Adding new functions with GIMP plugins

Plugins for Enhanced Effects

Several plugins reimplement existing GIMP tools, however, they always offer enhancements or expanded functionality. For example, tools normally act globally when they set an image's contrast. If they find a totally white pixel (255,255,255) and a totally black one (0,0,0) in the image, they'll reasonably interpret these as the maximum image contrast.

The human eye, however, interprets closely positioned contrast differences as more strongly contrasted than ones that are farther apart. The "Adaptive Contrast Enhancement" method takes this into account so that a completely different contrast distribution can occur in the image. The gimp-ace2 [11] plugin implements this method.

The unsharp mask (USM) option in the Filter menu under Improve … is the most often used tool to improve image sharpness. The Advanced Unsharp Mask (AUMASK) [12] feature provides a better version of this tool (Figure 2). Under Output mode: , you set how the plugin shows the results, as a new image, mask, or deltas. The Mask options set how AUMASK works, and the other parameters set the strength. With Export as new layer , you can render the result as a new layer while leaving the original layer unchanged.

Figure 2: The Advanced Unsharp Mask, with a "synthesizer" and enhanced settings, allows better control and results than the standard USM.

Focus Blur [13] has just the opposite result. This type of blur simulates or controls the depth of field (DOF) [14] according to an algorithm that is based on the human eye, and it can lead to some quite convincing results. It requires much more processing power than the standard Gaussian blur, though.

This plugin's special feature is the fact that the exact course of the blur is set with a mask (Figure 3), which naturally affects other areas as well. The weight of the mask is controlled by Focal depth and Peak Radius for Shining .

Figure 3: Focus Blur tries to simulate human eye perception. The blur can be adjusted using a self-defined "Use Depth map" mask.

Beautify

The Beautify plugin [15] installs in the Filter menu, under Beautify . It has three entries: Border (for various surface simulations, similar to Decorations ), Beautify… (for different color and character filters), and Skin whitening… (for editing skin tones). The plugin actions can also be performed manually but are presented in an orderly way with many variations, which makes work easier. Many of these actions modify the current layer, so it's best to test them on a transparent layer (for frames and effects) or a layer copy (for other actions) whose opacity can be adjusted. You can combine actions or apply them consecutively on the same layer.

The Border menu has three entries. Rip Border… edits the current layer by overlaying it with a same-sized pattern, Simple Border (Figure 4) works similarly, but it enhances the image with a simple picture frame. The image thereby is not reduced in size, but increases in total image size to the width of the frame. Many more frame shapes are available in Decorations in the Filter menu.

Figure 4: Many more effects and frames are available.

With Texture Border (Figure 5), you can apply a third group of overlays in the current layer that, for example, resemble splintered glass or folded, faded, or soiled paper or peeling paint. The texture quality is quite high so you can use the textures for larger images.

Figure 5: Beautify combines the filters in multiple windows, and each filter provides previews that can be adapted (left) and even combined.

These functions operate directly on the image layer. It's often a good idea, therefore, to copy the current later (with Ctrl+Shift+d) or generate a new working layer (with New from visible ) before applying the functions. The Reset button always undoes all the actions; a step-by-step Undo is not possible. The windows collects the tools onto the following six tabs:

Basic: Basic operations that adjust the images in subtle ways.
LOMO: Produces LOMO (lomography) effects in various combinations (see Figure 6). The HDR effects simulate them but only with limited success.

Figure 6: LOMO effects emerge with distinct vignettes and relatively intensive and dark colors.

Studio: Many color modifications, some of them in special modes (HSV, LAB), from "Elegant" to "Cold."
Fashion: Adorns images with different light reflections or water drops.
Art: Allows an "artistic" transformation of images, such as a line drawing or relief. The line drawings are of particularly good quality, often better than what other filters can produce (Figure 7).

Figure 7: Beautify creates line drawings of high quality.

Gradient: Superimposes various color gradients on the image.

The second menu item, Beautify… , contains a comprehensive tool for quite diverse and different manipulations. When loading the tool, GIMP first generates a set of previews for the various effects. You can then use them on your current layers.

The Skin Whitening… (Figure 8) menu item is used for lightening skin tones (the plugin's author is Chinese, where light skin stands for youth and beauty). Before using the filter, make a careful white balance, again working with a layer copy. It can also be a good idea to select a smaller image region to work with.