https://doi.org/10.2352/ISSN.2470-1173.2020.5.MAAP-033 © 2020, Society for Imaging Science and Technology Caustics and Translucency Perception Davit Gigilashvili, Lucas Dubouchet, Jon Yngve Hardeberg, Marius Pedersen; Department of Computer Science, Norwegian University of Science and Technology; Gjøvik, Norway; Abstract Caustics projected onto the surface carry very interesting in- formation regarding the material they are cast by. It has been ob- served in previous studies that caustics could be a widely used cue for translucency assessment by human subjects. We hypothesize that changing the reflectance properties of the surface an object is placed on, and removal of the caustic pattern might impact per- ceived translucency of the material. We conducted psychophysi- cal experiments to investigate the correlation among caustics, en- vironment colors and translucency perception, and found very in- Figure 1: The caustics might carry rich information regarding the teresting indications that materials appear less translucent under material properties. Even without looking at the objects them- the conditions where caustics are absent. selves, just by looking at the shadow and caustic pattern, we can deduce the color of the object, as well as some information about Introduction its light transmission properties. Illustration taken from [7]. According to Lynch [1], caustic is ”three dimensional en- velope of imperfectly focused rays” or ”two-dimensional pattern formed when a caustic falls on a surface.” According to Wand and Straßer, ”caustics occur if light is reflected (or refracted) at one or more specular surfaces, focused into ray bundles of a certain structure, and then received as patterns of light on a diffuse sur- face.” [2] As many translucent objects cast caustic patterns onto other surfaces, and particularly, onto the surfaces they are located on, we encounter this phenomenon on a daily basis - a glass of water projecting caustic pattern onto the table can be one of the simplest examples among many. It has been identified in the previous study [3] that caustics could be a significant cue for assessment of material subsurface light transport properties. The observers were asked to order objects from the Plastique [4] artwork collection. While many observers used translucency as a primary attribute for ordering, the caustic pattern cast through the object onto the white paper was widely used to assess translucency of the material. This phe- Figure 2: Caustics cast by sphere E is the only indicator that it is a nomenon is illustrated in Figure 1, where caustic is visible under translucent and non-opaque material. Illustration taken from [8]. translucent objects, while it is missing around the opaque one. In some cases, caustics can be the only cue for translucency translucent appearance, while Xiao et al. [10] extended the study perception. For instance, refer to Figure 2. While various cues to interactions among phase function, illumination directionality, provide information regarding light transmission properties of the and apparent translucency. spheres, caustics below sphere E is the only indicator that the ob- Rendering caustics in computer graphics is a computation- ject is not opaque. Moreover, indications have been found in [5] ally costly process. Although caustics might have negligible role that as the human visual system has proposedly limited ability to in some contexts, they play an important role in photorealism invert optics [6], and as many caustic patterns have high lumi- of some scenes [2, 11]. Kan´ and Kaufmann [12] have shown nance similar to specularities, internal and external caustics and that caustics increase perception of realism in augmented real- the glittering effect of the caustic highlights might be mistaken for ity, although it does not have the vital importance. However, Pa- specular highlights and thus, increase perceived glossiness. This padopoulos and Papaioannou [13] illustrate that caustics play a phenomenon is illustrated in Figure 3. very significant role in realistic appearance of underwater scenes. Little is known about the mechanisms of translucency per- While, on the one hand, some studies highlight importance ception, and factors contributing to that. Fleming and Bulthoff¨ of caustics in realistic appearance, and on the other hand, the stud- [6] proposed that translucency perception is a result of interpre- ies about translucency perception focus on the translucent object tation of simple image cues without inverting the underlying op- itself, to the best of our knowledge, no work has been done up- tics. Gkioulekas et al. [9] studied the role of phase function in to date to investigate the importance of the external caustics as a IS&T International Symposium on Electronic Imaging 2020 Material Appearance 033-1 even the roughest object has some degree of light transmission property. In this case, we expect relative judgement of translu- cency rather than an absolute one. Afterwards we had to render identical objects but without caustics in order to compare perceived translucency between the two setups. We considered six different ways of removing caus- tics: 1. Using a rendering technique that does not produce caustics (with caustics ”off”). However, the results would have been physically inaccurate. 2. Manually editing images in the graphics editor. This Figure 3: The caustics might contribute to glossiness perception. methodology will end in physically inaccurate and unreal- While the surface properties of all nine objects are identical, many istic results. subjects consider translucent ones more glossy, as caustics and back-reflected light are either mistaken for specular highlights, or 3. Rendering a fully opaque object of the identical shape, crop- increase total luminance and ”shininess” of the object. ping the translucent object, and placing into the render in place of the opaque object. This result is also physically inaccurate and unrealistic. cue for translucency perception. We have conducted a study to identify whether presence of caustics and the reflection properties 4. Varying refractive index that is directly correlated with the of the surface they are projected onto play any role in perceived caustics phenomenon. This is an interesting direction that translucency. The study revealed interesting trends that definitely we think of addressing in the future, but at this stage, we fo- deserve further follow-up in the future. cused on single material property for all test samples avoid- The paper is organized as follows: in the next section we ing an additional degree of freedom. present the experimental setup and stimuli generation process. Afterwards, the results are presented and discussed. Finally, we 5. Occluding caustics with other objects. The methodology outline the directions for the future work. is promising, but considering that we had to accommodate occlusion of objects with various shapes and sizes, judg- Experimental Setup ment of complex scenes that vary among images might have We conducted psychophysical experiments in order to ob- caused confusion among subjects, and also might be chal- serve whether presence or absence of caustics could impact per- lenging to interpret due to the unintended side-effects oc- ceived translucency of a given material. cluding objects bring into the scene. Stimuli 6. Making the floor most of the caustics are projected onto We rendered 30 images using Mitsuba Physically-based ren- fully absorbing black. Although we cannot remove inter- derer [14]. We used bidirectional path tracer to render glass ob- nal caustics this way, and some other cues are also impacted jects in 5 different shapes: sphere, cube, Stanford bunny, elephant, (e.g. lightness) in addition to caustics, the result is physi- and wineglass - all of them placed in the Cornell box. Each cally accurate, the scene structure remains the same (unlike object was rendered with 6 different degrees of transparency- the occlusion option), and understanding the impact from translucency. While intrinsic material properties remained the the surface color itself might have an application in the real same, translucency was manipulated using the alpha parameter, world. Therefore, we opted for the latter approach. which ”specifies the roughness of the unresolved surface micro- geometry using the root mean square (RMS) slope of the micro- Experimental Conditions facets” [14]. In other words, we manipulate light transmission We hypothesize that introduction of the black floor makes properties by changing surface scattering, while volume scatter- objects look less translucent. The example of the effects of the ing properties remain the same. The material property was loaded floor color is shown in Figure 6. In order to test the hypothesis, we from the .mtl material library. For each of the color channels, conducted an online user study (also referred to as ”psychophysi- ambient component was set to 0, while diffuse and specular com- cal experiment”) using QuickEval [15] web-based tool. We used ponents were set to 0.6 and 0.9, respectively. The refractive index category judgement psychometric scaling protocol, where ob- was set to 1.5. The alpha values were equidistantly sampled be- servers had to assign objects to one of the six categories vary- tween 0 and 1. ing from the most translucent to the least translucent, i.e. most The shapes are illustrated in Figure 4. The impact of the al- opaquish one. To facilitate decision-making for the observers, we pha value on the material appearance