Starting

Right-click on any image of this type and select Filters/Enhace/Fourier; this will start GFourier. You can perform a selection on the image before, and GFourier will only act on the selected zone. However, GFourier will internally work with the smallest rectangle containing the selected area (GIMP's plug-ins common behavior).

First of all, GFourier must calculate the Fourier Transform of the image, where each pixel of the image is considered to be an element of a real matrix; the Fourier Transform is then applied to this matrix. These transforms involve lots of calculations and may require some time; a progress bar will appear on screen. In case that you did not compile GFourier with FFTW support (see GFourier and FFTW for more information), a FFT algorithm will be used with images of dimensions that are powers of two, and a slower general algorithm with other dimensions.

Now, GFourier's main window will appear. It presents the results of the Fourier Transform and allows the user to apply filters, transform the data or calculate several operations.

Later, GFourier will calculate the Inverse Fourier Transform of the data. Because this inverse transform produces complex numbers, the module of the results is calculated. Optionally, this module is normalized or equalized; finally, it is converted to integer to produce an image.

Main window

The preview represents the data which results from the Fourier Transform. It is a gray bitmap where black pixels stand for zero-module vectors and white ones for vectors of maximum module. The scale used in this conversion is adjusted each time that the data is modified; this way, the full range of gray values is always used on each representation. The View/Invert options exchanges the meaning of black and white, and the View/Contrast option modifies the contrast. You can also choose what is this representation showing: the real part of each point, its imaginary part or the module; that is the View/Type option.

Buttons

• Ok
Use this button when you are satisfied with the modifications and you want to calculate the Inverse Fourier Transform of your data. If the File/Options/New Image item is active, a new window will appear with the results; otherwise, the base image will be updated.
• Preview
This button has the same use of the Ok button, but this one doesn't close this window. Use it when you want to have a look to your work and continue working on it.
• Cancel
It quits this window of GFourier immediately.
Warning: Cancel will not restore the image after a Preview, it just quits GFourier.

Menus

• File
• Ok
It is the same as the Ok button
• Preview
It is the same as the Preview button.
• Cancel
It is the same as the Cancel button
• Options
• New gfourier

When this option is selected, all modifications of the complex data (Filters, Transforms and Operations) will be performed on a new copy of the GFourier window. With the default option, data in the current window will be modified.
 
• New image

When this option is selected, a new window will be created to show the results of the Inverse Fourier Transform, when the Ok or Preview buttons are pressed. Default is to modify the original image (or selected zone).
 
• Auto-normalize

This options activates the auto-normalization of data after the Inverse Fourier Transform has been calculated and before data is converted to integers. There is a comment in the FAQ about this.
 
• Auto-equalize

This options activates the auto-equalization of the data after the Inverse Fourier Transform has been calculated and before data is converted to integers. There is a comment in the FAQ about this.
• Filters
• Design
This item opens the filters design window.
• Transforms
• Resize
Here you can change the size of the matrix that contains the complex data. If you make a dimension shorter, data will be cropped at the borders; if you make a dimension longer, data will be padded with zeros at the borders.
• Exchange quadrants
When the Fourier Transform of an image is calculated, coefficients corresponding to high frequencies are on the center of the data, while low frequencies are at the corners. However, this is not the best disposition for an easy use of that information. GFourier does a little trick before calculating the Fourier Transform,  which is equivalent to exchanging top-right quadrant with bottom-left quadrant, and top-left with bottom-right. This menu item cancels the effect of that trick.
• Conjugate
Use this transform to calculate the complex conjugate of each point in the matrix.
• Flip
This is equivalent to a flip in an image, but applied to the complex data.
• Rotate
This is equivalent to a rotation in an image, but applied to the complex data.
• Operations
• Add
You can open two GFourier windows (from the same or different images) and add the complex data contained in one window to the complex data contained in the other. This is the common sum of matrices.
Both matrices must have the same size.
• Multiplication
The results of this operation is the multiplication, element to element, of one matrix by the other one.
Both matrices must have the same size.
• Product
This is the dot product of matrices, where each element of the resulting matrix is the product of a row from the first matrix by a column from the second one.
The width of the first matrix must be equal to the height of the second one.
• View
The View menu changes the aspect of the preview, but the internal complex data is not modified.
• Contrast
The contrast of the representation can be changed from 0 (almost all the preview is medium gray) to 255 (only highest values of the data are not shown as black). Default value is 127, which produces an intermediate result.
• Invert
Usually, the Fourier Transform is represented using a gray scale where black means zero and white means maximum value. However, this option will invert this scale: white will mean zero and black will mean maximum value.
• Type
Here you can choose what will the preview represent: the real part of each point, the imaginary part or the module.
• Windows
• New Preview
This item creates an image containing the representation of the complex data, it is a 'photograph' of the preview. See the FAQ for more information about this.
• New GFourier
This item opens a new GFourier window, and copies the data contained in the current window to the new one. Modifications performed to one window will not affect the other.
• About
Here you can see the copyright notice.