Masked Ball

In GIMP, masks select parts of an image, just as you would with the selection tool to create a rectangular or circular selection in an image. GIMP draws "marching ants" around the selection when you do so and stores the selection information as a black-and-white image in which the white regions represent the selected parts of the image and the dark pixels represent unselected areas.

If the user draws a rectangle at the center of the image, the corresponding mask is a black image with a white rectangle at its center. And Masks can go even further: In contrast to mouse-based selections, masks can also define grayscales, in which the areas of the image are selected "softer" or "harder."

These grayscale images can be put to intelligent use as filters. Instead of painstaking manual selections, the user defines a mask image and GIMP automatically selects the parts of the original image that have corresponding light pixels in the mask image.

To define how the layer's pixels are to be applied to the image, GIMP lets you define a layer mask for any layer. The white areas of this grayscale mask make the layer pixels 100 percent opaque, whereas the black pixels in the mask make the layer 100 percent transparent; that is, they disable this part of the layer. For gray areas, GIMP applies the layer value to a certain degree, reflecting the value in the mask.

Figure 6 shows the Layers dialog with a big black circle that I added to the white background of the layer mask in the sepia layer as a test; the layer's overlay mode is set to "Normal." As you can see in Figure 7, GIMP does not change the original image in the black part of the mask but applies sepia to the white parts and ignores the image in the process. This approach is definitely not the right one, but a grayscale mask will get me closer to my goal.

Figure 6: A layer mask with a big black circle …

Figure 7: … leaves the image untouched where the mask is black, but completely colors the image sepia in the white parts of the mask.

Original as a Mask

What does the mask need to look like to apply a realistic looking sepia tint – that is, to open up the sepia floodgates for the darker parts of the original and leave them shut in the lighter areas? If a pixel in the original layer is black, the mask at this point has to be white, and the sepia tint will be applied 100 percent. The opposite also applies: If the original layer is white, the mask is black, and the sepia layer is not applied to the original layer at all.

For grayscales, the mask is somewhere in between. Black-to-white and white-to-black? The solution should now be obvious: The mask simply needs to invert the grayscale image of the original layer!

For this, edit_copy() in line 82 copies the image in the original layer into GIMP's internal cut-and-paste buffer, and line 85 drops this content on top of the previously created $layer_mask. This provides a reference to a floating section, which is inverted by line 90 and finally anchored in the layer mask by line 91.

To do this in GIMP by hand, you would click the original layer in the layer dialog, then change to the image window, press Select-All, then press Ctrl+C to copy the image content to the cut-paste buffer. This procedure is followed by a mouse click on the layer mask in the sepia layer (the second thumbnail in the layer line) and a return to the image window, where you need to select Paste to insert. This creates a floating section in the Layers dialog, which is fixed in place by clicking the anchor at the bottom. Finally, you would need to invert the colors in the layer mask by selecting Colors | Invert; after doing so, the Layers dialog would look similar to Figure 8.

Figure 8: The script creates a new layer in GIMP containing a sepia-colored area and its layer mask.

The final step is to flatten() (i.e. merge the two layers to create a single active one). This changes the order of the active layers and the script thus needs to call the get_active_layer() method (line 97) to obtain a reference to the remaining layer.

The results, with a nice coating of sepia, are shown in Figure 9, and for a more dramatic effect, as shown in Figure 10, the curves_spline function darkens the dark tones slightly, without touching the lighter ones. The six coordinates passed in to the function in line 102 define a graph, as shown in Figure 11. It manipulates the image as if the user had run GIMP's Curves against it. A linear curve in the Curves dialog leaves the image unchanged, whereas data points below the straight line darken all image pixels carrying that brightness value.

The gimp_file_save() function saves the results in a new file, which line 104 creates by appending -sepia to the original name.

Figure 9: A sepia tint applies the yellowish color mainly to the darker areas.

Figure 10: The final result for comparison: darkening the darker parts of the image in the Curves dialog for dramatic effect.

Figure 11: Reducing the dark tones but keeping the lighter ones gives a more dramatic image.

Installation

To install the required Perl modules, Gimp and Gimp::Fu, on Ubuntu Hardy Heron, enter:

sudo apt-get install libgimp-perl

Older Ubuntu versions have a bug, but in a previous article [5], I told you how to work around it. The Log4perl module (available from CPAN or in the liblog-log4perl-perl package with your Ubuntu distribution) shows the progress of the image conversion at the command line. If you prefer a less verbose script, you can comment out the call to easy_init() in line 10 of the script.

The documentation for GIMP's Perl interface isn't very detailed, but GIMP has an excellent procedure browser (Figure 12) that you can call via Xtns | Procedure Browser.

Figure 12: GIMP's Procedure Browser helps you find the right API function.

After doing so, type the part of the function name that you have already guessed, such as "load," "save," or "layer," and the procedure browser will return a list of available API functions with precisely documented parameters and return values. It is thus typically possible to find an API function for any action in GIMP's point-and-click user interface, empowering the user to automate steps in scripts instead of repeating them by hand.

The Author

Michael Schilli works as a software engineer with Yahoo! in Sunnyvale, California. He is the author of Goto Perl 5 (German) and Perl Power (English), both published by Addison-Wesley, and he can be contacted at mschilli@perlmeister.com. Michael's homepage is at http://perlmeister.com.