[Tig] sRGB vs. Rec. 709

Bob Friesenhahn bfriesen at simple.dallas.tx.us
Wed Nov 10 01:44:18 GMT 2010


On Tue, 9 Nov 2010, Rob Lingelbach wrote:
>
>> Hi Rob,
>> From TOH they share the same primaries, the difference sits in The définition of gamma curve.
>
> yes, thanks Cédric.  Whether or not the gamma curves for these 
> standards can be defined as one number is questionable.  The most 
> common number I hear for 709 is 2.35, and for sRGB is 2.2 .

Gamma uses a similar 2.2 curve for both (and primaries are the same) 
except that sRGB uses a linear segment below the linear intensity 
0.00313 (0.04045 luma) and the upper regions are equivalent to a 
somewhat higher gamma.  The most significant difference is surely that 
Rec. 709 is defined using limited video numeric ranges (originally for 
interfacing purposes) while sRGB uses the full numeric range as is 
typical for computer images.

The Wikipedia page (http://en.wikipedia.org/wiki/SRGB) points out that 
the sRGB gamma cannot be expressed as a single numerical value.  The 
sRGB definition is based on measurements made on many anchient 
computer CRT monitors to see how they actually behave and is not based 
on theory.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/


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