View Full Version : 4 ways to get Variable Blurs in GIMP

05-12-2008, 04:12 PM
Over in my Not so Random Coastline Tutorial (http://www.cartographersguild.com/showthread.php?t=875) I suggested using a variable blur to control the randomness of the coastline. I had a few pm's asking how to do this, so I will give you four ways (in four posts).

#1 - Feathered Selection, Bigger Blur
Start with your base shape:

Blur it the lesser amount (20 px here):

Use the lasso tool to make a selection around the area you want to have a larger blur. Set it to a 100px feather. Leave lots of space:

Blur it again, with the greater amount (100px here):

The result:

-Rob A>

05-12-2008, 04:17 PM
#2 Feathered Selection - Curves Adjust
Start with the base shape:

Blur it the larger amount (100px):

Use the lasso tool to make a selection around the area you want to have a smaller blur. Set it to a 100px feather:

use the curves dialog as shown. This pulls the shades closer to the middle. The advantage here is by moving the slope left and right you can move the tighter band in and out:

The result:

-Rob A>

05-12-2008, 04:25 PM
#3 Layer Masks

Start with the base shape:

Duplicate this layer. Blur the bottom one the lesser amount, and the upper one the greater amount (I'm not showing this, as it is the same as the other methods).

Create a layer mask on the upper layer and fill the layer mask with a B/W gradient. With layer masks, white is opaque, black is transparent. Here is the mask I used:

The final result, showing the layers and the layer mask:

Note: To use this, you will either need to merge the two layers together or do a "copy visible" then "Paste as new layer".

-Rob A>

05-12-2008, 04:35 PM
#4 Using the Focus Blur Plugin

This provided the "sweetest" results, creating a true variable blur, rather than a blend between two blurs, as the other three do. The plugin is computationally intensive, however. This sample took 4 minutes to run on my machine...

If running Linux, head over to the plugin home page (http://sudakyo.hp.infoseek.co.jp/gimp/fblur/focusblur_e.html) and get the plugin and install it as per the instructions there. There is also a windows binary mirror here (http://earth2marsh.com/fblur.zip) but I don't know for how long...

Start with the base image:

Create a new layer, and fill it with a gradient that will be the focal blur map:

Run the Focal Blur plugin. Set Focus to 255 (white in the map will be in focus, the more black, the more blurred). Select the distance map option, and select the map you made. The bigger the radius here, the longer it will take to run... start small and work it up. Here were my settings:

The result:

Notice how the blur blends smoothly from the narrow area to the wide area. Nice...

-Rob A>

05-14-2008, 11:24 AM
It can also be done with quick masks - very similar to the feathered lasoo method.

Start with the standard circle and run a light blur over it so you have something like this to start with:


Now click the quick mask button (circled in green on the following image). It should now look like this:


Now use the gradient tool to get a nice graded selection. Drawing over the image with white creates a see-through areas of the mask (the red) that will be interprested as the selection later on. Therefore using a gradien tool we can leave some areas black (unselected) and then as it fades to white, the area becomes selected. Using the gradient like this:


... should give a quick mask that looks like this:


Now click the quickmask button again to turn the quickmask into a selection. You should now have what looks like a rectangular selection around the bottom. With this selection in place, run the Gaussian Blur filter again at 100px to get the following: