Negative to positive

  • Thread starter Thread starter dps
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dps

I know most people would tell me to google for this, and believe me, I
did, but still it is not so clear to me. So here it comes: which formula
in which colorspace would you use to reverse an image from negative to
positive so as to reflect an efficient reversal for further color
post-processing?

thanks a lot!!!
 
I know most people would tell me to google for this, and believe me, I
did, but still it is not so clear to me. So here it comes: which formula
in which colorspace would you use to reverse an image from negative to
positive so as to reflect an efficient reversal for further color
post-processing?
Click to expand...

With a linear gamma, replace <r,g,b> with <1-r,1-g,1-b>, then subtract
the base color for the negative and finally normalize the white point.

I think you should profile the result of this operation, if you
want to use profiles.
 
Hello Phillip,

thanks for the reply. This will get you to a positive that is rather
"faded". The formula that should be used may be logarithmic rather than
linear. What I want to do is this: I have scanned my image with linear
gamma and subtracted the film base color. I now want to invert the image
but simply using the ~rgb's gives me a color fading. It is this fading
that I want to compensate for. If I scan as negative (and get a positive
image in my scanning sw) there i no such fading, though I have not
selected any film type or profile etc, even for the *simple* Minolta sw.
How do they do the inversion?
 
thanks for the reply. This will get you to a positive that is rather
"faded". The formula that should be used may be logarithmic rather than
linear. What I want to do is this: I have scanned my image with linear
gamma and subtracted the film base color. I now want to invert the image
but simply using the ~rgb's gives me a color fading. It is this fading
that I want to compensate for. If I scan as negative (and get a positive
image in my scanning sw) there i no such fading, though I have not
selected any film type or profile etc, even for the *simple* Minolta sw.
How do they do the inversion?
Click to expand...

What do you mean with 'color fading'? Low contrast or low saturation?
Low contrast can be fixed with gamma.

I don't know if I actually did the conversion in linear space. I did it
in Photoshop a couple of times (swap 0 and 255 in levels, exit
levels, re-enter levels, move black and white points for red, green
and blue individually. And change the gammas a bit to get the grey
point and the overall contrast right.)
 
Philip said:
With a linear gamma, replace <r,g,b> with <1-r,1-g,1-b>, then subtract
the base color for the negative and finally normalize the white point.

I think you should profile the result of this operation, if you
want to use profiles.
Click to expand...

I use such (or similar) workflow with negatives and it provides
me with by far best results:

1) Scan IT8 target on negative material -> 48bit positive raw file tiff
2) Generate *.icc profile from this raw file. I use Vuescan for this.
3) Scan your images with neutral gamma to positive 48bit raw files.
4) Scan raw files using your *.icc profile to generate images for
print or web posting.

Thomas
 
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