Restoring red back into blue/green underwater images
Red enhance method 2
This method is a GIMP version of the mandrake Photoshop method, I managed to find a post by meddler in the discussion forum of Scubaboard, which coverted the Photoshop action to GIMP.
The main difference with this method is that the grey layer has a Mode setting of Colour and the screen layer isn't then merged to another copy of the original.
The images below are a before blue/green image and an after red enhanced image using this method.
Script-Fu
There is a Script-Fu (macro) for this tutorial, file name med.scm on our Script-Fu downloads page, see the Script-Fu help page if you need instructions on loading and activating it. This will work on GIMP versions below 2.4, there is a seperate version for the new GIMP 2.4.
Duplicate original image
On the main GIMP toolbar, open the layers dialogue
File » Dialogues » Layers
In the layers dialogue click Duplicate and create two copies of your image.
Fill copy with FG colour and change mode
Make sure Background copy is selected in the layers dialogue, then double click on the Foreground colour in the main GIMP toolbar and type in the hexadecimal code 808080 (grey) in the HTML Notation option box.
On the image toolbar choose
Edit » Fill with FG Colour
In the layers dialogue change Background copy Mode to Colour.
Fill top layer with BG colour and change mode
Select Background copy#1, double click on the Background colour in the main GIMP toolbar and type in the hexadecimal code FF0000 (red) in the HTML Notation option box.
On the image toolbar choose
Edit » Fill with BG Colour
In the layers dialogue change Background copy#1 Mode to Multiply.
Merge down and change mode
Right click on the top red layer (Background copy#1) and choose Merge Down to the grey layer (Background copy).
Change the red and black layer (Background copy) Mode to Screen.
Adjust opacity and save
At this stage with the red and black screen image you will probably see a very red image. Adjust the Opacity of the red and black layer in the layers dialogue, until the image colours look correct.
If you have any whites in the image, you can use them as a datum to adjust to. Once they look correct the other colours should follow.
When you have achieved the result you want you can right click on any of the layers and choose Flatten Image and then save the image.
If you are not sure or want to work on the image again later, save the image in GIMP's native .xcf format. This saves all the layers and information, except for the undo history. Be warned though, these files are BIG, make sure you have plenty of hard disk space if you are doing this with lots of images.
