How to make a digital graduated neutral density filter--using FREE SOFTWARE!
Digital neutral density filters
Graduated neutral density filters balance the light of photos in difficult lighting scenarios, often the sky and a darker foreground.
This tutorial shows how to use GIMP, version 2.6.10, to created a graduated neutral density filter effect, similar to having used an actual filter.
GIMP is free software available for Windows, Mac, and Linux.
I took this photo during a hike on Shadow Mountain, in Phoenix, Arizona. The photo as a whole is dark, but I like the colors of the sky.
Here is the same photo after I brightened it a bit with the curves tool in GIMP.
I like the foreground in this bright photo better than in the original photo, but not the sky. The colors and contrast are gone from the sky in this version.
Bright photo with digital graduated neutral density filter
Here is the bright photo, after I applied a digital neutral density filter. The foreground is bright, but most of the colors of the sky are preserved.
The darkness of the sky and length of the transition to the brighter foreground are not fixed effects, but can be adjusted to taste.
How it works--graduated neutral density filter in GIMP
This tutorial assumes some basic knowledge of GIMP and/or similar photo processing software. I won't skip over any steps, but explaining how layers and layer masks work is beyond the scope of this tutorial. If you have any questions, ask them in the comment section, and I'll be glad to elaborate on exactly what is going on.
First, open your image in GIMP and duplicate the background layer. You can duplicate the layer by choosing Layer -> Duplicate Layer from the menu bar, or by clicking the duplicate layer icon in the layers dialog, which I prefer.
Next, brighten the duplicate layer using the curves tool, the levels tool, or some other method. What I do is choose Colors -> Curves... from the menu bar, then click and drag the middle of the diagonal line up and to the left, normally just a square or two away from center. The curves tool looks like this, while you're working with it:
Adding a Layer Mask
Next, add a white layer mask to the brighter layer. This is done by choosing Layer -> Mask -> Add Layer Mask... from the menu bar. When asked to select the type of layer mask, choose white. Your photo should now look something like this:
Note that the layers dialog shows two layers, a dark background layer covered by a lighter copy of the background layer with an associated white layer mask.
If you don't have this exact set-up, don't worry. Everything can be fixed!
If you have the darker layer on top, for example, you can move it to the bottom with the down arrow in the layers dialog. Please ask other fix-it questions in the comments section.
Creating the digital graduated neutral density filter--the blend tool
Make sure that the white layer mask is selected in the layers dialog by clicking on the icon for the layer mask in the layers dialog.
You will not see the layer mask that you are working on, nor should you. You will see the image. You could see the layer mask by right-clicking the layer mask in the layers dialog and choosing Show Layer mask, but don't do that; you should be looking at the image while you modify the layer mask, in order to draw the right gradient.
Select the blend tool, either by clicking on the blend tool icon in the tools layout on the GIMP main window, or by choosing Tools -> Paint Tools -> Blend from the menu bar.
Make sure that black is selected as the foreground color and white as the background color in the GIMP main window. These should be the default colors.
The blend tool draws a gradient, in this case from black to white, in the direction you choose, and of the length you choose. We want a vertical gradient from black to white on the layer mask, because we want the layer mask to block the light upper part of the lightened background copy, and reveal the light lower part of the background copy.
Imagine a vertical line from top to bottom in the middle of the image. If we click at the top of that line and drag to the bottom of the image, the blend tool will draw a gradient that is black at the top, but which starts to lighten immediately, until it is white at the bottom of the image.
This is an excellent gradient, and makes the final image look nice, but I chose to draw a gradient with a more abrupt transition.
Click on the imaginary vertical line farther down the image, around the top of the mountains, drag a short distance to just below the horizon, and release. That gives black above where you clicked, white below where you released the click, and a short transition zone from black to white between the clicks.
If you were to look at the type of gradient I drew, you might see this:
Explanation of effect
The upper black part of the layer mask makes the sky in the brightened copy of the background layer invisible. Therefore, in the final image, we see the sky in the original image.
The lower white part of the layer mask makes the foreground in the brightened part of the background layer visible, which is what we want.
The transition zone blends the two layers, so that the change from light to dark is not jarring. This is about as short a transition zone as you want, and of course, you may well want something different, based on taste.
If you don't like the effect of the blend tool after you've used it, click Edit -> Undo Blend from the menu bar, and try again.
You need to be look at the image, and not the layer mask, as you draw, because you have to know where to click and drag the blend tool, which you can only do by looking at the image. Also, you need to be able to see the changes the blend tool and layer mask create in the actual image, to decide whether to keep or discard them.
Your final image and layers dialog might look something like this:
If you have any questions, please ask them in the comment section at the bottom of the page. I can update this hub to make it more useful.