This extension adds a number of “advanced” blending operations that can be used to perform new color blending operations, many of which are more complex than the standard blend modes provided by unextended Vulkan. This extension requires different styles of usage, depending on the level of hardware support and the enabled features:

• If VkPhysicalDeviceBlendOperationAdvancedFeaturesEXT::advancedBlendCoherentOperations is FALSE, the new blending operations are supported, but a memory dependency must separate each advanced blend operation on a given sample. ACCESS_COLOR_ATTACHMENT_READ_NONCOHERENT_BIT_EXT is used to synchronize reads using advanced blend operations.
• If VkPhysicalDeviceBlendOperationAdvancedFeaturesEXT::advancedBlendCoherentOperations is TRUE, advanced blend operations obey primitive order just like basic blend operations.

In unextended Vulkan, the set of blending operations is limited, and can be expressed very simply. The BLEND_OP_MIN and BLEND_OP_MAX blend operations simply compute component-wise minimums or maximums of source and destination color components. The BLEND_OP_ADD, BLEND_OP_SUBTRACT, and BLEND_OP_REVERSE_SUBTRACT modes multiply the source and destination colors by source and destination factors and either add the two products together or subtract one from the other. This limited set of operations supports many common blending operations but precludes the use of more sophisticated transparency and blending operations commonly available in many dedicated imaging APIs.

This extension provides a number of new “advanced” blending operations. Unlike traditional blending operations using BLEND_OP_ADD, these blending equations do not use source and destination factors specified by VkBlendFactor. Instead, each blend operation specifies a complete equation based on the source and destination colors. These new blend operations are used for both RGB and alpha components; they must not be used to perform separate RGB and alpha blending (via different values of color and alpha VkBlendOp).

These blending operations are performed using premultiplied colors, where RGB colors can be considered premultiplied or non-premultiplied by alpha, according to the srcPremultiplied and dstPremultiplied members of VkPipelineColorBlendAdvancedStateCreateInfoEXT. If a color is considered non-premultiplied, the (R,G,B) color components are multiplied by the alpha component prior to blending. For non-premultiplied color components in the range [0,1], the corresponding premultiplied color component would have values in the range [0 × A, 1 × A].

Many of these advanced blending equations are formulated where the result of blending source and destination colors with partial coverage have three separate contributions: from the portions covered by both the source and the destination, from the portion covered only by the source, and from the portion covered only by the destination. The blend parameter VkPipelineColorBlendAdvancedStateCreateInfoEXT::blendOverlap can be used to specify a correlation between source and destination pixel coverage. If set to BLEND_OVERLAP_CONJOINT_EXT, the source and destination are considered to have maximal overlap, as would be the case if drawing two objects on top of each other. If set to BLEND_OVERLAP_DISJOINT_EXT, the source and destination are considered to have minimal overlap, as would be the case when rendering a complex polygon tessellated into individual non-intersecting triangles. If set to BLEND_OVERLAP_UNCORRELATED_EXT, the source and destination coverage are assumed to have no spatial correlation within the pixel.

In addition to the coherency issues on implementations not supporting advancedBlendCoherentOperations, this extension has several limitations worth noting. First, the new blend operations have a limit on the number of color attachments they can be used with, as indicated by VkPhysicalDeviceBlendOperationAdvancedPropertiesEXT::advancedBlendMaxColorAttachments. Additionally, blending precision may be limited to 16-bit floating-point, which may result in a loss of precision and dynamic range for framebuffer formats with 32-bit floating-point components, and in a loss of precision for formats with 12- and 16-bit signed or unsigned normalized integer components.

VK_EXT_blend_operation_advanced

Name String

VK_EXT_blend_operation_advanced

Extension Type

Device extension

Registered Extension Number

149

Revision

2

Extension and Version Dependencies

VK_KHR_get_physical_device_properties2 or Version 1.1

Contact

• Jeff Bolz jeffbolznv

Other Extension Metadata

Last Modified Date

2017-06-12

Contributors

• Jeff Bolz, NVIDIA