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Raw & Post Processing, Printing => Digital Image Processing => Topic started by: RFPhotography on October 09, 2012, 11:52:25 am

Title: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 09, 2012, 11:52:25 am
I'm trying to play around with RGB separations to create a colour image.  Since I don't have any true 'Prokudin-Gorsky' type negatives I'm taking an RGB image and separating each channel.  This website (http://www.loc.gov/exhibits/empire/making.html) has a bit of information on the process but I'm obviously missing something.

Starting with my RGB image, I've highlighted each channel and copied it into a new greyscale document.  Then I've created a new greyscale document, as the link above outlines, and pasted each of the separations in as layers with red on the top, green in the middle and blue on the bottom.  What I don't know is what comes next.  How does this get converted into an RGB image?

Thanks.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: Stephen G on October 09, 2012, 01:11:20 pm
Once you've aligned the grayscale Red, Green and Blue layers (which you don't need to do, of course, in your example) then create a new RGB document and copy each of the grayscale layers over to the channels of the new RGB document.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 09, 2012, 01:25:28 pm
How funny, I did this exercise a while back after seeing this exhibit mentioned on the web.

1) Use the composite 3 panel B&W image, extract each panel of the three-image composite and paste each image onto its own layer in a RGB document.

2) make sure you figure out which panel corresponds to which color channel (RGB).

3) Select all three layers and use Edit > Auto-align layers... to register them.

4) Apply a gradient adjustment layer above each of the three B&W panels, and clip the gradient adjustment to the B&W under it.

5) For the appropriate channel, make the gradient adjustment layer's gradient go from black to pure color (for example, 0,0,0 to 255,0,0 for the red gradient adjustment layer).  If you get funky results, reverse the gradient (check the "reverse" tick box).  You can apply a different "filter" color by adjusting the RGB value of the end of the gradient.

6) Working form the bottom to the top layer, the base (bottom) layer blend mode NORMAL, 100% opacity.  Middle = NORMAL, 50%.  Top = NORMAL, 33%.
6a) Put a curves adjustment above everything to balance luminosity.

OR

7) Set the blend modes of the middle and top layers to LINEAR DODGE (ADD) and opacity 100.  No curves adjustment for luminosity necessary.

Have fun!

Kirk

Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 09, 2012, 01:32:34 pm
Here's what I came up with when I did this exercise previously, after cloning out color plate defects, etc. and balancing color and tone.

(http://kirkt.smugmug.com/Photography/Photo-of-the-Day/Composite-FINAL-copy/641220172_3vrA6-X3.jpg)

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 09, 2012, 01:39:06 pm
Here is a link to hi-res scans of the original B&W image composite:

http://www.loc.gov/pictures/resource/prok.01887/

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: jeremypayne on October 09, 2012, 02:11:22 pm
Here's an older discussion of the same topic ...

http://www.luminous-landscape.com/forum/index.php?topic=37705.0
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 09, 2012, 02:32:09 pm
Ah ha!  Pasting back into the respective channels in the new document makes much more sense. 
Title: Re: Combining RGB Separations to Create Colour Image
Post by: Kumar on October 10, 2012, 04:54:10 am
There's a long discussion here (http://www.largeformatphotography.info/forum/showthread.php?48583-Color-photography-with-black-and-white-film)

Kumar
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 10, 2012, 09:50:39 am
You can also replace the gradient adjustment layers in the above layer stack with Photo Filters, uncheck the "Preserve Luminosity" box, and crank the filter up to 100% density.  This gives you a little more of an intuitive approach in changing the RGB value of the filter through which the B&W is being processed and seems to give better color reproduction.

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 10, 2012, 09:53:01 am
Pasting the respective separations back into the designated channel in an RGB document works just fine.  It's easy and simple.  No need to make the process unnecessarily complex.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 10, 2012, 10:41:56 am
... 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

Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 10, 2012, 11:26:16 am
Given what I'm experimenting with, as outlined in the original post, the possibility of a different coloured 'filter' is not possible.

That aside, I looked at your method and can't see how it could possibly work.  I tried it, in fact, and it doesn't seem to.  I wonder if you've written your methodology the way some old grandmothers write recipes and have left out an ingredient or two.  ;D

What I end up with is either a muddy cyan colour obscuring the actual image if the grad layers are left at 100% opacity or a muddy cyan cast over the image if the grad layers are changed to the 66/33 opacity levels as the green and red image layers.  But the image itself is not turned into a colour image. 

Are the grad layers from 100% opacity to 0% (black to colour) or 100% to 100% (black to colour).  Again, I've tried both and neither works.

Why a grad as opposed to a simple colour fill layer?

The Photo Filters makes more sense.  The grad doesn't make a lot of sense to me.  The Photo Filter actually does work somewhat.  A lot of additional work needs to be done to get a proper colour balance and luminance level (H/S, Curves, Levels, Color Balance).  But better than the grad which doesn't work at all.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 10, 2012, 12:17:37 pm
Hopefully this will give you a better recipe than "season to taste!"  ;)

First attachment is the three source panels from the single negative linked above in a previous post (Library of Congress).  The assumption is that red, green and blue filters were used for the separation, so, if we make some assumptions about the subject, we can infer which plate is which.  If we assume a caucasian-like skin tone, then blue will give us a dark tonal value for skin, green a mid tone and red a light tone.  This helps as a first pass guess for the recombination of the separate plates in the negative

Second attachment is a stack using Photo Filters.  The key to the adjustment layer approach is to make sure that each adjustment layer is CLIPPED to the B&W image below it so that the adjustment (gradient, photo filter, etc.) will only affect the layer directly below it and not the whole stack.  For anyone using PS that is not aware of how to clip the layer, make sure the adjustment layer is directly above the layer you want to operate on, hold down the Option (ALT on PC) key and hoever over the border between the two layers in the layers panel - the cursor will change to an "L" with a downward pointing arrow - click on the border and the adjustment layer will become indented in the stack, with the arrow to its left indicating that the layer is clipped to the one below it.

Photo filter settings: 100% density (or season to taste) and uncheck "preserve luminosity".  Filter layer blend mode: normal, 100% opacity.

Layer blend modes: bottom layer: normal, 100% opacity - middle and top layers: linear dodge (add), 100% opacity.

Give this a shot and see if it works.  I understand this is more complicated than necessary for the approach where you extract the channels from a color image using the B&W representation in the PS Channels panel.  In that case, all plates are registered and the channel color is exactly known.  However, if you want to composite Prokudin-Gorsky' type negatives, you may want a little more control over the compositing process.

Hope this helps!

kirk



Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 10, 2012, 12:25:21 pm
What I end up with is either a muddy cyan colour obscuring the actual image if the grad layers are left at 100% opacity or a muddy cyan cast over the image if the grad layers are changed to the 66/33 opacity levels as the green and red image layers.  But the image itself is not turned into a colour image.  

Are the grad layers from 100% opacity to 0% (black to colour) or 100% to 100% (black to colour).  Again, I've tried both and neither works.

Why a grad as opposed to a simple colour fill layer?

The opacity of the gradient layers should be 100%, normal blend mode.  The B&W image layers (in normal blend mode only) have the different opacity assigned (bottom=100, middle=50, top=33).  If you use linear dodge on the plate layers, all are 100% opacity.  Again, make sure you clip each gradient layer to the associated B&W plate.

The gradient approach is just a different method of mapping color to the tones in the B&W plates.

I agree, the photo filter method is better.

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 10, 2012, 01:26:32 pm
Yeah, I understand the opacity of the grad layers (or photo filter).  It was a question of the opacity of the grad itself.  When you create a grad you have colour stops and opacity stops.  You're setting the colour stops to black and red, green or blue.  It's the opacity at each end of the scale that's the question.  Standard is 100 to 0.  But that wouldn't really seem to have much of an effect.  It would depend on whether black was at the 100 opacity end or the colour was at the 100 opacity end.

The Photo Filter method works much better and when used with the Linear Dodge blend mode on the top two layers is pretty good.  The only thing I had to do after that was apply an H/S adjustment layer.  With the Normal blend mode and lowered layer opacity, much more work had to be done with additional adjustment layers - again, simpler is better. 

Still don't get the grad idea and can't get it to work.  The Photo Filter is a nice twist though.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 10, 2012, 03:17:11 pm
Both the begin and end stop of each gradient is 100% opacity.  Here is a capture of the blue B&W with the blue gradient adjustment layer, pictured in the screenshot, affecting the B&W plate - similar to what you would see in the blue channel in an RGB image, if you enable "Show channels in color" in Photoshop preferences.  You could add intermediate stops and change the uniformity of the gradient, increasing contrast in that color channel if you so desire.

The adjustment layer method just makes things non-destructive - if, like with the Prokudin-Gorsky images, I don't know exactly what they are supposed to look like in the final comp, I like being able to configure a few different plate mappings (filters, gradients, etc.) and toggle among them to see the effect of per-channel adjustments.

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: Bart_van_der_Wolf on October 11, 2012, 04:56:35 am
Both the begin and end stop of each gradient is 100% opacity.  Here is a capture of the blue B&W with the blue gradient adjustment layer, pictured in the screenshot, affecting the B&W plate - similar to what you would see in the blue channel in an RGB image, if you enable "Show channels in color" in Photoshop preferences.  You could add intermediate stops and change the uniformity of the gradient, increasing contrast in that color channel if you so desire.

Hi Kirk,

A small remark, if I may. Gradients with 'Smoothness' set to 100% will exhibit a bit of an s-curve to them. For a truely linear gradient, which may be partcularly useful in this case for increased control, Smoothness is best set to 0%.

It's easy to check that behavior when a 256 pixel horizontal gradient is made, going from black to white. Then the horizontal pixel position should correspond with the pixel value.

Cheers,
Bart
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 11, 2012, 09:00:05 am
Makes sense about the "S" curve nature of a smooth gradient - the gradient techniques tends to produce more punchy color, so this would explain it.  Thanks Bart!

Attached are two renderings, one with smoothness of all three gradients set to 100 and one with all three gradients set to 0.  The 100 smooth gradients image has more color contrast, as Bart's comment would predict.

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 11, 2012, 11:32:00 am
Here's one using the Photo Filter adjustment layers.  The unaltered RAW file is included for comparison.  The combined version has a bit of a red cast but that's easily corrected.  Overall not bad.  Switching the blue and red filters to Deep Blue and Deep Red gives it a more neutral balance.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: kirkt on October 11, 2012, 06:40:26 pm
Yeah - if you click on the color swatch for "blue" or "red" filters, etc. the RGB values are not pure 0,0,255 or 255,0,0.  Deep red is 242,0,0, for example.  You can specify a custom color if you want, and make it 255,0,0 for pure red, for example.  The way the black and white channel image is rendered will also affect the final composite as well (dot gain, LStar, etc.).

fun, eh?!

kirk
Title: Re: Combining RGB Separations to Create Colour Image
Post by: papa v2.0 on October 30, 2012, 10:02:02 am
... 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.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 30, 2012, 11:09:48 am
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?
Title: Re: Combining RGB Separations to Create Colour Image
Post by: Gulag on October 30, 2012, 01:16:58 pm
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.
Title: Re: Combining RGB Separations to Create Colour Image
Post by: RFPhotography on October 30, 2012, 03:24:49 pm
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.