VK_QCOM_ycbcr_degamma(3) Manual Page

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Not ratified

This extension allows implementations to expose support for “sRGB EOTF” also known as “sRGB degamma”, used in combination with images using 8-bit Y′C_BC_R formats. In addition, the degamma can be selectively applied to the Y (luma) or CrCb (chroma).

VK_KHR_sampler_ycbcr_conversion adds support for Y′C_BC_R conversion, but allows texture sampling in a non-linear space which can cause artifacts. This extension allows implementations to expose sRGB degamma for Y′C_BC_R formats, which is performed during texture filtering, allowing texture filtering to operate in a linear space.

1) Which Y′C_BC_R formats support the degamma feature?

RESOLVED: For implementations that support the extension, each format that contains 8-bit R, G, and B components and supports either VK_FORMAT_FEATURE_MIDPOINT_CHROMA_SAMPLES_BIT or VK_FORMAT_FEATURE_COSITED_CHROMA_SAMPLES_BIT must support degamma.

Since non-compressed Vulkan sRGB formats are already limited to 8-bit components, and since Adreno supports degamma for all 8bit Y′C_BC_R formats, this extension does not introduce a new VK_FORMAT_FEATURE* bit for the degamma feature.

2) On which Y′C_BC_R components is the degamma applied?

RESOLVED: While degamma is expected to be applied to only the Y (luma) component, the extension provides the ability to selectively enable degamma for both the Y (luma) and/or CbCr (chroma) components.

3) Should degamma be enabled for the sampler object or for the image view object?

RESOLVED: Both. This extension extends VkSamplerYcbcrConversionCreateInfo and the specification already requires that both sampler and view objects must be created with an identical VkSamplerYcbcrConversionCreateInfo in their pNext chains.

4) Why apply the “sRGB” transfer function directly to Y′C_BC_R data when it would be more correct to use the “ITU transfer function”, and do so only after the values have been converted into non-linear R’G’B'?

RESOLVED: Y′C_BC_R is frequently stored according to standards (e.g. BT.601 and BT.709) that specify that the conversion between linear and non-linear should use the ITU Transfer function. The ITU transfer function is mathematically different from the sRGB transfer function and while sRGB and ITU define similar curves, the difference is significant. Performing the “sRGB degamma” prior to range expansion can introduce artifacts if the content uses VK_SAMPLER_YCBCR_RANGE_ITU_NARROW encoding. Nevertheless, using sRGB can make sense for certain use-cases where camera YCbCr images are known to be encoded with sRGB (or a pure gamma 2.2) transfer function and are known to use full-range encoding.

For those use-cases, this extension leverages the GPU ability to enable sRGB degamma at little cost, and can improve quality because texture filtering is able to occur in linear space.

VkPhysicalDeviceYcbcrDegammaFeaturesQCOM, VkSamplerYcbcrConversionYcbcrDegammaCreateInfoQCOM

For more information, see the Vulkan Specification

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