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Simulating Fog with The GIMP

Before	After

In this tutorial I'll show you how to simulate fog or clouds.

The Procedure

My original image, loaded into The GIMP.

I thought it might look good with some patchy fog.

Duplicate the image (Ctrl+D).

Now we are going to make some clouds and transfer them to the original image. In the duplicate, run Filters/Render/Clouds/Solid Noise.

Parameter settings:

I usually use the "Time" option for the randomizing seed.
For "clumpy" fog, check the "Turbulent" option. The smaller the Detail, X Size and Y Size settings, the bigger the clumps. You can see the values I used in this example at right.
For less clumpy fog, uncheck "Turbulent".

If you don't like the result Rerun it (Alt+Shift+F) and change the parameters until you get a cloud effect that is close to what you want.

Filters/Render/Clouds/Plasma also works well, especially for making misty fog. You need to Desaturate (Image/Colors/Desaturate) afterward.

Apply any other filters or manipulations you want to the fog (Levels/Curves, blur, add noise, etc.). A gaussian blur is usually a good idea. Here I did a Gaussian Blur with a radius of 60.

Keep playing until your fog looks pretty good in B&W.

Open the Layers dialog and make sure that the original image is listed in the drop-down box. Right-click on the Background layer and select New Layer (there is also a button for this in the bottom button bar of the Layers dialog (

Here I named the new layer "Fog". It doesn't matter what you choose for the fill.

Go to the image window containing the fog. Select All and Copy (Ctrl+A then Ctrl+C). Now back to the working image window and Paste (Ctrl+V). In the Layers dialog, anchor the pasted image into the Fog layer.

Now run Filters/Colors/Color To Alpha. Choose black as the color, as shown at right.

When the filter finishes, you should see your fog in the image.

In the Layers dialog, adjust the opacity of the Fog layer to suit your taste. Here I dropped it down a bit to 90 percent.

You can continue to edit the fog to suit your needs once you see it in the image. The erase tool with the right brush can be handy to clear out certain areas. Since you are editing the Fog layer only, the image is unsullied.

Consider adding a layer mask to remove large portions of fog. You will want to do something similar to the layer mask in the digital split neutral density filter tutorial.

Here I wanted the fog to end just above the water and not too far into the sky, so I added a layer mask with a gradient.

That's it! Click the "eye" next to the Fog layer in the Layers dialog on and off to quickly view the effects with and without the fog.

Tips

Another possibility instead of running the Color To Alpha filter step is to run Levels on the Fog layer, setting the OUTPUT to 128 at the low end; then set the blend mode of the layer to Overlay. I've tried this and not been quite as happy with the results, but YMMV.
Fog makes a great addition to colored backgrounds.

Other Examples

Another version without the "Turbulent" option in the Solid Noise filter. Notice how the fog is less clumpy.

Last modified: Wed Dec 10 22:23:38 HST 2003

Text and/or images are available for personal or commercial use under certain conditions.

Contact Eric Jeschke