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Author Topic: Combining RGB Separations to Create Colour Image  (Read 4589 times)
papa v2.0
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« Reply #20 on: October 30, 2012, 09:02:02 AM »
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... unless the filters through which the B&W images were shot were not pure 255 Red, Green or Blue.  Then the color derived from the B&W pasted into the channels will be incorrect.

The above process gives you the flexibility to prescribe the color of the filter through which color information will be derived from each B&W so you do not introduce problems into the composite that you have to undo with further adjustments.  It also provides some insight into how the filtered B&W images are combined to get a full-color comp. 

The above process is not complex, but go with what works for you.  As with most things in Photoshop, there are several ways to accomplish the final result.  This topic has been discussed a few times here, but I figured I'd share my experience with the process because you asked.

Have fun!

kirk






simply change the XYZ values of the RGB channels in CUSTOM RGB set up in photoshop and save as an icc profile.

if anyone knows the of the filter set likely to have been used at the time the values my be available.
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RFPhotography
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« Reply #21 on: October 30, 2012, 10:09:48 AM »
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Unless I'm mistaken, PS only allows changing of X and Y values.  But where would one find those values for a given photographic filter?
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Gulag
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« Reply #22 on: October 30, 2012, 12:16:58 PM »
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Unless I'm mistaken, PS only allows changing of X and Y values.  But where would one find those values for a given photographic filter?

From the RGB drop down box select Custom at the top, and you will be presented with the platter as mentioned above.
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“For art to be art it has to cure.”  - Alejandro Jodorowsky
RFPhotography
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« Reply #23 on: October 30, 2012, 02:24:49 PM »
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Yes, I know that.  And as I said, unless I'm mistaken, you're only offered the X and Y values to input.  The image below illustrates.  That still doesn't deal with the issue of where these values are to be obtained for a particular filter.

I'm not even certain how, or convinced that, doing this would make a difference.  I'm not seeing how this would change the effective 'colour' of the red, green and blue filters (channels) that are used to recombine the separations.  Within different colour spaces you have differing RGB primaries.  But in all colour spaces, you still have full saturation 255 for each of the channels.  That's what needs to be changed if the filters are not exactly full red, green and blue.  I don't see how changing the primaries; effectively changing the boundaries of the space, alters the full saturation value of 255.  I understand that Red at 255 in sRGB will be different from Red at 255 in ProPhoto but it's still 255, full red.  And it's not necessarily the saturation that needs to be changed but perhaps the hue, as was pointed out earlier and why, I think it was Kirk, suggested using colour fill layers so that the hue of the filter could be changed if and as needed.  If just the saturation level of the filter (channel) needs to be changed it would seem that would be more easily done via Kirk's method than by creating a new colour space.
« Last Edit: October 30, 2012, 05:15:49 PM by BobFisher » Logged
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