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Update README.md
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richgel999 authored Jan 21, 2025
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Expand Up @@ -165,7 +165,7 @@ Note the .EXR reader we're using is [TinyEXR's](https://github.com/syoyo/tinyexr

Alternatively, LDR images (such as .PNG) can be compressed to an HDR format by specifying `-hdr`, `-hdr_6x6`, or `-hdr_6x6i`. By default LDR images, when compressed to an HDR format, are first upconverted to HDR by converting them from sRGB to linear light and scaled to 100 [nits](https://en.wikipedia.org/wiki/Candela_per_square_metre) (candelas per square meter). The sRGB conversion step can be disabled by specifying `-hdr_ldr_no_srgb_to_linear`, and the normalized RGB linear light to nit multiplier can be changed by specifying `-hdr_ldr_upconversion_nit_multiplier X`.

Note: If you're compressing LDR/SDR image files to an HDR format, the codec's default behavior is to convert the 8-bit image data to linear light (by undoing the sRGB transfer function). It then multiplies the linear light RGB values by the LDR->HDR upconversion multiplier, which is in [nits (candela per sq. meter)](https://en.wikipedia.org/wiki/Candela_per_square_metre). In previous versions of the codec, this multiplier was effectively 1 nit, but it now defaults to 100 nits in all modes. (The typical luminance of LDR monitors is 80-100 nits.) To change this, use the "-hdr_ldr_upconversion_nit_multiplier X" command line option. (This is done because the HDR 6x6 codecs function internally in the [ICtCp colorspace](https://en.wikipedia.org/wiki/ICtCp). LDR/SDR images must be upconverted to linear light HDR images scaled to a proper max. luminance based off how the image data will be displayed on actual SDR/HDR monitors.)
Note: If you're compressing LDR/SDR image files to an HDR format, the codec's default behavior is to convert the 8-bit image data to linear light (by undoing the sRGB transfer function). It then multiplies the linear light RGB values by the LDR->HDR upconversion multiplier, which is in [nits (candela per sq. meter)](https://en.wikipedia.org/wiki/Candela_per_square_metre). In previous versions of the codec, this multiplier was effectively 1 nit, but it now defaults to 100 nits in all modes. (The typical luminance of LDR monitors is 80-100 nits.) To change this, use the "-hdr_ldr_upconversion_nit_multiplier X" command line option. (This is done because the HDR 6x6 codecs function internally in the [ICtCp HDR colorspace](https://en.wikipedia.org/wiki/ICtCp). LDR/SDR images must be upconverted to linear light HDR images scaled to a proper max. luminance based off how the image data will be displayed on actual SDR/HDR monitors.)

### Some Useful Command Line Options

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