The following paper was originally published in the
Proceedings of the USENIX Fourth Annual Tcl/Tk Workshop
Monterey, California, July 1996.

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Introduction

Visualizing images or volumes helps extract qualitative information and quantitative measurements from raw data sets. Visualization software is thus becoming critical in virtually every domain of engineering. However, despite the number of packages available from commercial vendors and from the public domain, it is extremely difficult to find a package that fulfills the needs of a research laboratory dealing with large data sets. Indeed, all the packages are implemented based on the following scheme.

• First, the file formats need to be decoded, and the raw data read in. The data can then be formatted or extracted. This step includes operation such as subsampling, interpolation or dimensional reduction, e.g when a slice is extracted from a 3D volume.
• Next, the data are normalized, typically to 8- or 16-bit integers, and displayed in a window after the colors have been allocated. Using predefined color look-up tables, stretching the histogram, reducing the image dynamic range and providing a colorbar help identify the relevant features of the data set.
• Numerical values, extracted data or regions of interest can then be written back into a file.
• At any point, the user may interact with a graphical user interface (GUI) or issues commands to an interpreter, e.g. to change the color look-up table, look at pixel values, etc.

Practical experiments showed that none of the all command-line oriented (VIEW, IDL, PWave, Matlab, etc), dataflow-oriented (AVS, Explorer,..) or ``self-contained'' (Analyze, 3DVIEWNIX, etc) visualization packages could handle large data sets whose size exceed both the size of the memory and swap space. The user is thus compelled to manually extract smaller pieces of the data sets, which can be time-consuming and inefficient. It is thus mandatory to tightly link the data extraction and data visualization steps, typically by reading and visualizing one slice at a time instead of loading a 3D volume in memory before visualizing its 2D slices.

The efficiency of interactive visualization is also limited by the GUI design, which can almost never be customized by the user. Even when the source code is available, adding or removing features is very difficult if not impossible, as the GUIs are implemented as monoliths of hundreds of thousands of lines. However, the user needs to control the way the colors are allocated, e.g., interactive thresholds, linear and non-linear colormaps. Similarly, extracting profiles, histograms or non-rectangular regions of interest make quantitative measurements possible. Off-line data transforms or extractions are in our opinion too cumbersome in a research environment. In addition, unsupervised automatic segmentation performs poorly when the data are noisy. The alternative, that is the use of manual and semi-automatic segmentation techniques, is however limited by the lack of flexibility of most graphical interfaces.

The considerations above led us to implement a new visualization tool, geared to large data sets. Since we could not afford, nor had the experience required to write a self-contained application in X-Motif, our approach was to divide the GUI into small reusable components by relying on Tcl/Tk. Tcl/Tk is now used by thousands of users in every domain of graphical and engineering applications. Indeed, Tcl/Tk provides simple ways to ``glue'' different modules together and it can be extended easily, in contrast to other GUI builders.

However, the use of Tcl/Tk in the signal and image processing community is scarce. Two main reasons can explain this situation. First, efficient data management mechanisms and number-crunching capabilities in image processing are generally believed to require high-performance languages, in contrast to Tcl which only handles character strings. Next, the photo image widget was not designed for interactive visualization and its flexibility is very limited, mainly because image processing applications are not the main focus of Tcl/Tk developers.

In this paper, we describe the implementation of a new gray-scale image widget. By focusing on data management and color allocation problems, we were able reach a level of performance which compares favorably with other image processing and visualization packages. A transparent overlay mechanism provides a link to manual and semi-interactive segmentation techniques. We will show how this widget helped us build VISU, a flexible VISUalization software described below. VISU is made up of several standalone applications which communicate which each other over the network. The power and flexibility of these hypertools in image and volume visualization will be described with some examples. A biomedical application will be presented.