Exploring the GIMP Oilify Filter

The Oilify filter in the GIMP gives a painterly effect to pictures. You can use the filter both with photos and other art created or imported into the GIMP. The official documentation is here. I found it a bit lacking. So, I put together this post and a companion video.

You find this filter in the Artist group, in the drop down menu of the Filters tab (Filters>Artistic>Oilify). When one opens the filter this is what you see:

When first I opened it, I was intimidated, because the documentation doesn’t mention what “Aux Inputs” are, or how to use them. So, I set out to try and figure it out. They manage how two of the basic adjustments are applied. So, I’ll start by demonstrating the basic adjustments, then circle back to the Aux Inputs.

GIMP Oilify filter basic adjustments

The documentation defines the four basic adjustments. Since a picture is worth a thousand words, I’ll run through them here with two example photos: A hellebore that has fine details, but not a lot of contrast. And a pansy with high contrast, details

Mask Radius

This is basically brush size. A low number, a small brush, gives a more detailed image. And a higher value, a large brush, produces a more simplified, or abstract result.

For these examples I left the Exponent at 8, the Number of Intensities at 128 and the Intensity mode turned on.

Exponent

Next comes the Exponent adjustment. The Oilify filter documentation calls this “density of the brush mask used to paint the oily render”. I think of it as the crispness of the brush strokes. If the exponent is 1, the lowest value, then the image is soft and rather blurry. When it is at its maximum, 20, the shape of the strokes is crisply defined. As the value gets higher it seems to make less and less difference to the result. As you can see below: The difference between 1 and eight is greater than the difference between eight and twenty.

For these examples I used a Mask Radius of 10, left the Number of Intensities at 128 and the Intensity mode turned on.

Number of Intensities

The last slider adjustment is the number of intensities. The documentation calls this “Size of the histogram: default is 128. Reducing the number of intensities results in a loss of details.” I think of it as tonality within the colors. If the number is low (8 is the minimum value) the colors are flatter and when it is high (256 is the maximum) there is more definition of the lightness and darkness of the colors. Examples work better than words:

For the above examples I used Mask Radius of 10, Exponent of 8 and the Intensity mode turned on.

Intensity Mode

This adjustment is a check box. You are choosing to use an algorithm or not. With the box checked you get a more realistic effect and with it unchecked the colors get a little wild. Also, notice that the effect is more dramatic for lower values for the number of intensities.

For the examples above the Mask Radius was 10, the Exponent was 8 and the Intensity mode was turned off (box was not checked).

How to use Aux Inputs in the GIMP Oilify Filter

You can specify an image (preferably of the same size as your primary image) to use as an auxiliary input. The Aux Input image controls how the Mask Radius is applied: for black it is at its lowest value (1) and for white it is at the value you provide using the slider. The Aux 2 Input controls how the Exponent is applied, similarly black is the lowest value (1) and white is at the value you provide using the slider.

To demonstrate how they work I created a set of square test patterns:

Aux Input only

Using Aux Input, with no Aux 2, you can see that where the pattern is black the mask radius is at the minimum ,and the image is detailed. And where the pattern is white the slider value for Mask Radius is applied. Where there is a gradient the effect phases in. When there is no Aux 2 Input the Exponent value acts in tandem with the Mask Radius.

Aux Input and Aux 2 Input

When you specify both Aux Input and Aux 2 Input. The the Aux Input controls the Mask Radius and the Aux 2 Input controls the Exponent. Where Aux 1 is black there is not any effect from Aux 2. A few examples (if you click on a photo you can see it larger):

So there you have it. Using these inputs allows you to come up with a variety of effects.

If you haven’t yet gotten the GIMP you might check out this post: Get Going with the GIMP. This page: Nuts & Bolts & Tools contains a few posts about other creative ways to use the GIMP. If you would like to share your creations I host the One-to-Three Photo Processing Challenge monthly.