Re: [Gimp-user] double exposure

Am 05.11.2012 20:01, schrieb Richard Gitschlag:
PS:  Since linear RGB pixels values are typically gamma-encoded representations of some absolutely
recorded brightness, I suppose technically for a double-exposure you would have to un-encode the
gamma (e.g. apply a Levels adjustment of 0.45 or so), merge the two source layers using Addition
blending, then re-encode (e.g. 2.2 Levels adjustment).  I did some experiments using two layers with
black-to-white gradients rotated 90 degres from each other, and the result of those adjustments was
indeed pretty close to the "Screen" blending mode.  That's pretty interesting.
this similarity is not by accident:

gamma curves can pretty closely resemble logarithms within the given value range and resolution:^0.8%2C+ln%28x%2B1%29%2Fln%282%29+from+x%3D0+to+1

Adding the logarithms of two layers and subsequent un-logarithmising
gives the product of the layers:
ln(a) + ln(b) = ln(a*b)
a*b = exp( ln(a)+ln(b) )

The screen blend mode f(a,b) = 1-(1-a)*(1-b)  can be read as multiplication,
to be performed with inverted input layers and followed by inversion of the result.

best regards,

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