What is the advantage of an RGB working space that uses an L* tone response curve?
Asked 8/1/2014
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Some RGB working spaces, such as LStar-RGB and ProStarRGB, use an L* tone response curve instead of a simple gamma curve. What practical advantage does an L* TRC offer compared with gamma-based spaces like sRGB or Adobe RGB, especially when editing tonal contrast?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
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The intention behind L* was to make for a device independent colour space with a more neutral or 'flat' representation than available with Adobe RGB or sRGB. This is done by replacing the gamma with a set of calculated luminance values for the grey axis.
The L* colour space is said to exhibit less compression or blocking in when you make changes such as applying an S-Curve to increase contrast.
Colour guru Dan Margulis seems keen on L* (certainly over Adobe RGB, sRGB and ProPhoto RGB) writing:-
I believe that everyone agrees that if we wanted something with a gamut that approximates that of sRGB, there would have been better choices than sRGB. And for sure, if we had wanted something to give us approximately what Adobe RGB does, then Adobe RGB is a poor choice. But in spite of the fact that these are clearly undesirable defaults, they have won out.
Source: http://www.ledet.com/margulis/2007HTM/ACT06-Lstar.htm
Originally by user14028. Source · Licensed CC BY-SA 4.0
user14028
12y ago
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An L* tone response curve is intended to make the RGB working space behave more like a perceptually uniform, device-independent space. Instead of a simple gamma curve, it uses luminance-based values along the gray axis, giving a more neutral or “flatter” tonal distribution.
The claimed practical benefit is better handling of tonal edits: because tones are distributed more in line with human perception, edits such as adding contrast with an S-curve may show less compression, blocking, or bunching in the tones than in gamma-based spaces like sRGB or Adobe RGB.
So the main argument for L* is not a bigger gamut, but smoother and more natural tonal editing behavior. Whether that is worthwhile depends on your workflow, but the supposed advantage is improved tone spacing and fewer artifacts during strong tone adjustments.
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