How to add the effect of a graduated neutral density filter on a photo using the Gimp graphics editor

A graduated neutral density filter (GND) is a filter used for cameras to darken an area of the shot that is significantly brighter than the rest of the shot. like a picture of a dark forest and the bright sky, or a bright snow field and darker mountain. This howto shows you howto emulate the use of this filter on your digital photo. With a good graphics editor like Gimp you can balance the bright and dark portions of your picture without it being obvious that you modified it.

Limitations
Using graphics editors to add a graduated neutral density filter effect has its limitations. It cannot correct for washed-out (bright) or underexposed black (dark) regions of an image whereas an actual GND filter has the potential to bring the scene into the dynamic range of the camera.

In general digital cameras are better at capturing darker tones rather than lighter tones, so darker regions are more easily corrected, while washed out regions are almost always unrecoverable. Knowing this, it is better underexpose you pictures (make it darker), when you plan on using a graphics editor to add this effect.

Before and after pictures using a Grad ND Filter
Effect of applying a Grad ND Filter Effect

What is a Grad ND filter
A graduated neutral density filter is a filter used for cameras to darken an area of the shot that is significantly brighter than the rest of the shot. like a picture of a dark forest and the bright sky, or a bright snow field and darker mountain.

The filter comes in many types, basically can be separated in to two categories. A hard edge is use when there is an abrupt change in brightness. For example a field with a horizon to a bright sky. A soft edge is a wider smoother change from light to dark. This is used when the light and dark portions are not distinctly separated. For example a mountain and sky. A soft edge filter is less noticeable than a hard edge. It also has the benefit of making the sky more intense with the sky darkens on the top. Below is a picture of what a hard edge and soft edge filter would look like.
 * Types
 * Hard Edge
 * Soft Edge

Graduated Neutral Density Filter

Initial Image

 * Open up your picture in Gimp
 * Tip: Save it now under a different name so it won't alter the original
 * In the picture below the sky is fine but the foreground is way too dark
 * Tip: in cases where you think that a portion will be too bright, always take a darker Shot
 * Cameras have a limited range of light and dark. Much worse than human eyes.  Most of the time this results it the bright portions to be washed out.  This means that the pictures content in that area will be lost and there is no way to recover it.  This howto wont help much, maybe changing white to two or three colors near white, but not good.  So always take a darker picture.  Darker portions usually have much more content that is seems.  Sometimes then full content will be recoverable.

Step 1

 * Duplicate the image, to create two identical layers
 * Right click on the background image in the layers tab.
 * In the menu click Duplicate Layer
 * The copy with uprear like below, rename it if you wish by a Right Click and click Edit Layer Attributes

Step 2

 * Create an mask for the copied layer
 * Right click on the new copied layer in the layer tab
 * Click Add layer mask
 * Select White (full opacity) and click OK
 * A white square will pop-up next to your copied layer as shown below

Step 3

 * Adding a radiant to the mask
 * Click the Gradiant Tool
 * Make the Foreground Color white, and the Background Color black
 * Click the eye next to the Background Layer to hide the original layer
 * Select the mask of the copied layer. The selected layer/mask has a white border, all unselected layers/masks will have black border.
 * Click and drag on top of your picture to create a gradient. The longer the drag the slower gradient, the shorter the drag the sharper the gradient. The gradient can be produced at any point and any direction. This can be done as many times as u need until to get it right. Each time you make a gradient, it removes the old one, and replaces it.

Step 4
Click on copy of the image
 * Notice the white border



Step 5

 * Click Layer in the menu bar.
 * then color
 * Brightness/Contrast
 * Adjust the levels as desired.

Step 6

 * Click the eye of the original image to see the final result

Step 7

 * Click the eye of the copy to see the original image and see if your changes acturally look better

Step 8
You can continue to adjust the contrast, brightness, and gradient, or anything else.
 * Just remember
 * Click the mask for the gradient
 * Click the copy of the image for the bightness and contrast of the graded copy of the image
 * CLick the original image layer to adjust anything for the background
 * The white border is for the selected layer and mask. The black border is the non-selected parts.

Step 9

 * You can do may more things with the image and gradients. The following steps show you how to add a second gradient.  You  can also add the effect of a graduated color filter.  See: Howto add the effect of a graduated color filter on a photo using the Gimp graphics editor

Step 11

 * You can also change the order of the layer to see if that makes the image look better.

Results
=See Also=
 * Guide to photography
 * Howto use camera filters for photography
 * Howto add the effect of a graduated color filter on a photo using the Gimp graphics editor

From HowTo Wiki, a Wikia wiki.