I have prepared-well BC7 tetxure, how to convert it to astc format (e.g., ASTC_12x12) directly? Not decompress BC7 -> compress ASTC, It's too slow.
Is there any algorithm or tool that can convert BC7 directly to standard ASTC format? No high image quality required
There are some transcoding tools from Khronos which compress to a UASTC intermediate format, which can be transcoded efficiently into BC7 or ASTC on the target device once you know what you need. The bitrate choices are somewhat limited - transcoding needs the destination formats to have broadly similar compression ratios - so the most direct conversion would be to a 4x4 encoding which is 8bpp in both formats.
I'm not aware of any tools that do direct conversion from BC7 to ASTC.