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Improve README clarity for XUASTC LDR options

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Updated README to enhance clarity and emphasize specific options for XUASTC LDR compression.
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Rich Geldreich
2026-01-24 20:57:55 -05:00
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@@ -242,19 +242,24 @@ Compressing and Unpacking .KTX2/.basis Files
`basisu -xuastc_ldr_6x6 -quality 75 -effort 4 x.png`
An alias for `-xuastc_ldr_6x6` is `-ldr_6x6i` (where 'i'="intermediate"). All [14 standard ASTC block sizes](https://developer.nvidia.com/astc-texture-compression-for-game-assets) are supported, from 4x4-12x12: 4x4, 5x4, 5x5, 6x5, 6x6, 8x5, 8x6, 10x5, 10x6, 8x8, 10x8, 10x10, 12x10 and 12x12. The XUASTC LDR to BC7 transcoder has special optimizations for several common block sizes: 4x4, 6x6 and 8x6. When transcoding XUASTC LDR at these particular block sizes, most XUASTC blocks are *directly* transcoded to BC7, skipping the real-time analytical bc7f encoding step.
An alias for `-xuastc_ldr_6x6` is `-ldr_6x6i` (where 'i'="intermediate"). All [14 standard ASTC block sizes](https://developer.nvidia.com/astc-texture-compression-for-game-assets) are supported, from 4x4-12x12: 4x4, 5x4, 5x5, 6x5, 6x6, 8x5, 8x6, 10x5, 10x6, 8x8, 10x8, 10x10, 12x10 and 12x12. The **XUASTC LDR to BC7 transcoder has special optimizations for several common block sizes: 4x4, 6x6 and 8x6**. When transcoding XUASTC LDR at these particular block sizes, most XUASTC blocks are *directly* transcoded to BC7, skipping the real-time analytical bc7f encoding step.
More ASTC and XUASTC LDR specific options:
More XUASTC LDR specific options (most of these also apply to standard ASTC):
- The options `-xuastc_arith`, `-xuastc_zstd` (the default), and `-xuastc_hybrid` control the XUASTC LDR profile used. The arithmetic profile trades off transcoding throughput for 5-18% better compression vs. the Zstd profile, and the hybrid profile is a balance between the two.
- The options `-xuastc_arith`, `-xuastc_zstd` (the default), and `-xuastc_hybrid` control the **XUASTC LDR profile used**. The arithmetic profile trades off transcoding throughput for 5-18% better compression vs. the Zstd profile, and the hybrid profile is a balance between the two.
- `-ts` or `-srgb` enables the sRGB profile (the default), and `-tl` or `-linear` enables the linear profile.
- `-ts` or `-srgb` enables the **sRGB profile (the default)**, and `-tl` or `-linear` **enables the linear profile**.
- `-weights X Y Z W` sets the unsigned channel error weights, used to favor certain channels during compression.
- `-weights X Y Z W` sets the unsigned integer **channel error weights**, used to favor certain channels during compression.
- Another set of XUASTC specific options overrides the windowed RDO behavior (windowed RDO is a separate and optional perceptual optimization vs. Weight Grid DCT): `-xy` enables and `-xyd` disables windowed RDO. By default, if Weight Grid DCT is not enabled (i.e. `-quality` isn't specified, or its set to 100), windowed RDO is disabled. Windowed RDO is automatically enabled if the quality level is less than 100, unless `-xyd` is specified. Also see the tool's help text for additional windowed RDO options: `-ls_min_psnr`, `-ls_min_alpha_psnr`, '-ls_thresh_psnr`, '-ls_thresh_alpha_psnr`, etc.
- Another set of XUASTC specific options overrides the **windowed RDO behavior** (windowed or bounded RDO is a separate and optional perceptual optimization vs. Weight Grid DCT): `-xy` enables and `-xyd` disables windowed RDO. By default, if Weight Grid DCT is not enabled (i.e. `-quality` isn't specified, or is set to 100), windowed RDO is disabled. Windowed RDO is automatically enabled if the quality level is less than 100, unless `-xyd` is specified. Also see the tool's help text for additional windowed RDO options: `-ls_min_psnr`, `-ls_min_alpha_psnr`, '-ls_thresh_psnr`, '-ls_thresh_alpha_psnr`, etc.
- `-xs` disables 2-3 subset usage and `-xp` disables dual plane usage (slightly higher compression and faster transcoding to BC7).
- `-higher_quality_transcoding`: Permits slower but higher quality transcoding
- `-no_deblocking`: Disables all adaptive deblocking on larger ASTC block sizes (faster)
- `-force_deblocking`: Always use adaptive deblocking filter, even for block sizes <= 8x6 (slower)
- `-stronger_deblocking`: Use stronger deblocking when it's enabled (same performance)
- `-fast_xuastc_ldr_bc7_transcoding` and `-no_fast_xuastc_ldr_bc7_transcoding`: Controls faster direct XUASTC->BC7 transcoding (defaults to enabled, which is slightly lower quality)
- To compress an LDR sRGB image to a standard ASTC LDR 6x6 .KTX2 file, using effort level 4 (valid effort levels 0-10):