Creating trichrome images in Gimp 2.8 ?

[pdeeh]

Hello -- I shoot both film and digital and also recently joined APUG. I'm hoping there might be the guidance and expertise I need here ...

I've recently been interested to create trichrome images from three separate monochrome images shot using red, green and blue filters.

Having obtained the filters, taken the photographs and scanned the negs, I'm now struggling to create trichromes in Gimp 2.8.

I'm not at all competent in using Gimp, as I generally do all I need to in LR, and I don't have access to PS (even if I could, I wouldn't know how to use it)

I found a tutorial (in French) on the web (I shan't post the address as this is my first post here, and doing so will probably trigger the spam filter) - but apart from the shortcoming of Bing or Google translation, it is a few years old & the Gimp has changed somewhat in it's dialogues over the years.

Search as I might, I can't find more explicit information, and wondered if anyone knows of (or could create?) a brief clear tutorial in English that could step a naive Gimp user through the process?

 

[Peltigera]

I would have thought you could open one image and then open the other two images as layers (from the FILE menu) into the same image. You should then be able to use the "RECOMPOSE" option in the COLOUR menu.

 

[pdeeh]

Thanks for replying.

Unfortunately Gimp tells me I can only use the "recompose" function if the image was originally generated using "decompose" ...

I may have made a small amount of progress inasmuch as by loading up the three images as layers, and using greyscale mode, I can then get the "colour/components/compose" option, which gives a promising colour mixing menu ... I worked with a lot of software in quite a technical role for years, yet Gimp and PS still both defeat me utterly

I'll plough on ...

 

[Retro]

This is how to do it in Photoshop (as described by Scott Bilotta in his Yahoo History of Photography Group):

The "goal is to create an image where the red record is copied to the red channel, the green to green and blue to blue. An easy way to do this in Photoshop is with the Merge Channels command. Merge Channels will take 3 separate same-size, flattened grayscale images and create a new RGB image. The command copies each image to one of the RGB channels. The result is a full-color photograph."

 

[pdeeh]

I expect that is very useful information for competent users of Photoshop.

However after posting the same request on a Flickr Trichromy group, someone picked up the baton and wrote a clear explanation of one method of doing this in Gimp 2.8. I have had brief go using that method and it works rather well.

The major obstacle for me is now generating scans that are well-aligned enough.

The method is outlined at Brian Hefele's blog Trichromes in GIMP • PH.BRHFL

 

[Pioneer]

Great stuff here. Thanks for the link.

 

[brh]

Stumbled in here after seeing the trackback link to my article - glad it was helpful, pdeeh!

Couple of things - the 'compose' or 'merge channels' method probably works a lot better if you're doing this on digital for the Harris Shutter effect. I pretty much shoot film exclusively, and when you have to deal with alignment (and realignment, and realignment, and so on) as well as spotting out dust and such, it's a real pain to be constantly decomposing and composing your image into appropriate channels, when you can do it as layers and work on it as though it's a 'normal' image instead.

To pdeeh - I often shoot an additional frame unfiltered just for luminance info. Sometimes I can get the color layers lined up well enough that really all I need is sharp luma. In PS (I don't have GIMP in front of me to see how this would be achieved) you can set a luminance blend mode on the topmost layer - color info is retained from the lower layers, luma from this one. Since our eyes or brains or what-have-you are better at detecting luminance contrast than chromatic, the end result is a much sharper looking photo.

Cheers - brian