Combined volume and surface rendering with global illumination caching

We present a combined volume and surface rendering technique with global illumination caching. Our approach uses volumetric path tracing to compute the global illumination volume and local shading models for rendering the isosurface. By joining both visualization approaches, we have enhanced the display and illumination of the surfaces while preserving physically realistic illumination of the participating media. To achieve real-time performance and avoid recomputing the image when the camera view changes, we compute the global illumination volume incrementally and defer the projection to a later step. We evaluated our technique by comparing different local shading models for isosurface rendering with the result of full volumetric path tracing and with the non-caching variant of our technique. Results show that the caching and non-caching variants perform comparably well, while the caching variant has the added beneﬁt of being camera-view-independent. Additionally, we show that our approach emphasizes the surfaces within volumes better than volumetric path tracing.


Introduction
In visualization, physically correct lighting contributes a lot to the accurate perception of details in the data, which is especially evident when rendering polygonal geometry.In this case, lighting contributes enormously to the correct perception of shapes, depth, and mutual arrangement of objects in the scene.When rendering volumetric data with different degrees of transparency, lighting is even more important, as the penetration of light through the substance allows for the recognition of details in the data.With simple and fast methods, which neglect physical correctness, such visualization may come at the cost of depth perception.
Volumetric data are used in a variety of scientific fields such as medicine, where such data are captured using various radiological techniques (computed tomography [22], magnetic resonance imaging [13], three-dimensional ultra-sound [16], positron emission tomography [2]), meteorology, where such data can be captured by satellites and/or radars, astronomy, where volumetric data can also be captured in various ways [45] (by light or radio telescopes or with gravity wave detectors), all scientific fields using microscopy (transmission tomography [23], cryoelectron tomography [26]), physics, where such data are mostly the results of simulations, etc.All these areas have a common need for clear and accurate visualization of the captured data, which provides a good insight into their structure and details.
Often we want to visualize parts of volumetric data with the same properties (e.g., the same tissue densities), which can be represented by isosurfaces.In the past, researchers have presented many ways for direct and indirect isosurface rendering.Examples of indirect rendering involve calculating isosurfaces in the first step and presenting them as hardware-friendly geometry (e.g., a triangle list), while a common example of direct rendering is ray tracing [39].While modern null-collision-based volume rendering techniques are more than fit for rendering participating media, they lack the capability of efficiently rendering isosurfaces since they correspond to large discontinuities in density.A suitable transfer function can simulate the solid appearance of an isosurface, but at a considerable cost of rendering efficiency.Until a general volume rendering method emerges that can efficiently handle such discontinuities, combining volume and surface rendering methods is a necessity.
The goal of our work is to produce an interactive approach for displaying surface illumination while preserving the physically realistic illumination of the participating media.We obtain this by combining the two prevalent rendering approaches: isosurface rendering, which can indicate a specifically selected property in the data, and volumetric path tracing (VPT), which adds details that can only be attained with a global illumination technique.By ensuring that the individual techniques used in our method are physically based, we retain a high degree of realism without any computationally expensive work.Additionally, we include global illumination caching, which makes the method usable in interactive and exploratory scenarios.As a result, the method provides a good overview of the structures and details in the data while being fully interactive, thus enabling high-quality exploratory visualization in many scientific fields.
The main contributions of our work are: • combining volumetric path tracing with locally illuminated isosurfaces in order to emphasize the desired surfaces in the volume, and achieve interactive but physically based visualization; • adding global illumination caching to achieve real-time performance and interactivity; and • comparison of the caching and non-caching variants of our technique with full volumetric path tracing and the use of different local lighting models for isosurface shading.
In Sect.2, we present the related work and differentiate our contributions from it.In Sect.3, we present our approach and compare it with selected existing ones.The results and evaluation of our method in comparison with the chosen techniques are presented in Sects.4 and 5.In Sect.6, we present the conclusions and give possible extensions and upgrades as part of further work.

Related work
The first approaches for interactive rendering of volumetric data were not physically based and were based on ad hoc techniques such as maximum intensity projection and emission-absorption model by Max [33].Due to their simplicity and speed, they are still widely used in practice today, despite having undergone many upgrades.However, their simplicity is also the reason why they are mostly inadequate for more advanced visualizations, as they do not enable a good perception of the depths and shapes of individual structures in the data due to the lack of global illumination or its approximations such as volumetric ambient occlusion [41].
These shortcomings were the reason for the development of the first physically based approaches [10,44], which were soon adapted for interactive use by Parker et al. [38].An overview of the existing methods for imaging medical data is presented in the work of Tiede et al. [46].Physically based approaches for calculating illumination in volumetric data include path tracing by Kajiya and Von Herzen [21] and radiosity calculation by Rushmeier and Torrance [42], which do not achieve better output quality or an increase in speed.An overview of the most commonly used methods for volumetric imaging in practice can be found in the work of Engel et al. [11].
When rendering volumetric data, however, we are most often really interested in what surfaces (isosurfaces) are present in the data.The most known approach for their calculation is the so-called marching cubes algorithm by Lorensen and Cline [32], using predefined templates to convert an isosurface into a triangular grid geometry that is most suitable for real-time rendering on graphics hardware.The approach has been upgraded and adapted several times, and the most well-known upgrades include marching tetrahedra [9,48], implicit surface polygonization by Bloomenthal [4], and multi-level partition of unity implicits by Ohtake et al. [36].A standard graphics pipeline can be used to represent the resulting geometry, but the disadvantage is the need for an additional step to calculate the geometry each time the isovalue is changed.Direct approaches do not have this disadvantage, as they take into account the isovalue during the rendering process itself.The most commonly used approach for rendering isosurfaces is ray tracing [5,39].While it is possible to perform such rendering in real time with modern graphics hardware, it is still not physically correct due to local illumination.
Ray marching isosurface rendering of surfaces within the volume data was first introduced by Levoy [28], where local shading was used during the accumulation of every nontransparent sample along the ray.This method was and still is a basis for many direct volume rendering techniques, as it offers great speed and a good approximation of a physically correct result.An adaptation of ray marching-sphere tracing-was introduced by Hart [15], assuring that one does not penetrate the implicit surface.Hart's method also approximates cone tracing [25] for antialiased rendering.
Voxels that represent the isosurface can also be considered as points in the 3D space-a point cloud-which can be used for surface reconstruction.A survey on surface reconstruction from point clouds is presented by Berger et al. [3].
The first physically based volume rendering approach [21], formalized with the rendering equation and its Monte Carlo solution by Kajiya [20], was later extended with support for visual mapping by Drebin et al. [10].An unbiased approach to path tracing using delta tracking introduced by Yue et al. [54] was integrated into the implementation of 123 Content courtesy of Springer Nature, terms of use apply.Rights reserved.an interactive progressive volumetric rendering system by Kroes et al. [27].A unified delta tracking framework was presented by Galtier et al. [14].An overview of modern physically based Monte Carlo-based methods was presented by Novák et al. [35].
Combining multiple rendering techniques to achieve the desired results is not a new concept.Tietjen et al. [47] introduced combined surface and line rendering with ray marching for emphasizing the objects of interest within volumes.Their approach expects a segmented volume as well as volumetric and mesh representations of objects and allows users to render the final image in the desired style.Andersen et al. [1] present hybrid fur rendering where they join rendering of explicit hair strand geometry and rendering of volume texture of hair densities using ray marching of a prismatic shell volume with dynamic resolution.Their method creates a more detailed and soft fur appearance than either of the individual approaches.Isenberg et al. [18] present an observational study on how non-photorealistic rendering of 3D objects can be compared with traditional hand-drawn sketches.Bruckner and Gröller [6] present a style transfer approach for volumetric rendering for achieving sketch-like results.Often the desired results are achieved by carefully designing an appropriate transfer function.An overview of transfer function design is presented by Ljung et al. [30].Most of the above methods aim to emphasize certain features in a volume using different approaches.Xu et al. [53] present a survey of feature-enhancing volume visualization techniques and propose their own approach.While they are not aiming to emphasize surfaces but rather certain voxel features, it is not directly comparable with our approach.They could be used together to emphasize features as well as surfaces in the volumes.
Even with the rapid development of GPU technology, realtime rendering of complex volumetric data with an unbiased physically based approach is not possible without additional steps such as denoising presented by Iglesias-Guietan et al. [17], super-resolution presented by Weiss et al. [51] or global illumination caching, such as radiance caching by Jarosz et al. [19], irradiance caching by Ribardieere et al. [40] and Khlebnikov et al. [24], and transmittance caching by Weber et al. [50].In our approach, we implemented irradiance caching for computing a global illumination volume, allowing us to use faster ray casting techniques to render the data while also displaying the isosurfaces.
There are several possible alternatives to our approach.The directional occlusion shading model [43] and its multidirectional extension [37] could be adapted for use with isosurfaces and a local shading model, although we would be severely limited by the illumination setup since these methods can only compute illumination in scenes with a single directional light source with a specific direction with respect to the camera.On the other hand, deep shadow maps [31] do not have such limitations, but again, only directed light sources can be simulated.Our approach has no such limitations.

Overview
We first present a baseline non-caching technique for combined volume and surface rendering.An outline of the technique is shown in Fig. 1.For every pixel, we first extract the isosurface depth and calculate local illumination, then we pass this information to the path tracing module.In the path tracing module, the rays are cast from the camera into the volume, but only the light contributions gathered up to the isosurface are taken into account and blended with the local isosurface illumination contribution.
Next, we present the caching technique, which is an extension of the non-caching technique.An outline of the caching technique is presented in Fig. 2. We compute the global illumination of each voxel with path tracing and store it in a global illumination volume.Both the global illumination volume and local illumination contribution are taken into account during the ray marching step when accumulating the illumination along the ray.
individual steps from both techniques are presented in the following subsections.

Isosurface depth extraction
The isosurface separates the inside and outside regions where V is greater than or less than isovalue ρ, respectively.Let S be the isosurface of the volume V such that for every s ∈ S V (s) = ρ.To display the isosurface with local illumination, only the visible part of the isosurface is needed, which we store in the depth buffer G.Each pixel G i j in the depth buffer stores the distance to the isosurface along a viewing ray through that pixel, where the value 0 corresponds to the nearest intersection n with the volume, and the value 1 corresponds to the farthest intersection f with the volume.We are essentially solving the following equation to find the G i, j : To compute the depths of individual pixels G i j , we implement a simple stochastic process, in which we iteratively store the nearest point on the ray belonging to the inside region by randomly selecting new points along the ray, closer to the camera than the currently stored one, and checking whether it still belongs to the inner region or not.A single iteration of this process is described by Algorithm 1. Unlike classic techniques used for isosurface extraction, such as ray marching, the described technique only requires to sample the volume once per iteration.This allows it to be performed parallel to rendering, giving quicker results and improving interactivity.

Algorithm 1: Single iteration of the stochastic isosurface depth extraction
foreach pixel p i j in the final image do r ← ray cast from c through pixel p i j ; n ← near intersection between r and V ; f ← far intersection between r and V ; ξ ← random value between 0 and G i j ;

Local illumination
For the local illumination of the isosurface, we used three different models: Lambert's, Phong's and Disney's BRDF [7], with the gradient of the volume as the normal vector.We use one directional light in our demonstration.For the non-caching variant of the technique, we extract the depth for each pixel G i j and use it to reconstruct the location of the isosurface inside the volume for the corresponding viewing ray.We sample the neighborhood of that location to compute the gradient and then, apply local illumination.For the caching variant of the technique, the isosurface location is computed during the ray marching, and the local illumination is computed in the same way as in the non-caching variant.

Global illumination
Light transport in a transparent medium is described by the radiative transfer equation [8].It is composed of four terms, describing the change in radiance L due to emission, absorption, out-scattering, and in-scattering of the light traveling through an infinitesimal volume at the point x in the direction ω: where σ a and σ s are the absorption and scattering coefficients, respectively, L e is the emission, and f p is the phase function, which describes the directional distribution of scattering.Integrating Eq. ( 2) along the direction ω up to the background at depth d gives the volume rendering equation, where the radiance contribution L o at every point x t = x−tω along the ray is weighted by the transmittance T : If we substitute the integral with a Monte Carlo estimate [12], we get the volumetric path tracing algorithm: where p(t) is an arbitrary probability density function along the ray.We generate the samples for the Monte Carlo simulation by first generating the free-flight paths of the photons from the camera to an interaction with the medium, simulating the absorption or scattering event, and then repeating until we hit a light source.For the purposes of analytical and unbiased sampling of the free-flight paths, we homogenize the medium by adding a fictitious component with a density σ n .The fictitious medium does affect light transport because it does not absorb light and exhibits perfect forward scattering: By adding the above equation to Eq. ( 2), we have to update the solution in Eq. ( 4) and the transmittance in Eq. ( 6) accordingly.By choosing σ n such that σ = σ a +σ s +σ n is a constant, the transmittance T (t) = e −σ d is analytically invertible, the free-flight distance sampling with an exponential probability density function p(t) = σ T (t) can be unbiased, and as a bonus, we can avoid transmittance evaluation in Eq. ( 7).This process, known as Woodcock tracking or delta tracking [52], was later generalized to arbitrary values of σ n by Galtier et al. [14].We used it in our work to compute the global illumination volume by starting the tracking in the voxels.

Composition
There is a difference in how the composition is done for the non-caching and caching variants of the technique.
In the non-caching variant, the composition is combined with the path tracing procedure; when a ray passes the isosurface, we access the precomputed local illumination of the isosurface at the path's origin pixel.The path is then redirected toward the light source.
Such an approach cannot be employed in the caching technique as the isosurface depth changes with the position of the camera, while the global illumination is computed independently of the camera position.To project the resulting global illumination volume on the screen and compute the isosurface, we employ ray marching with front-to-back alpha compositing [33], terminating the ray at the isosurface or once the accumulated opacity reaches a high value or the ray exits the bounding volume.

Evaluation and results
We evaluated the techniques on two volumes: a CT scan of the abdomen and pelvis 1 of size 512 × 512 × 174, and a CT scan of a rainbow wrasse 2 of size 198 × 470 × 432.The results were rendered in 1024 × 1024 pixel resolution.All  tests were performed on a desktop computer with an AMD Ryzen 5 3600X 6-Core processor, 16 GB of RAM, and an Nvidia Titan Xp graphics card with 12 GB of RAM.The prototype application was running in the Google Chrome web browser version 99.
We evaluated the proposed techniques using different local illumination models (Lambert, Phong, and Disney).The volumes were illuminated with a white directional light originating from a lower frontal corner of the scene.To evaluate the convergence of the proposed methods over time, we used four different metrics: PSNR, SSIM [49], LPIPS [55], and VMAF [29], with respect to a converged reference image, which we acquired separately for each of the three shading models using the non-caching variant after 30 min of rendering time.The convergence rates for both volumes are plotted in Fig. 3.Both variants show a similar steady convergence in the first 5 s after which it slows down.LPIPS and SSIM even show an advantage in the first few seconds compared to the non-caching variant.The graphs show that the caching variant converges after around 15-20 s, after which point the metrics show a high similarity to the reference image.We make up for the difference with interactivity, which is only available when using the caching variant.The graphs in Fig. 3 show that different shading models have only a limited impact on the metrics, with Phong shading performing the worst.
We measured the similarity between the rendering results acquired with the caching and non-caching variants of the technique.We used the same image similarity metrics as for the convergence rate evaluation: PSNR, SSIM, LPIPS, and VMAF.Similarly, we used three different local illumination models: Lambert, Phong, and Disney.The resulting graphs in Fig. 4 show high similarity between the images rendered with the two variants, which supports the claim that the caching variant is capable of producing good quality images while also enabling interactive use.
Additionally, we measured the frame rate for both variants.The frame rate ranges from 10 to 30 frames per second for both variants, depending on the data, the view, and the rendering settings.However, most of of the computation time in the caching variant is spent on the light transport simulation, which may be suspended after reaching sufficient convergence.According to Fig. 3, this happens after 15-20 s, after which point the light transport simulation can be suspended, leaving ray marching and local illumination as the only remaining computational efforts.Ray marching alone is capable of running at much higher frame rates, and we measured frame rates of up to 100 frames per second in our test cases.
To evaluate how the cache precision and size affect the final rendering, we compared results obtained using a fullresolution cache, and 32-bit precision floats with results obtained using a full-resolution cache and 16-bit precision floats, half-resolution cache and 32-bit precision floats and Fig. 3 Convergence rates of the two technique variants using different local illumination models in the first 30 s of rendering measured with different image similarity metrics and evaluated on two volumes: a a CT scan of a rainbow wrasse, and b a CT scan of the abdomen and pelvis.For the reference image in each of the above graphs, we used the converged result obtained with the respective variant of the rendering method and a corresponding local illumination model quarter-resolution cache and 32-bit precision floats on two volumes using PSNR and SSIM metrics as presented in Table 1.The results in Table 1 show that there is almost no difference between rendering outputs.
For qualitative evaluation, we present renderings of both technique variants using different local illumination models (Lambert, Phong, and Disney) after a 30-s run time.In Fig. 5, we show the images of the volumes we used for convergence evaluation and zoom-ins of the selected regions.In the top two rows, we show the resulting images of the abdomen and pelvis volume from left to right using: path tracing, noncaching variant using the Lambert, Phong, and Disney local illumination, followed by the caching variant of the presented technique for the same local illumination models.The bottom two rows show the same techniques applied to the volume of the rainbow wrasse.For the path-traced image, the transfer function was specifically set to emphasize the isosurface, where we used a high-opacity transfer function as a substitute for the isosurface.Note that this approach results only in an approximation of the isosurface and offers minimal control over its appearance.The user can still define the transfer function differently.We let the path tracing simulation run for 30 min for adequate convergence.We present and discuss the qualitative results in Sect. 5.
Next, we present the rendering results for a CT scan of a backpack 1 of size 512 × 280 × 374 in Fig. 6.We show the converged output of both technique variants and their difference image, which shows there are no major differences between the proposed technique variants.In Fig. 7, we show how the presented techniques converge in 30 s for the rainbow wrasse volume.In line with the results from Fig. 3, the images rendered with the cached variant catches up with the noncached variant after the first few seconds.
For qualitative comparison of the presented methods with path tracing, we present the CT scan of a head with upper torso 3 of size 512 × 512 × 460 in Fig. 8.In the figure, we show a comparison between the path-traced images and our approach together with the difference images between variants of our method and the path tracing.The difference images show that the selected isosurfaces are additionally emphasized and that the use of the Disney shading model adds additional detail to the isosurface.This illustrates that the desired surfaces are distinguishable better with the presented method than with volumetric path tracing, while still preserving global illumination in the volume.
The memory demands of the presented techniques are as follows: • unmodified path tracing requires 14 float values per pixel of the output image; 3 https://www.osirix-viewer.com/resources/dicom-image-library/.

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Content courtesy of Springer Nature, terms of use apply.Rights reserved.This makes the caching variant of the presented technique drastically more memory-demanding than the path tracing or non-caching variant of the presented technique.However, today's high-performance GPUs typically come with a high amount of memory, making this issue less severe.The size of the global illumination volume increases proportionally   We conducted further tests on data from various sources, as depicted in Fig. 9.The data include the Chameleon Dataset4 (top), acquired using Computed Tomography, the Hand dataset5 (middle), obtained with magnetic resonance imaging, and Virgo cluster simulation dataset6 (bottom).The first column shows volumetric path tracing results, the second shows the non-cached version of the presented method, and the third column shows the results of the cached version of the presented method.The results of each method are presented, highlighting both their benefits and limitations and are discussed in the following section.

Discussion
We must first point out that while all the presented methods in this paper are interactive (even path tracing), only the caching variant of our method retains the information even throughout the camera view changes, while the noncaching variant and path tracing methods start rendering from scratch. 7While path tracing and the non-caching variant of our method need to restart rendering after every parameter change-let that be the camera position or the isovalue-the cache is cleared only once the lighting conditions or the transfer function change, or when a new volume is loaded.The caching variant, however, takes a longer time to converge (see Figs. 3, 7).The illumination cache is updated while rendering; hence, no precomputation time is needed and no delay is experienced by the user.Consequently, the user can interact with the scene immediately after loading the volume or changing the transfer function, while illumination changes Our tests reveal comparable quality between the caching and non-caching variants of our method on the test volumes.While there are bigger differences at the start of the rendering (in the first 5 s), the results are later on par with each other (see Figs. 5, 7).Since memory consumption remains the single largest differentiating factor between the two variants, the decision about which variant to use can be resolved automatically by the application, given the available GPU memory and the size of the volume.
From Fig. 5, one can see that the shapes and their edges are much more pronounced when rendered with one of our techniques.More details can be seen on the isosurfaces in comparison with the path-traced image due to the local illumination.Both properties are beneficial for the perception of the structures within the volume, especially in cases where path tracing can substantially obscure them (see the structures inside the eye of the detailed view of the rainbow wrasse).The second row shows that the shadows are preserved with all techniques.Since shadows are essential for spatial per-ception, this means that our method is not inferior in this regard.Moreover, since the volume and isosurface illumination are stored separately, the method can easily be extended by adding support for enhancing either of these two contributions, which may be valuable in certain application scenarios.
While there are smaller differences between the results in Fig. 5, we can see a bit more pronounced differences between the caching and non-caching variants of the presented technique in the example shown in Fig. 6, where the caching variant exhibits notably less contrast in the semi-transparent regions than the non-caching variant.We suspect that this is a consequence of the compositing step, where we use ray marching, which may significantly underestimate the transmittance due to Jensen's inequality.There are no noticeable differences between the results using different resolutions of the global illumination volume, which could be caused by light leaking.
The comparison of the presented methods to traditional volumetric path tracing for rendering data from various domains highlights both their advantages and disadvantages.
With regards to the Chameleon dataset Fig. 9, top, the presented methods demonstrate a clear improvement over regular volumetric path tracing.The bones exhibit more distinct features in our results compared to volumetric path tracing, and these results could not be achieved even with extensive modification of the transfer function.
A similar pattern is observed for the Hand dataset Fig. Figure 9, middle, where our methods produce more pronounced tumor details compared to volumetric path tracing.The downside is that the cached variant of our method is not capable of preserving shadowing details, which are retained in both the non-cached version of our method and the traditional volumetric path tracing.
However, the Virgo cluster simulation dataset Fig. 9, bottom reveals a limitation of our method in rendering small objects with intricate shadow details.The non-cached version of our method provides the best depth perception, with distant objects occluded by the participating gas clouds.The structure of the gas clouds is best visible in both volumetric path tracing and the non-cached variant of our method, but is lost in the cached method due to the absence of shadows.

Conclusion
In this work, we presented two variants of a novel technique of combined volume and surface rendering, a non-caching and caching one, of which the latter enables real-time rendering whereas the former does not.We evaluated both techniques, showing their suitability for interactive use.The resulting images show that the quality of the approaches is comparable, with only minor differences in performance.Additionally, the caching variant allows for interaction with the camera without having to recompute the image from scratch.
As a future extension of the presented technique, we will consider support for multiple isosurface rendering.For complete global illumination, one could also extend the method to take into account the parts of the volume within the regions enclosed by the isosurface.One could adapt the technique to exploit different global illumination techniques such as diffusion and incorporate a different composition of volume and surface rendering.Furthermore, the whole rendering pipeline could be replaced with by combining a NeRFbased approach [34] designed for surface rendering and Deep Direct Volume Rendering [51] for volume rendering.Further improvements could also target isosurface extraction. 1.

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Fig. 1
Fig. 1 An overview of the non-caching technique for combined volume and surface rendering

Fig. 4
Fig. 4 Image similarity between the caching and non-caching variants with respect to rendering time, computed with different image similarity metrics

Fig. 5
Fig. 5 Converged renders of the CT scan of the abdomen and pelvis (1st row) its crop (2nd row), and a rainbow wrasse volume (3rd row) and its crop (last row).From left to right the images are showing: path traced image (1st column), non-caching variant with Lambert (2nd col-

Fig. 6 Fig. 7
Fig. 6 Comparison of CT scan of a backpack rendered using non-caching variant (left) and caching variant (middle) of the presented technique, and a difference between them (right)

Table 1
PSNR and SSIM metrics of rendering results using 16-bit precision, half-resolution, and quarter-resolution on the backpack and rainbow wrasse volumes