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# 5.4.4 Calculate weave maps

The most complex function in the software, *threadCount*, allows direct calculation of weave and angle maps of your own canvas X-rays. The input to the *threadCount* function is an X-ray of a canvas in tif format; the output is a file containing data that represents the vertical and horizontal thread density estimates and the vertical and horizontal angle estimates. These data files have been precomputed for the full set of Vermeer’s canvases, and so there is no need to use *threadCount* when studying the work of Vermeer.

On the other hand, it is possible to use the functions *displayWeaveMaps* and *align* with any canvas, by first calculating the thread data. There are several choices to be made, which correspond to various properties of the Fourier Transform. Since the software is slow (there are a lot of calculations), it is helpful to choose the parameters wisely. The interface for *threadCount* is shown in Figure 5.19. At startup, there is no image loaded and it is necessary to select a *.tif canvas image file using the *load X-ray file* button. The two input boxes labeled *horizontal mean* and *vertical mean* must be filled with estimates of the thread counts (in threads/cm) as derived from manual thread counts. The *block size* specifies the size of the regions used in the Fourier calculations, for example, 0.5 means that the regions are 0.5 x 0.5 cm squares. The overlap specifies how many regions touch each pixel: an overlap of 2 with a 0.5 cm block size means that successive regions are offset by 0.25 cm. The main effect of the block size (called *FFT size *in the interface) is to give a smoother picture for 1024 while giving a quicker calculation for 512. The resolution, in dots per inch (DPI), must also be specified in order for the function to be able to translate pixels into physical units.

*Figure 5.18: The interface for the *threadCount *function, which calculates the weave and angle maps, and writes the results to a file for later display with *displayWeaveMaps *and *align*.*

Once the options are selected, press *Calculate,* and prepare to wait; the progress bar (shown empty on the right hand side of Figure 5.18) will move slowly. When finished, a data file is written (to the same directory as *threadCount*) with a name such as threadcountL07(0.5-2).m. This indicates that it is a thread count file of canvas *The Milkmaid (*L07) with parameters block size ½ and overlap 2. It is a text file containing four matrices; each matrix is an arrays of numbers, one for each region, containing the values of the Horizontal and Vertical counts and angles.

Chapter 6 shows how the weave and angle maps can be applied to study the canvases of Vermeer.