Thanks for the relevant doco link.

So it sounds like I was deluded in thinking I was altering R, G, B 
levels independently with the histogram.

If I get time I might have a look at contributing some code. (I have 
also been wanting to add a loupe to lighttable.)

Russell

On 2013-02-04 08:27, johannes hanika wrote:

> On Mon, Feb 4, 2013 at 9:57 AM, Russell Edwards <[email protected]> 
> wrote:
>> Hi,
>>
>> I have some digital photos of faded old colour prints. The R, G, B
>> histograms are squashed and shifted with respect to eachother due to 
>> the
>> different fading characteristics of the different pigments in the 
>> paper.
>>
>> It seems to me the best way to fix what I am seeing would be if I 
>> could
>> apply the exposure or levels module to the R, G, B modules
>> independently.
>>
>> What's the closest feature in darktable to achieve such a thing?
>>
>> I can kind of do it dragging the histogram but that is extremely 
>> slow
>> and jerky, and I can't find which module actually implements that
>> processing, nor documentation of it, so am hesitant to use it.
>
> http://www.darktable.org/usermanual/ch03s04s05.html.php#idp619488
>
>> I can partly do what I need by playing with channel mixer or color
>> correction, but it is not flexible enough to really do what I need 
>> to
>> do, which to first order is move the left end of the red histogram 
>> to
>> the left and the right end of the red histogram to the right.
>>
>> Thanks in advance for any help you can offer.
>>
>> Russell
>>
>>
>> 
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