Unity3D - 关于ASTC压缩格式的一些资料搜集

From Qualcomm_Adreno_OpenGL_ES_Developer_Guide：

QTI recommends the following strategy for selecting a texture compression format:
1. Use ASTC compression if it is available. 如果可用使用ASTC
2. Otherwise, use ETC2 compression if available. 否则, 如果可用则使用ETC2
3. Otherwise, select the compression format as follows:否则
  a1. ATC if using alpha
  b1. ETC if not using alpha
ASTC is a newer format and may not be supported or optimized on all content reation pipelines. Also the sRGB formats for ASTC are more efficiently handled on Adreno hardware than the RGBA formats.
ASTC是一种较新的格式，可能不支持或优化所有内容reation管道。此外，ASTC的sRGB格式在Adreno硬件上的处理效率高于RGBA格式。

From ARM® Guide for Unity Developers Optimizing Mobile Gaming Graphics：

ASTC texture compression is an official extension to the OpenGL and OpenGL ES graphics APIs.
ASTC纹理压缩是OpenGL和OpenGL ES图形API的官方扩展。
ASTC can reduce the memory required by your application and reduce the memory bandwidth required by the GPU.
ASTC可以减少应用程序所需的内存，并减少GPU所需的内存带宽。
ASTC offers texture compression with high quality, low bitrate and has many control options. It includes the following features:
ASTC提供高质量，低比特率的纹理压缩，并具有许多控制选项。 它包括以下功能：
• Bit rates range from 8 bits per pixel (bpp) to less than 1bpp. This enables you to fine-tune the tradeoff of file size against quality.
• Support for 1 to 4 color channels.
• Support for both low dynamic range (LDR) and high dynamic range (HDR) images.
• Support for 2D and 3D images.
• Support for selecting different combinations of features.
Selecting specific settings for all of your textures improves the visual quality of your project and avoids unnecessary texture data at compression time.

The following table shows the compression ratio for the available ASTC block sizes in Unity for an RGBA 8 bits per channel texture with a 1024x1024 pixel resolution at 4 MB in size.
选择所有纹理的特定设置可提高项目的视觉质量，并避免在压缩时不必要的纹理数据。
下表显示Unity中可用ASTC块大小的压缩比，每个通道纹理的RGBA 8位，大小为4 MB的1024x1024像素分辨率。

Table 4-1 Compression ratios for the ASTC block sizes available in Unity

ASTC block size	Size	Compression ratio
4x4	1 MB	4.00
5x5	655 KB	6.25
6x6	455 KB	9.00
8x8	256 KB	16.00
10x10	164 KB	24.97
12x12	144 KB	35.93