)$] +
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| latexmath:[$A=\max(A_{s0},A_d)$]
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|==========================================
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In this table, the following conventions are used:
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- latexmath:[$R_{s0},G_{s0},B_{s0}$] and latexmath:[$A_{s0}$] represent
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the first source color R, G, B, and A components, respectively.
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- latexmath:[$R_d,G_d,B_d$] and latexmath:[$A_d$] represent the R, G, B,
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and A components of the destination color. That is, the color currently
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in the corresponding color attachment for this fragment/sample.
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- latexmath:[$S_r,S_g,S_b$] and latexmath:[$S_a$] represent the source
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blend factor R, G, B, and A components, respectively.
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- latexmath:[$D_r,D_g,D_b$] and latexmath:[$D_a$] represent the
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destination blend factor R, G, B, and A components, respectively.
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The blending operation produces a new set of values latexmath:[$R, G,
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B$] and latexmath:[$A$], which are written to the framebuffer attachment. If
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blending is not enabled for this attachment, then latexmath:[$R, G, B$] and
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latexmath:[$A$] are assigned the values of
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latexmath:[$R_{s0},G_{s0},B_{s0}$] and latexmath:[$A_{s0}$].
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If the color attachment is fixed-point, the components of the source and
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destination values and blend factors are each clamped to
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latexmath:[$[0,1\]$] or latexmath:[$[-1,1\]$] respectively for an unsigned
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normalized or signed normalized color attachment prior to evaluating the
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blend operations. If the color attachment is floating-point, no clamping
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occurs.
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The pname:colorWriteMask member of slink:VkPipelineColorBlendAttachmentState
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determines whether the final color values latexmath:[$R, G, B$] and
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latexmath:[$A$] are written to the framebuffer attachment.
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pname:colorWriteMask is any combination of the following bits:
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include::../enums/VkColorComponentFlagBits.txt[]
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If ename:VK_COLOR_COMPONENT_R_BIT is set, then the latexmath:[$R$] value is
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written to color attachment for the appropriate sample, otherwise the value
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in memory is unmodified. The ename:VK_COLOR_COMPONENT_G_BIT,
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ename:VK_COLOR_COMPONENT_B_BIT, and ename:VK_COLOR_COMPONENT_A_BIT bits
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similarly control writing of the latexmath:[$G, B,$] and latexmath:[$A$]
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values. The pname:colorWriteMask is applied regardless of whether blending
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is enabled.
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If the numeric format of a framebuffer attachment uses sRGB encoding, the R,
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G, and B destination color values (after conversion from fixed-point to
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floating-point) are considered to be encoded for the sRGB color space and
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hence are linearized prior to their use in blending. Each R, G, and B
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component is converted from nonlinear to linear as described in the
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``KHR_DF_TRANSFER_SRGB`` section of the Khronos Data Format Specification.
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If the format is not sRGB, no linearization is performed.
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If the numeric format of a framebuffer attachment uses sRGB encoding, then
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the final R, G and B values are converted into the nonlinear sRGB color
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space before being written to the framebuffer attachment as described in the
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``KHR_DF_TRANSFER_SRGB`` section of the Khronos Data Format Specification.
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If the framebuffer color attachment numeric format is not sRGB encoded then
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the resulting latexmath:[$c_s$] values for R, G and B are unmodified. The
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value of A is never sRGB encoded. That is, the alpha component is always
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stored in memory as linear.
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[[framebuffer-logicop]]
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== Logical Operations
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The application can: enable a _logical operation_ between the fragment's
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color values and the existing value in the framebuffer attachment. This
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logical operation is applied prior to updating the framebuffer attachment.
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Logical operations are applied only for signed and unsigned integer and
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normalized integer framebuffers. Logical operations are not applied to
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floating-point or sRGB format color attachments.
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Logical operations are controlled by the pname:logicOpEnable and
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pname:logicOp members of slink:VkPipelineColorBlendStateCreateInfo. If
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pname:logicOpEnable is ename:VK_TRUE, then a logical operation selected by
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pname:logicOp is applied between each color attachment and the fragment's
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corresponding output value, and blending of all attachments is treated as if
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it were disabled. Any attachments using color formats for which logical
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operations are not supported simply pass through the color values
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unmodified. The logical operation is applied independently for each of the
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red, green, blue, and alpha components. The pname:logicOp is selected from
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the following operations:
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include::../enums/VkLogicOp.txt[]
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The logical operations supported by {apiname} are summarized in the
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following table in which
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* latexmath:[$\lnot$] is bitwise invert,
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* latexmath:[$\land$] is bitwise and,
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* latexmath:[$\lor$] is bitwise or,
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* latexmath:[$\oplus$] is bitwise exclusive or,
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* latexmath:[$s$] is the fragment's latexmath:[$R_{s0}, G_{s0}, B_{s0}$]
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or latexmath:[$A_{s0}$] component value for the fragment output
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corresponding to the color attachment being updated, and
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* latexmath:[$d$] is the color attachment's latexmath:[$R, G, B$] or
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latexmath:[$A$] component value:
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.Logical Operations
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[width="75%",options="header",style="verse",align="center"]
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|==========================================
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|Mode | Operation
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|ename:VK_LOGIC_OP_CLEAR | latexmath:[$0$]
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|ename:VK_LOGIC_OP_AND | latexmath:[$s \land d$]
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|ename:VK_LOGIC_OP_AND_REVERSE | latexmath:[$s \land \lnot d$]
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|ename:VK_LOGIC_OP_COPY | latexmath:[$s$]
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|ename:VK_LOGIC_OP_AND_INVERTED | latexmath:[$\lnot s \land d$]
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|ename:VK_LOGIC_OP_NO_OP | latexmath:[$d$]
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|ename:VK_LOGIC_OP_XOR | latexmath:[$s \oplus d$]
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|ename:VK_LOGIC_OP_OR | latexmath:[$s \lor d$]
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|ename:VK_LOGIC_OP_NOR | latexmath:[$\lnot (s \lor d)$]
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|ename:VK_LOGIC_OP_EQUIVALENT | latexmath:[$\lnot (s \oplus d)$]
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|ename:VK_LOGIC_OP_INVERT | latexmath:[$\lnot d$]
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|ename:VK_LOGIC_OP_OR_REVERSE | latexmath:[$s \lor \lnot d$]
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|ename:VK_LOGIC_OP_COPY_INVERTED | latexmath:[$\lnot s$]
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|ename:VK_LOGIC_OP_OR_INVERTED | latexmath:[$\lnot s \lor d$]
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|ename:VK_LOGIC_OP_NAND | latexmath:[$\lnot (s \land d)$]
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|ename:VK_LOGIC_OP_SET | all 1s
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|==========================================
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The result of the logical operation is then written to the color attachment
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as controlled by the component write mask, described in
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<<framebuffer-blendoperations,Blend Operations>>.
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