ASTC HDR纹理压缩官方规范：KHR_texture_compression_astc_hdr详解
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更新于2024-07-15
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标题："KHR_texture_compression_astc_hdr.pdf"文档提供了关于高级采样纹理压缩（ASTC High Dynamic Range, HDR）的官方规格，这是由ARM、AMD、NVIDIA等多家公司共同贡献并得到Khronos Group支持的一项技术。该扩展旨在优化纹理数据的存储和处理，特别是在游戏和图形应用中，对于高动态范围图像的支持至关重要。
"KHR_texture_compression_astc_hdr"是OpenGL和OpenGLES标准中的一个扩展，其目标是解决HDR（高动态范围）纹理的压缩问题，提高图形质量和性能。HDR图像能够提供更丰富的色彩和细节，但传统的8位或16位纹理格式无法满足其需求，因此ASTC HDR应运而生，它采用固定功能硬件支持，可以提供高质量的无损或低损压缩，同时减少内存带宽消耗。
文档涉及的关键知识点包括：
1. **名称与字符串表示**：GL_KHR_texture_compression_astc_hdr是OpenGL中的扩展名称，用于识别和调用这一特性。同时，GL_KHR_texture_compression_astc_ldr可能是另一个相关的LDR（低动态范围）压缩方案。
2. **联系人与贡献者**：列举了多位来自ARM、AMD、NVIDIA等公司的专业人士，他们在ASTC HDR的开发过程中发挥了关键作用，如Sean Ellis、Jon Leech等。
3. **知识产权**：版权信息表明该规范在2012年至2016年间由Khronos Group拥有，并指出有关版权条款可在Khronos官网找到。
4. **更新策略**：Khronos Group对扩展规格的更新遵循特定流程，即优先处理OpenGL和OpenGLES Working Groups确认的重要问题和bug。对于核心规范的改动，首先会在最新版本中体现，然后逐渐回滚到扩展文档。
5. **适用场景**：ASTC HDR主要用于提升游戏引擎和图形应用程序中HDR纹理的压缩效率，特别是对于支持高动态范围渲染的场景，如现代游戏、虚拟现实（VR）和增强现实（AR）应用。
"KHR_texture_compression_astc_hdr.pdf"文档详细介绍了如何在OpenGL和OpenGLES中利用ASTC HDR压缩技术，以优化对高动态范围图像的支持，提升了图形质量与系统性能。开发者可以通过这个规范了解如何集成和使用这种高效的纹理压缩方法。
determine the effective weight to be used for each texel position. This allows
very low bit rates to be used with acceptable quality. This can also be used
to more efficiently encode blocks with low detail, or with strong vertical
or horizontal features.
For blocks which have a mixture of disparate colors, a single line in the
color space is not a good fit to the colors of the pixels in the original
image. It is therefore possible to partition the texels into multiple sets,
the pixels within each set having similar colors. For each of these
"partitions", we specify separate endpoint pairs, and choose which pair of
endpoints to use for a particular texel by looking up the partition index
from a partitioning pattern table. In ASTC, this partition table is actually
implemented as a function.
The endpoint encoding for each partition is independent.
For blocks which have uncorrelated channels - for example an image with a
transparency mask, or an image used as a normal map - it may be necessary
to specify two weights for each texel. Interpolation between the components
of the endpoint colors can then proceed independently for each "plane" of
the image. The assignment of channels to planes is selectable.
Since each of the above options is independent, it is possible to specify any
combination of channels, endpoint color encoding, weight encoding,
interpolation, multiple partitions and single or dual planes.
Since these values are specified per block, it is important that they are
represented with the minimum possible number of bits. As a result, these
values are packed together in ways which can be difficult to read, but
which are nevertheless highly amenable to hardware decode.
All of the values used as weights and color endpoint values can be specified
with a variable number of bits. The encoding scheme used allows a fine-
grained tradeoff between weight bits and color endpoint bits using "integer
sequence encoding". This can pack adjacent values together, allowing us to
use fractional numbers of bits per value.
Finally, a block may be just a single color. This is a so-called "void
extent block" and has a special coding which also allows it to identify
nearby regions of single color. This may be used to short-circuit fetching of
what would be identical blocks, and further reduce memory bandwidth.
C.2.5 LDR and HDR Modes
-------------------------
The decoding process for LDR content can be simplified if it is known in
advance that sRGB output is required. This selection is therefore included
as part of the global configuration.
The two modes differ in various ways.
-----------------------------------------------------------------------------
Operation LDR Mode HDR Mode
-----------------------------------------------------------------------------
Returned value Vector of FP16 values, Vector of FP16 values
or Vector of UNORM8 values.
sRGB compatible Yes No
LDR endpoint 16 bits, or 16 bits
7
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