Redefining A in RGBA: Towards a Standard for Graphical 3D Printing

Advances in multimaterial 3D printing have the potential to reproduce various visual appearance attributes of an object in addition to its shape. Since many existing 3D file formats encode color and translucency by RGBA textures mapped to 3D shapes, RGBA information is particularly important for practical applications. In contrast to color (encoded by RGB), which is specified by the object's reflectance, selected viewing conditions and a standard observer, translucency (encoded by A) is neither linked to any measurable physical nor perceptual quantity. Thus, reproducing translucency encoded by A is open for interpretation. In this paper, we propose a rigorous definition for A suitable for use in graphical 3D printing, which is independent of the 3D printing hardware and software, and which links both optical material properties and perceptual uniformity for human observers. By deriving our definition from the absorption and scattering coefficients of virtual homogeneous reference materials with an isotropic phase function, we achieve two important properties. First, a simple adjustment of A is possible, which preserves the translucency appearance if an object is re-scaled for printing. Second, determining the value of A for a real (potentially non-homogeneous) material, can be achieved by minimizing a distance function between light transport measurements of this material and simulated measurements of the reference materials. Such measurements can be conducted by commercial spectrophotometers used in graphic arts. Finally, we conduct visual experiments employing the method of constant stimuli, and derive from them an embedding of A into a nearly perceptually uniform scale of translucency for the reference materials.Comment: 20 pages (incl. appendices), 20 figures. Version with higher quality images: https://cloud-ext.igd.fraunhofer.de/s/pAMH67XjstaNcrF (main article) and https://cloud-ext.igd.fraunhofer.de/s/4rR5bH3FMfNsS5q (appendix). Supplemental material including code: https://cloud-ext.igd.fraunhofer.de/s/9BrZaj5Uh5d0cOU/downloa

A framework for realistic 3D tele-immersion

Meeting, socializing and conversing online with a group of people using teleconferencing systems is still quite differ- ent from the experience of meeting face to face. We are abruptly aware that we are online and that the people we are engaging with are not in close proximity. Analogous to how talking on the telephone does not replicate the experi- ence of talking in person. Several causes for these differences have been identified and we propose inspiring and innova- tive solutions to these hurdles in attempt to provide a more realistic, believable and engaging online conversational expe- rience. We present the distributed and scalable framework REVERIE that provides a balanced mix of these solutions. Applications build on top of the REVERIE framework will be able to provide interactive, immersive, photo-realistic ex- periences to a multitude of users that for them will feel much more similar to having face to face meetings than the expe- rience offered by conventional teleconferencing systems

Pushing the Limits of 3D Color Printing: Error Diffusion with Translucent Materials

Accurate color reproduction is important in many applications of 3D printing, from design prototypes to 3D color copies or portraits. Although full color is available via other technologies, multi-jet printers have greater potential for graphical 3D printing, in terms of reproducing complex appearance properties. However, to date these printers cannot produce full color, and doing so poses substantial technical challenges, from the shear amount of data to the translucency of the available color materials. In this paper, we propose an error diffusion halftoning approach to achieve full color with multi-jet printers, which operates on multiple isosurfaces or layers within the object. We propose a novel traversal algorithm for voxel surfaces, which allows the transfer of existing error diffusion algorithms from 2D printing. The resulting prints faithfully reproduce colors, color gradients and fine-scale details.Comment: 15 pages, 14 figures; includes supplemental figure

Haemoglobin and size dependent constraints on swimbladder inflation in fish larvae

In developmental studies of fish species (especially physostomians) it could be demonstrated, that the lack of haemoglobin during larval and juvenile stages is a relatively common phenomenon. Generally it is linked with body translucency. In representatives of the families Galaxiidae, Osmeridae and Clupeidae, partly reared, partly observed immediately after being caught in the wild, it turned out, that this condition coincides with a considerable delay in swimbladder inflation. To determine the moment of its first inflation, larvae placed in a hermetic chamber were observed under a dissecting microscope. While lowering the pressure, the expanding swimbladder showed whether or not its content is really gaseous. The reason postulated to be responsible for the delayed inflation is that larvae lacking haemoglobin do not have the possibility of oxygen transport to their buoyancy organ by means of the blood. Apart of this, capillarity force calculations and body force estimations show that with decreasing size the constraints linked with surface tension increase overproportionally. While in larger sized larvae like trout we could demonstrate inflation by swallowing air, in species with small larvae this was not the case. Below a certain size, even in physostomians, the ductus pneumaticus is no alternative to the blood pathway for swimbladder inflation

Influence of thermal tempering processes on color characteristics of different monolithic computer-assisted design and computer-assisted manufacturing ceramic materials

The optical properties of dental restoration were influenced by the sintering parameters. This study investigated the effects of different tempering processes on optical properties of three monolithic Cad-Cam ceramics. 135 monolithic material bars (4 mm width, 14 mm length, 1.2 mm thickness) were prepared from yttria-stabilized tetragonal zirconia polycrystalline (inCoris TZI, I), zirconia-reinforced lithium silicate (Vita Suprinity, V), and lithium disilicate glass (e.max CAD, E) ceramics, with different tempering processes through slow (S), normal (N), and fast (F) cooling (n=15). The color appearance (?EW), translucency parameter (TP), contrast ratio (CR), and opalescence parameter (OP) were determined. ANOVA and Bonferroni?s multiple comparisons were determined for significant difference (?=0.05). The grain sizes were microscopically examined by scanning electron microscope. The phase transformation of zirconia was determined using X ray diffraction. The mean±sd of ?EW, TP, CR, OP were 74.15±0.46, 1.26±0.15, 0.977±0.006, 1.02±0.12 for IS; 74.00±0.83, 1.27±0.19, 0.977±0.007, 1.02±0.12 for IN; 74.44±0.64, 1.70±0.08, 0.965±0.003, 1.30±0.07 for IF; 73.35±1.32, 2.44±0.24, 0.958±0.006, 2.10±0.20 for VS; 66.37±0.88, 4.05±0.3, 0.911±0.010, 3.18±0.20 for VN; 67.02±0.65, 3.79±0.17, 0.919±0.006, 3.01±0.13 for VF; 60.01±0.30, 5.53±0.17, 0.821±0.006, 2.71±0.06 for ES; 60.18±0.23, 5.49±0.17, 0.822±0.006, 2.66±0.05 for EN; and 59.82±0.26, 5.36±0.06, 0.826±0.002, 2.64±0.07 for EF. The color parameters were significantly affected by type of materials, tempering processes, and their interactions (p<0.05). Phase transformation from t?m related with tempering procedure for zirconia. Rapid thermal tempering process of Y-TZP resulted in larger grain size and t?m phase transformation leading to higher translucency. To achieve optimum translucency, a fast thermal tempering process was suggested for inCoris TZI and IPS e.max CAD, whilst a normal tempering process was recommended for Vita Suprinity

Single-shot layered reflectance separation using a polarized light field camera

We present a novel computational photography technique for single shot separation of diffuse/specular reflectance as well as novel angular domain separation of layered reflectance. Our solution consists of a two-way polarized light field (TPLF) camera which simultaneously captures two orthogonal states of polarization. A single photograph of a subject acquired with the TPLF camera under polarized illumination then enables standard separation of diffuse (depolarizing) and polarization preserving specular reflectance using light field sampling. We further demonstrate that the acquired data also enables novel angular separation of layered reflectance including separation of specular reflectance and single scattering in the polarization preserving component, and separation of shallow scattering from deep scattering in the depolarizing component. We apply our approach for efficient acquisition of facial reflectance including diffuse and specular normal maps, and novel separation of photometric normals into layered reflectance normals for layered facial renderings. We demonstrate our proposed single shot layered reflectance separation to be comparable to an existing multi-shot technique that relies on structured lighting while achieving separation results under a variety of illumination conditions

Perception based heterogeneous subsurface scattering for film

Many real world materials exhibit complex subsurface scattering of light. This internal light interaction creates the perception of translucency for the human visual system. Translucent materials and simulation of the subsurface scattering of light has become an expected necessity for generating warmth and realism in computer generated imagery. The light transport within heterogenous materials, such as marble, has proved challenging to model and render. The current material models available to digital artists have been limited to homogeneous subsurface scattering despite a few publications documenting success at simulating heterogeneous light transport. While the publications successfully simulate this complex phenomenon, the material descriptions have been highly specialized and far from intuitive. By combining the measurable properties of heterogeneous translucent materials with the defining properties of translucency, as perceived by the human visual system, a description of heterogeneous translucent materials that is suitable for artist use in a film production pipeline can be achieved. Development of the material description focuses on integration with the film pipeline, ease of use, and reasonable approximation of heterogeneous translucency based on perception. Methods of material manipulation are explored to determine which properties should be modifiable by artists while maintaining the perception of heterogenous translucency

Colored fused filament fabrication

Fused filament fabrication is the method of choice for printing 3D models at low cost and is the de-facto standard for hobbyists, makers, and schools. Unfortunately, filament printers cannot truly reproduce colored objects. The best current techniques rely on a form of dithering exploiting occlusion, that was only demonstrated for shades of two base colors and that behaves differently depending on surface slope. We explore a novel approach for 3D printing colored objects, capable of creating controlled gradients of varying sharpness. Our technique exploits off-the-shelves nozzles that are designed to mix multiple filaments in a small melting chamber, obtaining intermediate colors once the mix is stabilized. We apply this property to produce color gradients. We divide each input layer into a set of strata, each having a different constant color. By locally changing the thickness of the stratum, we change the perceived color at a given location. By optimizing the choice of colors of each stratum, we further improve quality and allow the use of different numbers of input filaments. We demonstrate our results by building a functional color printer using low cost, off-the-shelves components. Using our tool a user can paint a 3D model and directly produce its physical counterpart, using any material and color available for fused filament fabrication

Role of sintered temperature and sintering time on spectral translucence of nano-crystal monolithic zirconia

Sintering process is accountable for aesthetic appearance of zirconia restoration. This study appraised the effect of different sintering procedure via sintered temperatures and sintering times on spectral translucence of monolithic zirconia. One hundred and thirty five monolithic zirconia specimens (width, length, thickness = 10, 20, 1.5 mm) were prepared from yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP, Ceramill®) and unintentionally divided into nine groups to be sintered at different temperatures [decreasing- (SD, 1350°C), regular- (SR, 1450°C), and increasing- (SI, 1550°C) sintering temperature] and different sintering times [shortening- (HS, 60 min), regular- (HR, 120 min), and prolong- (HP, 180 min) sintering time]. Spectral translucence was determined by using spectrophotometer and calculated for translucency parameter (TP). The surface topography and grain size were evaluated by using a scanning electron microscope (SEM). Crystalline structures of monoclinic (m) and tetragonal (t) phases were determined by using the X-ray diffraction (XRD). An analysis of variance (ANOVA) was used to determine for significant differences of translucence upon different sintering processes (?=0.05). The mean, standard deviation of TP were 3.22±0.12 for SRHP, 3.14±0.18 for SIHS, 3.04±0.17 for SRHR, 2.94±0.18 for SRHS, 2.93±0.17 for SIHR, 2.67±0.15 for SIHP, 1.91±0.17 for SDHP, 1.34±0.21 for SDHR and 0.10±0.01 for SDHS. Spectral translucence was significantly affected by altering sintering temperatures and holding times (p<0.05). Enlargement of grain size and increasing t?m phase metamorphosis related with upraising sintered temperatures and extending sintered holding times were signified. Altering sintering parameters affected spectral translucence of zirconia. Upraising sintered temperature to SR and prolonging sintering time to HP were advocated to enhance spectral translucence of nano-crystal monolithic zirconia, and advised to accomplished aesthetic appearance of restoration in clinical practice

Affordable spectral measurements of translucent materials

We present a spectral measurement approach for the bulk optical properties of translucent materials using only low-cost components. We focus on the translucent inks used in full-color 3D printing, and develop a technique with a high spectral resolution, which is important for accurate color reproduction. We enable this by developing a new acquisition technique for the three unknown material parameters, namely, the absorption and scattering coefficients, and its phase function anisotropy factor, that only requires three point measurements with a spectrometer. In essence, our technique is based on us finding a three-dimensional appearance map, computed using Monte Carlo rendering, that allows the conversion between the three observables and the material parameters. Our measurement setup works without laboratory equipment or expensive optical components. We validate our results on a 3D printed color checker with various ink combinations. Our work paves a path for more accurate appearance modeling and fabrication even for low-budget environments or affordable embedding into other devices