// Copyright 2023 The Khronos Group Inc.
//
// SPDX-License-Identifier: CC-BY-4.0

include::{generated}/meta/{refprefix}VK_QCOM_ycbcr_degamma.adoc[]

=== Other Extension Metadata

*Last Modified Date*::
    2023-07-31
*IP Status*::
    No known IP claims.
*Interactions and External Dependencies*::
  None
*Contributors*::
  - Jeff Leger, Qualcomm
  - Jonathan Wicks, Qualcomm

=== Description

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

`apiext:VK_KHR_sampler_ycbcr_conversion` adds support for {YCbCr}
conversion, but allows texture sampling in a non-linear space which can
cause artifacts.
This extension allows implementations to expose sRGB degamma for {YCbCr}
formats, which is performed during texture filtering, allowing texture
filtering to operate in a linear space.

include::{generated}/interfaces/VK_QCOM_ycbcr_degamma.adoc[]

=== Issues

1) Which {YCbCr} 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
ename:VK_FORMAT_FEATURE_MIDPOINT_CHROMA_SAMPLES_BIT or
ename: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 {YCbCr} formats,
this extension does not introduce a new VK_FORMAT_FEATURE* bit for the
degamma feature.

2) On which {YCbCr} 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 slink:VkSamplerYcbcrConversionCreateInfo and the
specification already requires that both sampler and view objects must be
created with an _identical_ slink:VkSamplerYcbcrConversionCreateInfo in
their pNext chains.

4) Why apply the "`sRGB`" transfer function directly to {YCbCr} 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*: {YCbCr} 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 ename: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.

=== Version History

  * Revision 1, 2023-07-31 (Jeff Leger)