Grunge is a popular style for print work. Generating grunge is a very personal thing – much like the music form – so this texture example may or may not produce what you might call grunge. This technique also requires the use of a plugin that is not included in the stock GIMP distribution: Twist. You’ll need to download, compile and install this manually. Fortunately, building this package is simple on Linux. Just run make clean; make; make install. If GIMP is already running you’ll need to restart it. GIMP registers plugins when it starts so whenever you add a new one you have to restart the program.
Grunge effects start with clouds generated from the Plasma filter (Filters->Render->Plasma). Less cloud fill is probably better for a grunge look that doesn’t overpower the rest of the image. Desaturate the Plasma rendering (Colors->Desaturate).
Use the bump map option of the Emboss filter (Filters->Distorts->Emboss) on this layer. Use a low azimuth and elevation (25-35), and a moderately high depth (60-70). Name this layer Bump Map which will become the layer over which everything else will be blended. Duplicate this layer. The duplicate layer should be named Bump Map copy. Change this to GIMPressionist.
The next part of the texture will come from the GIMPressionist plug-in (Filters->Artistic->GIMPressionist). This plug-in is very complex but can produce some fantastic textures. We’re going to apply this plugin to the GIMPPressionist layer. Select the Cubism preset and click on the Apply button in the GIMPressionsist dialog. This sets the initial parameters to something useful but these need a slight modification. First, change the Paper to canvas2.pgm. Change the brush to spunge02.pgm. You can scale it up using the Aspect Ratio if you like but this isn’t required. Next make sure the Orientation span angle is set to 360 with a start angle > 20 and using Random orientation. In the Size tab, leave the Sizes value at 1, with a minimum of 4 and a maximum of 13 using the Value size setting. Finally, set the Placement Stroke density to 15 and make them Evenly distributed. When you click on OK the filter will begin processing. Don’t be surprised if it takes a while to finish. You can Update the preview before you do so, but that will just add to the time it takes to finish this step.
Switch back to the Bump Map layer in the Layers dialog. Use the Select by Color tool from the Toolbox, then click and drag anywhere on the canvas window to select any small, non-contiguous region of the this layer (not the duplicate layer we just worked on). Add a transparent layer (Layer->New) above this layer and fill the selection with black. Clear the selection (Select->None). Move the new layer to the top of the layer stack. Set the layer mode for this layer to Soft Light. Name this layer Bump Map Selection.
Now we can use the Twist filter (Filters->Distorts->Twist) on the GIMPPressionist layer. In the Twist filter window, select the Cosines function and Effect 4 in this filter. You shouldn’t need to change any other parameters. Apply these settings.
Use Select by Color on the GIMPPressionist layer to select a random region. Again, create a transparent layer above this and fill the selection with black in this new layer. Be sure the new transparent layer is moved to the top of the layer stack. The order of layers and blend modes should be:
1. Black filled GIMPPressionist Selection layer with layer Mode set to Normal.
2. Black filled Bump Map Selection layer with layer Mode set to Soft Light.
3. GIMPPressionist layer with layer Mode set to Normal
4. Bump Map layer with layer Mode set to Normal. In this example this background layer won’t be visible. Variations on this process may blend it with the other layers.
Variations of blend modes and possibly the order of layers may help produce the effect you want. Also, flipping the direction of the original or duplicate layer may help. Color can be added to any layer or a top layer of color can be added with the layer Mode set to Color or perhaps some other blend mode.