1 Verifying Volume Rendering Using Discretization Error Analysis Tiago Etiene, Daniel J¨onsson,Timo Ropinski, Carlos Scheidegger, Jo~aoComba, L. Gustavo Nonato, Robert M. Kirby, Anders Ynnerman, and Cl´audioT. Silva UUSCI-2013-001 Scientific Computing and Imaging Institute University of Utah Salt Lake City, UT 84112 USA March 12, 2013 Abstract: We propose an approach for verification of volume rendering correctness based on an analysis of the volume rendering integral, the basis of most DVR algorithms. With respect to the most common discretization of this continuous model (Riemann summation), we make assumptions about the impact of parameter changes on the rendered results and derive convergence curves describing the expected behavior. Specifically, we progressively refine the number of samples along the ray, the grid size, and the pixel size, and evaluate how the errors observed during refinement compare against the expected approximation errors. We derive the theoretical foundations of our verification approach, explain how to realize it in practice and discuss its limitations. We also report the errors identified by our approach when applied to two publicly-available volume rendering packages. 1 Verifying Volume Rendering Using Discretization Error Analysis Tiago Etiene, Daniel Jonsson,¨ Timo Ropinski, Carlos Scheidegger, Joao˜ Comba, L. Gustavo Nonato, Robert M. Kirby, Member, IEEE, Anders Ynnerman, and Claudio´ T. Silva, Fellow, IEEE Abstract—We propose an approach for verification of volume rendering correctness based on an analysis of the volume rendering integral, the basis of most DVR algorithms. With respect to the most common discretization of this continuous model (Riemann summation), we make assumptions about the impact of parameter changes on the rendered results and derive convergence curves describing the expected behavior. Specifically, we progressively refine the number of samples along the ray, the grid size, and the pixel size, and evaluate how the errors observed during refinement compare against the expected approximation errors. We derive the theoretical foundations of our verification approach, explain how to realize it in practice and discuss its limitations. We also report the errors identified by our approach when applied to two publicly-available volume rendering packages. Index Terms—discretization errors, volume rendering, verifiable visualization, verification, testing F 1 INTRODUCTION analysis [38] which we can apply after deriving the expected behavior of the algorithms under observation. In the last several decades, the visualization and graphics communities have developed a wide range of volume rendering To allow the verification of volume rendering algorithms, techniques. As they are used in several different disciplines we start with an analysis of the volume rendering integral and of science, and thus form a basis for new scientific insights, the most common discretization of this continuous model – it is essential to assess their reliability and identify errors. Riemman summation. This analysis gives us insight into the Furthermore, the increasing complexity of volume rendering expected behavior of the observed algorithms, which is essential algorithms makes the correctness of the algorithm itself as well to perform verification [14]. In this sense, our main assumption, as its potentially error-prone implementations complementary serving as a foundation for the proposed verification approach, and equally important issues. Being that volume rendering is is that discretization errors of the implementations under essential in areas such as medical imaging, where accuracy verification should behave as the errors introduced by the and precision play a crucial role, a formal methodology for discretization of the volume rendering integral. Based on assessing correctness is highly desirable [18], [32]. While this, we can mathematically derive the expected behavior verification has been widely adopted in many different branches from the discretization of the volume rendering integral and of computer science – see model checking [3], fuzzing [12], verify existing implementations through convergence analysis and convergence analysis [38] – there has not been significant by comparing their actual behavior to the expected behavior. work accomplished on a formalized praxis for asserting the Based on the results of this comparison, we can assess the correctness of visualization techniques. In this article we correctness of the implementation under verification. To get present a new verification approach for direct volume rendering further insights about deviations from the expected behavior, we techniques as well as its theoretical background. We use the present an investigation of the sensitivity of this method. We can word verification in the same sense as Babuska and Oden [1]: demonstrate that our methodology is capable of increasing the “verification is the process of determining if a computational confidence in volume rendering algorithms. To our knowledge, model, and its corresponding numerical solution, obtained by the proposed approach is the first step towards the verification discretizing the mathematical model (with corresponding exact of DVR algorithms. Thus, it can be seen as an important solution) of a physical event, and the code implementing the contribution towards a formal verification methodology of computational model can be used to represent the mathematical volume rendering techniques [35]. Our main contributions are: model of the event with sufficient accuracy”. The presented • we derive the theoretical foundations necessary for methodology is based on order-of-accuracy and convergence verifying volume rendering with order-of-accuracy and convergence analysis. We analyze the volume rendering • Etiene and Kirby are with University of Utah, USA E-mail: ftetiene,[email protected] integral and its (common) discretization using Riemann • Jonsson,¨ Ropinski and Ynnerman, are with Linkoping¨ University, Sweden summation to derive an algorithm’s expected behavior E-mail: fdaniel.jonsson,timo.ropinski,[email protected] when being subject to parameter changes; • Nonato is with Universidade de Sao˜ Paulo, Brazil • E-mail: [email protected] we explain how to exploit these theoretical foundations to • Comba is with Universidade Federal do Rio Grande do Sul, Brazil perform a practical verification of implemented volume E-mail: [email protected] rendering algorithms, such that it can be easily used for • Scheidegger is with AT&T Labs – Research E-mail: [email protected] the verification of existing volume rendering frameworks; • Silva is with NYU Poly, USA • we discuss the limitations of the proposed concepts by E-mail: [email protected] analyzing frequently occurring errors and by documenting those errors we could identify when applying the pre- 2 Fig. 1. (a) shows the result of our verification procedure for dataset refinement. The blue line corresponds to the initial behavior, which deviates from the expected slope (solid dark line). After fixing the issues, we obtain the orange curve, with a slope closer to the expected one. (b) and (c) show a human torso, displaying the blood vessels and the spine, before and after our changes. (d) shows the difference between (b) and (c). sented methodology to two widely used volume rendering frameworks, VTK [39] and Voreen [27] (see Figure 1). 2 RELATED WORK Critical decisions in fields such as medical imaging often rely on images produced by volume rendering algorithms, where it is of utmost importance that the results are correct [4]. The multitude of algorithm components and their interactions make this guarantee a challenge. As a consequence, many authors focus on specific aspects of the problem such as Fig. 2. Left: the volume rendering of the torso dataset numerical aspects of the evaluation of the volume rendering using an incorrect trilinear interpolation. Middle: Same integral, shading, transfer functions, and interpolation schemes. dataset with the correct interpolation. Right: difference The quality of volume rendering has always been of central between the two images. interest to the community, and relying on visual inspection is a common practice. Meissner et al. [26] evaluate volume rendering techniques using the human visual system as a The use of a verification framework has only recently been reference while, more recently, Smelyanskiy et al. [40] present discussed in scientific visualization, despite the vast literature on a domain expert guided comparison scheme. While those verification in computer science. Globus and Uselton [11] first approaches are valuable, the need for a more systematic pointed out the need to verify not only visualization algorithms evaluation is discussed in several papers [11], [15], [16], [18]. but also their implementations, and Kirby and Silva suggested See Pommert and Hohne¨ [32], [33] for a survey. a research program around verification [18]. The verification Among several aspects to consider in the correctness of of isosurface algorithms was discussed by Etiene et al. [8], [9], volume rendering algorithms, one of the most important is the where a systematic evaluation identified and corrected problems approximation of the volume rendering integral. The solution in several implementations of isosurface extraction techniques. with linearly interpolated attributes is presented by Williams Zheng et al. [47] address CT reconstruction and interpolation and Max [44], with further discussions on its numerical errors in direct volume