Putting the Red Back in Underwater Photos

(Last updated March 8, 2014)

This tutorial will use GIMP to colour correct under water pictures by restoring the reds that are absorbed by the water in underwater pictures. The idea behind this technique is to create a red tinted version of the original image and overlay it with the original. And then adjusting the transparency of the red tinted layer to achieve the desired result.

All the credit for this tutorial goes to tankedup-imaging.com. I've mostly just updated the screenshots and UI differences for GIMP 2.8. However, I've added an additional step at the end to adjust the levels of the image. I've found that images taken underwater can sometimes be a bit faded and that adjusting the levels can fix the image contrast.

Some of my results by using this technique. Higher res images and the blog post here.


Overview




1. Create 3 Duplicate Layers of the Original Image


The Layers dialogue should already be open by default. If it isn't, open it from Windows > Dockable Dialogs > Layers (or Ctrl+L).

Right click the layer of your image and select Duplicate Layer (or use the shortcut Shift+Ctrl+D). Repeat this 2 more times so that you have a total of 4 layers.

       


[Layers] The GIMP documentation describes layer modes as a mechanism that permits complex color changes in the image. They are often used with a new layer which acts as a kind of mask. For example, if you put a solid white layer over an image and set the layer mode of the new layer to "Saturation", the underlying visible layers will appear in shades of gray. My translation of this is: Layers are different images stacked one on top of the other, and allows these images to be changed both independently as well as interact with one another.

2. Fill the Second Layer with Grey (808080)


Click the Foreground colour in the Toolbox dialogue and enter 808080 (grey) in the HTML notation box.


Select the second layer (i.e. "<image> copy #1.jpg"). In the Edit menu, select Fill with FG Color (or use shortcut Ctrl+,). The image in the main window may not change, but you can see that it took effect in the Layers dialogue.

     
 
Back to Overview

3. Change Top Layer Mode to Value and Merge Down


Select the top layer (i.e. "<image> copy #2.jpg") and change the Mode from Normal to Value. The image will become greyscale.



Right click the top layer (i.e. "<image> copy #2.jpg") and select Merge Down. You should end up with 3 layers, with the top one being a greyscale version of the original.



[Layer Mode: Value] The GIMP documentation says the Layer Value mode in particular "uses the value of the upper layer and the saturation and hue of the lower layer to form the resulting image. You can use this mode to reveal details in dark and light areas of an image without changing the saturation." As a beginner to image manipulation, this was all mumbo jumbo to me.

Fortunately, I found another site that translates this into terms a bit easier to comprehend. It describes Value mode as where "the color and saturation of the layer below are kept. The colors take on the lightness or darkness of the selected layer. Here we just see the highlights and shadows of the top layer." [pare and focus]

Back to Overview

4. Duplicate New Layer, Fill with Red (FF0000), and Change Top Layer Mode to Multiply


Right click the top layer (should now be "<image> copy #1.jpg") and select Duplicate Layer (or use the shortcut Shift+Ctrl+D) to create a new greyscale layer.



Click the Background colour in the Toolbox dialogue and enter FF0000 (red) in the HTML notation box.



Select the top layer (should now be "<image> copy #2.jpg"). In the Edit menu, select Fill with BG Color (or use shortcut Ctrl+.). The image should now be solid red.



With the top layer (i.e. "<image> copy #2.jpg") still selected, change the the Mode from Normal to Multiply.




[Layer Mode: Multiply] The multiply mode "multiplies the colors in the selected layer with colors in the layer below it. The result will be darker than either the selected layer or the layer below." [pare and focus]

Back to Overview

5. Merge Down, Change Top Layer Mode to Screen, and Merge Down again


Right click the top layer (i.e. "<image> copy #2.jpg") and select Merge Down. You should end up with 3 layers, with the top one being a solid red.



With the top layer (i.e. "<image> copy #1.jpg") still selected, change the Mode from Normal to Screen. You should now be able to see that the reds have been restored to the image (although perhaps a bit more than you would like - we will correct this in the next step).



Right click the top layer (i.e. "<image> copy #2.jpg") and select Merge Down. You should end up with just 2 layers.




[Layer Mode: Screen] The screen mode lightens an image without "burning out" any parts of the image. "Burned out" parts of an image appear completely white, and is sometimes what happens with the divide layer mode. [pare and focus]

Back to Overview

6. Adjust Opacity of Tinted Layer


The redness of the image can then be changed by adjusting the Opacity of the layer. Simply drag the slider in the Opacity bar until you get the desired redness restored to the image.


Back to Overview

7. Auto Adjust the Image Levels


The image may look a little faded after the red has been restored. To fix this, with the top layer still selected open the Levels dialogue by going to Colors > Levels....



In the Levels dialogue, simply click on the Auto button to let GIMP fix the image contrast for you. You can uncheck/check the Preview checkbox to see the before/after effect on the image.



You can also try playing with the other sliders in the Levels dialogue to tweak your image some more if you're still not getting the desired result. Perhaps in the future I will look into how to use this dialogue more "professionally" :).

Back to Overview

8. Save/Export the Image


When you're done with your adjustments, you can save your image in GIMP's native format (with .xcf extension) by going to File > Save As.... This will preserve the 2 layers that you currently have, just in case you want to go back later to make any more adjustments.

To save it in a more standard format, you can export the image by going to File > Export As.... This allows you to save the image as a JPEG (or other common image format) to post on the web or share with others.
And that's it!  8 simple steps to fix your blue/green filled underwater photos!  If you have any questions or have any issues with this tutorial, post a comment below and I will try to help.

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