Basic LRGB workflow part four - non-linear combination to create LRGB - Barry Wilson

Basic LRGB workflow part four - non-linear combination to create the LRGB image

Creating a mask for RGB / Luminance combination

I have experimented with a variety of methods of combing the Luminance image with the RGB image: each have their benefits.  However I am currently favouring this method as it gives a great deal of control on how much Luminance is added at each combination.

The basic concept is to protect the RGB image and to combine a portion of the Luminance iteratively such that an optimum balance of the RGB colour is preserved at the same time as the detail from the Luminance is added.

This can be achieved in a number of ways, eg (non-exhaustive list):

1. Creating a clone of the RGB image and using the LRGB Combination process to add the entirety of the Luminance to the RGB image, resulting in two images - an RGB and a LRGB.

2. These two images are then combined as a simple direct proportion using the Pixelmath formula in the Workflow tutorials.

3. As a variation to this, you can iteratively combine ever decreasing proportions: iteration one combine 50% of the Luminance to create LRGB_1; then for iteration two, combine 30% of the Luminance into image LRGB_1 to create image LRGB_2; then for iteration three, combine 15% of the Luminance into LRGB_2 to create image LRGB_3.  And so on.

4. Rather than use the LRGB Combination process use instead the CieLab mode of the Channel Combination process (this uses a different colour space to achieve the same combination goal and works better at preserving colur in my opinion).  Deselect "a" and 'b" and select the Luminance image in "L".

5. Repeat the steps above.

6. Use the method I describe below.

The mask is created by applying a PixelMath value for each pixel the new mask window to the Luminance image window (it could be any window and doesn't need to be the Lumimance window, as long as it has the same dimensions it will work).

Each pixel has a value between 1 (white) to 0 (black).  In this example I am choosing to assign a pixel value of "0.15" to each pixel in the window. Apply the Instance to the Luminance window ensuring you have selected "Create new image" and given the "Image Id" some meaningful name.

If you accidentally forget to select "Create new image" before you apply the PixelMath process to the Luminance window, you will see the Luminance window turn almost black as you have transformed every pixel value to 0.15.  Don't panic, simply right click on the Luminance window and selecet "Undo".

The Mask015

Add Mask015 to RGB image

There are a couple of ways to add a mask to protect an image.  I generally right click on the image window and select the context menu option.

Select the mask

Next, we add the Luminance using the CieLab colour space from the Channel Combination process.

Open the process, select "CieLab", deselect "a" and "b" and then select the luminance image.

Luminance addition in CieLab mode

You can see a mask is applied becuase the image tab has changed to show a coloured background.

Apply the triangular instance to the RGB image.  Repeat until you have achieved a result which balances the detail from the luminance and retains the RGB colour.

Luminance addition in CieLab mode to produce LRGB image

Of course if you want even finer control, you can create a mask using 0.1 pixel value or even 0.05.

We now want to recover some of the colour and enhance some of the features of this striking galaxy, such as its dusty spiral arms and dark dust lanes.

Curves boost

Use the Curves Transformation tool to boost the colour using the "C" and "S" curves.

Notice the small adjustment.

Activating the preview window enables you to see the effectbefore you click the square apply icon.

Resultant boosted LRGB image