Chapter 10
Software Process

An outstanding feature of ITK is the software process used to develop, maintain and test the toolkit. The Insight Toolkit software continues to evolve rapidly due to the efforts of developers and users located around the world, so the software process is essential to maintaining its quality. If you are planning to contribute to ITK, or use the Git source code repository, you need to know something about this process (see 2.1 on page 24 to learn more about obtaining ITK using Git). This information will help you know when and how to update and work with the software as it changes. The following sections describe key elements of the process.

10.1 Git Source Code Repository

Git) is a tool for version control. It is a valuable resource for software projects involving multiple developers. The primary purpose of Git is to keep track of changes to software. Git date and version stamps every addition to files in the repository. Additionally, a user may set a tag to mark a particular of the whole software. Thus, it is possible to return to a particular state or point of time whenever desired. The differences between any two points is represented by a “diff” file, that is a compact, incremental representation of change. Git supports concurrent development so that two developers can edit the same file at the same time, that are then (usually) merged together without incident (and marked if there is a conflict). In addition, branches off of the main development trunk provide parallel development of software.

Developers and users can check out the software from the Git repository. When developers introduce changes in the system, Git facilitates to update the local copies of other developers and users by downloading only the differences between their local copy and the version on the repository. This is an important advantage for those who are interested in keeping up to date with the leading edge of the toolkit. Bug fixes can be obtained in this way as soon as they have been checked into the system.

ITK source code, data, and examples are maintained in a Git repository. The principal advantage of a system like Git is that it frees developers to try new ideas and introduce changes without fear of losing a previous working version of the software. It also provides a simple way to incrementally update code as new features are added to the repository.

The ITK community use Git, and the Google web software tool Gerrit ( to facilitate a structured, orderly method for developers to contribute new code and bug fixes to ITK. The Gerrit review process allows anyone to submit a proposed change to ITK, after which it will be reviewed by other developers before being approved and merged into ITK. For more information on how to contribute, please visit For information about the Git-based development workflow adopted by ITK, see the Appendix ?? on page ??. DRAFT-ITK

10.2 CDash Regression Testing System

One of the unique features of the ITK software process is its use of the CDash regression testing system ( In a nutshell, what CDash does is to provide quantifiable feedback to developers as they check in new code and make changes. The feedback consists of the results of a variety of tests, and the results are posted on a publicly-accessible Web page (to which we refer as a dashboard) as shown in Figure 10.1. The most recent dashboard is accessible from Since all users and developers of ITK can view the Web page, the CDash dashboard serves as a vehicle for developer communication, especially when new additions to the software is found to be faulty. The dashboard should be consulted before considering updating software via Git.



Figure 10.1: On-line presentation of the quality dashboard generated by CDash.

Note that CDash is independent of ITK and can be used to manage quality control for any software project. It is itself an open-source package and can be obtained from

CDash supports a variety of test types. These include the following.

All source and test code is compiled and linked. Any resulting errors and warnings are reported on the dashboard.
Some ITK tests produce images as output. Testing requires comparing each test’s output against a valid baseline image. If the images match then the test passes. The comparison must be performed carefully since many 3D graphics systems (e.g., OpenGL) produce slightly different results on different platforms.
Problems relating to memory such as leaks, uninitialized memory reads, and reads/ writes beyond allocated space can cause unexpected results and program crashes. ITK checks run-time memory access and management using Purify, a commercial package produced by Rational. (Other memory checking programs will be added in the future.)
All classes in ITK are expected to print out all their instance (i.e., those with associated Set and Get methods) and their internal variables correctly. This test checks to make sure that this is the case.
Each class in ITK should have a corresponding unit test where the class functionalities are exercised and quantitatively compared against expected results. These tests are typically written by the class developer and should endeavor to cover all lines of code including Set/Get methods and error handling.
There is a saying among ITK developers: If it isn’t covered, then it’s broke. What this means is that code that is not executed during testing is likely to be wrong. The coverage tests identify lines that are not executed in the Insight Toolkit test suite, reporting a total percentage covered at the end of the test. While it is nearly impossible to bring the coverage to 100% DRAFT-ITK because of error handling code and similar constructs that are rarely encountered in practice, the coverage numbers should be 75% or higher. Code that is not covered well enough requires additional tests.

Figure 10.1 shows the top-level dashboard web page. Each row in the dashboard corresponds to a particular platform (hardware + operating system + compiler). The data on the row indicates the number of compile errors and warnings as well as the results of running hundreds of small test programs. In this way the toolkit is tested both at compile time and run time.

When a user or developer decides to update ITK source code from Git it is important to first verify that the current dashboard is in good shape. This can be rapidly judged by the general coloration of the dashboard. A green state means that the software is building correctly and it is a good day to start with ITK or to get an upgrade. A red state, on the other hand, is an indication of instability on the system and hence users should refrain from checking out or upgrading the source code.

Another nice feature of CDash is that it maintains a history of changes to the source code (by coordinating with Git) and summarizes the changes as part of the dashboard. This is useful for tracking problems and keeping up to date with new additions to ITK.

10.2.1 Developing tests

As highlighted, testing is an essential part of ITK. Regression testing on a regular basis allows ITK to meet high code quality standards, and to enable reproducible research. Code coverage reported daily in CDash allows us to systematically measure the degree to which the ITK source code is reliable. Therefore, writing tests, and improving current tests and the testing infrastructure is crucial to ITK.

There are a number of scenarios when writing tests:

In either case, the tips and tools described in Section 9.4 were developed to improve and facilitate the process. DRAFT-ITK

10.3 Working The Process

The ITK software process functions across three cycles—the continuous cycle, the daily cycle, and the release cycle.

The continuous cycle revolves around the actions of developers as they check code into Git. When changed or new code is checked into Git, the CDash continuous testing process kicks in. A small number of tests are performed (including compilation), and if something breaks, email is sent to all developers who checked code in during the continuous cycle. Developers are expected to fix the problem immediately.

The daily cycle occurs over a 24-hour period. Changes to the source base made during the day are extensively tested by the nightly CDash regression testing sequence. These tests occur on different combinations of computers and operating systems located around the world, and the results are posted every day to the CDash dashboard. Developers who checked in code are expected to visit the dashboard and ensure their changes are acceptable—that is, they do not introduce compilation errors or warnings, or break any other tests including regression, memory, PrintSelf, and Set/Get. Again, developers are expected to fix problems immediately.

The release cycle occurs a small number of times a year. This requires tagging and branching the Git repository, updating documentation, and producing new release packages. Although additional testing is performed to insure the consistency of the package, keeping the daily Git build error free minimizes the work required to cut a release.

ITK users typically work with releases, since they are the most stable. Developers work with the Git repository, or sometimes with periodic release snapshots, in order to take advantage of newly-added features. It is extremely important that developers watch the dashboard carefully, and update their software only when the dashboard is in good condition (i.e., is “green”). Failure to do so can cause significant disruption if a particular day’s software release is unstable.

10.4 The Effectiveness of the Process

The effectiveness of this process is profound. By providing immediate feedback to developers through email and Web pages (e.g., the dashboard), the quality of ITK is exceptionally high, especially considering the complexity of the algorithms and system. Errors, when accidentally introduced, are caught quickly, as compared to catching them at the point of release. To wait to the point of release is to wait too long, since the causal relationship between a code change or addition and a bug is lost. The process is so powerful that it routinely catches errors in vendor’s graphics drivers (e.g., OpenGL drivers) or changes to external subsystems such as the VXL/VNL numerics library. All of these tools that make up the process (CMake, Git, and CDash) are open-source. Many large and small systems such as VTK (The Visualization Toolkit