In this paper, we describe a new wavelet-based data processing that performs simultaneously compression and fast multidimensional interpolation of hemispherical angular features. This algorithm is applied for display aspect simulation to allow predicting the rendering of any content on a given display under any  illuminations  conditions.  Such  rendering  is  based  on  a complex set of data composed of emitting display properties as a function of angle as well as its reflective behavior (BRDF). This record was migrated from the OpenDepot repository service in June, 2017 before shutting down

Boher, P

Collomb-Patton, V

Leroux, T

English

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Predicting Perceptions: Proceedings of the 3rd International Conference on Appearance, pp. 68-73, Edinburgh, UK, ISBN 978-1-4716-6869-2, April 2012. Predicting Perceptions: The 3rd International Conference on Appearance, 17-19 April, 2012, Edinburgh, UK. Conference Proceedings Publication ISBN: 978-1-4716-6869-2,  Pages: 68-73 @2012 Authors & Predicting Perceptions. All Rights Reserved.  Wavelet-Based Processing of Angular Measurements: Application to Realistic Display Aspect Simulation   V Collomb-Patton ELDIM R&D engineer 1185, ancienne route d’epron 14200 Herouville - France (+33) 2 31 02 76 94 00 vcollomb-patton@eldim.fr  P Boher ELDIM R&D manager 1185, ancienne route d’epron 14200 Herouville - France (+33) 2 31 02 76 94 00 pboher@eldim.fr  T Leroux ELDIM CEO 1185, ancienne route d’epron 14200 Herouville - France (+33) 2 31 02 76 94 00 tleroux@eldim.fr    ABSTRACT In this paper, we describe a new wavelet-based data processing that performs simultaneously compression and fast multidimensional interpolation of hemispherical angular features. This algorithm is applied for display aspect simulation to allow predicting the rendering of any content on a given display under any  illuminations  conditions.  Such  rendering  is  based  on  a complex set of data composed of emitting display properties as a function of angle as well as its reflective behavior (BRDF).  General Terms Algorithms, Measurement, Performance, Design, Human Factors.  Keywords Display  aspect  simulation,  viewing  angle,  wavelet  transform, compression, multidimensional data interpolation, Tree diagram geodesic sphere, Fourier optics, BRDF.   1.  INTRODUCTION Nowadays  flat  panel  displays  consumers  are  more  and  more skilled  in  technology  and  demanding.  It  becomes  obvious  for anyone now that any flat panel display absolutely has to be tested under realistic conditions what means using it in the final place it is made for: outside ambient for mobile phone, in place in dashboard    for    embedded    displays    on    cars    or    plane. Characterization in darkroom has now proven its insufficiency.  In this paper we will describe a software processing tool that uses measured data to simulate display rendering under realistic conditions, taking into account most of the display imperfections as well as environmental inescapable factors (sun light, dashboard shape). This is useful for color/contrast quality assessment and provides a wise help for display design.  Very few papers tackle the subject of image quality of displays under physico-realistic conditions. Farrell [1] proposed an evaluation of image quality from display simulation using three functions: the intensity response, the spectral response and the point spread response.  However results are presented on axis only and don’t take into account the specific non-uniform angular response in the case of flat panel displays. They cannot be used for the simulation of the display aspect for a user positioned anywhere  in  front  of  the  display  -meaning  each  pixel  of  the display is seen from a different (θ,φ) couple of angles-.          The angular behavior of a display is a key feature of its overall performances. ELDIM has introduced the Fourier optic instrument twenty  years  ago  to  allow  fast  and  accurate  angular characterization in terms of luminance, but also contrast, color and color purity [2-3]. Collecting such data makes it possible to simulate the display aspect for various positions of the observer (cf. fig 1). A first approach has been proposed in [4-5] analyzing the homogeneity of a display switched on in a uniform white. The usefulness of cartography of viewing angle measurements along the display has also been discussed.  Our concern here is about the screen aspect while displaying a given  content  (not  simply  full  ON/OFF  state  but  a  full  color image) and giving consideration to ambient light in order to have a realistic outlook of a given image.  For the emissive behavior of the display, we therefore need not only ON/OFF state angular data but also R, G, B angular behaviors at multiple grey levels. To tackle the physic-realistic part of the simulation we need also to take into account the BRDF of the display surface. Defining a given ambient we can then simulate the unwanted ambient reflections  that  corrupt  contrast  and  color.  A  little  difference occurs here since for the emissive part we can only focus on X, Y, Z  components  of  each  R,  G,  B  primaries.  In  the  case  of reflectance we have to consider it as a function of wavelength since the reflective phenomenon varies strongly with λ. Because of the required amount of data we use wavelet-based tree decomposition.   Data   are   thus   strongly   compressed   while remaining easily and  quickly accessible  for use  in  ray tracing software.   2.  MEASUREMENT TECHNIQUE Fourier optics based viewing angle instruments (fig 2.) have long been  recognized  as powerful tools to  measure these  important characteristics. Last generation  of instruments are now able to measure the luminance and the color up to ±88° for all the azimuths on large spot size up to 6mm. Fourier optics has the capacity to transform the angular response in spatial information that  can  be  imaged  by  a  2D  sensor  (cf.  fig  3).  The  optical mounting patented by ELDIM allows adjusting the measurement spot   size   independently   than   the   angular   aperture.   The consequence is that the measurement spot size varies with the angle.  It  is  the  so-called  “cosine  compensation”.  This  unique feature allows getting a system transmittance above 60% even for very grazing angles. ELDIM range of equipment includes a reflective option that allows measuring the display BRDF [6].   3.  MEASUREMENT EXAMPLE As a first approximation we use here measurements at the center of  the  display  only  and  suppose  a  good  homogeneity  of  the angular behavior on the display surface. Since we have to evaluate Predicting Perceptions: Proceedings of the 3rd International Conference on Appearance, pp. 68-73, Edinburgh, UK, ISBN 978-1-4716-6869-2, April 2012. separately R, G and B sub pixels, we need to use a big enough spot size to ensure accurate measurement: at least ten of ON State sub-pixel  elements  need  to  be  included  in  the  spot  size.  The display is measured in Red, Green and Blue and this for different grey levels (for example {0, 31, 63, 95, 127, 159, 191, 223, 255}). We can see on fig 6. some important variations versus grey level. Data  have  obviously  to  be  compensated  for  black  state  what require also a measurement for a black pattern.  The reflective display properties are measured with the same instrument  including the reflective option. The display is  then illuminated  across  the  Fourier  optics  with  a  white  collimated beam along the desired incoming direction and the measured reflectance  is  normalized  using  a  measurement  on  a  reference white in the same illumination conditions. Measurement are made for different incidence angles and wavelength (cf fig 7) and results are assumed azimuth independent (isotropy of the display).   4.  WAVELET-BASED MODEL Many attempts have been done to build accurate models that correctly render angular reflective behavior of materials. None of them is universal enough to fit simultaneously any display emissions and display reflectances (specular, diffuse, haze…). That’s why we choose to follow the work of L. Claustres [7] that had to cope with the same issues for the BRDF of surfaces in realistic scene rendering. Instead of choosing a model that is material-dependent  and  requires  to  be  able  to  cope  with  the inverse problem (in order to identify the parameters of the model), we decide to start directly from the data and represent them by their wavelet coefficients allowing to compress strongly the data.   4.1  Angular wavelets A large amount of work has been published on wavelet transform since  1975  when  Morlett  has  proposed  at  first  his  time-scale representation which was shortly applied by Mallat to multi-level analysis. Many wavelets have then been designed on R and extended to R² : Daubechies, Morlett., Adelson,   Battle-lemarie, Odegard   Angular   display   emission   and   display   BRDF   are however defined onto the sphere S instead of R². Data processing makes it possible to use previous advanced wavelet basis by shifting from R² to S (and back) but the work is tedious. An alternative option is to use specific wavelets defined on S: they are not many, all derived from the original Haar one but have proven to be of adequate use in our case.  We therefore propose to make a tree decomposition of the sphere using a recursive subdivision of triangles as described in figure 8. At  each  step  a  (father)  triangle  is  subdivided  into  4  small (children) triangles. This means that the number of triangles used to map the half sphere depends on the decomposition level as follows: Nb triangles = 4i+1 , where i is the level of decomposition. Figure 4 shows the number of triangles and the angular resolution as   a   function   of   the   decomposition   level.   At   a   given decomposition level, each leaf triangle corresponds to an entering or outgoing direction (aimed by the triangle center). The highest the subdivision of the sphere is, the smallest the deformation of the sphere is  and the best angular resolution  can be obtained. Figure 9 represents the decomposition level 0, 1, 2.  During the wavelet processing step –based on Haar multi-level data description-, each father triangle (step i) is evaluated as the mean of all its 4 children (step i+1). This average value is also stored in the central children whereas the “detail” information is stored in the other 3 children triangles as the difference between initial  value  and  average  value.  The  process  starts  from  the highest decomposition level and goes up recursively into the decomposition tree. If no compression is performed this allows to perform the inverse process to find back exactly the original value for each leaf triangle. Fig 5 shows an example of reconstruction from a wavelet compressed set of data at level 3.   4.2  Spectral Data As discussed previously any processing on BRDF requires taking care of the wavelength. For each incoming direction and outgoing direction we can therefore store a complete spectrum that can be compressed using a wavelet scheme. At that step, it is a well- known transform in the R space that is not correlated to the S transform. It can use any of the advanced wavelet basis previously mentioned.   4.3  Main key points The interest of this approach lies into:  A high compression efficiency: able to manage large data sets. Final files are 1% to 10% from initial ELDIM file (depending on the sphere subdivision level).  A multi-resolution approach: a reconstruction at different levels of accuracy is possible  The reconstruction efficiency: the reconstruction of the signal is done in a logarithmic time according to the number of samples. The major point is that data is reconstructed locally: the whole wavelet tree does not need to be inversed.  Easy interpolation: this processing allows interpolation of data simultaneously on entering and outgoing direction as well as on the spectral dimension.  Additional denoising: small and localized signal variations are automatically reduced by the compression   5.  CONCLUSION We have described the process used to compress and decompress the experimental data and explained its adequacy to display aspect simulation. The full software package is available for free for all potential users.   6.  REFERENCES [1]   J.  E.  Farrell  ,  “A  display  Simulation  Toolbox  for  image quality evaluation”, J. of Display Tech., vol. 4, 262, 2008 [2]   T. Leroux, ”Fast contrast vs viewing angle measurements of LCDs”, Eurodisplay, Strasbourg, 447 (1993) [3]   T. Leroux, C. Rossignol, “Fast analysis of LCD contrast and color coordinates”, SID, Orlando, 33.2, 739, 1995 [4]   P. Boher, Fall (2005). Study of LCD emission features yields clues to display aspect, , Display Devices [5]   P. Boher, V. Gibour, T.Leroux , December 6-9 (2005). Can we predict LCD aspect using local viewing angle measurement, , IDW, Takamatsu, Japan [6]   P. Boher, T. Leroux, “Optical characterization of e-papers using multispectral Fourier optics instrument”, 2nd CIE Expert Symposium on Appearance, 8-10 September, Ghent, Belgium (2010) [7]   L Claustres , “Wavelet-Based Modeling of Spectral BRDF Data”, Optical Engineering, Vol. 43, N° 10, 2327-2339, October 2004           Predicting Perceptions: Proceedings of the 3rd International Conference on Appearance, pp. 68-73, Edinburgh, UK, ISBN 978-1-4716-6869-2, April 2012.      Figure 1:  Display simulation parameters.     Figure 3: Schematic diagram of the ELDIM patented Fourier optics mounting for large angular aperture and independent control of the measurement spot size.          Figure 2: ELDIM Fourier optic equipment.         Figure 4:  Angular resolution and number of leaf triangles as a function of the sphere decomposition level  Figure 5:  White state luminance emission as measured (left), reconstructed with subdivision level 3 without interpolation (center) and with barycentric interpolation (right)   Predicting Perceptions: Proceedings of the 3rd International Conference on Appearance, pp. 68-73, Edinburgh, UK, ISBN 978-1-4716-6869-2, April 2012.         Figure 6: Viewing angle luminance measurements at six different levels for the red state.      Figure 7: BRDF measured on a display surface at 550nm for six different incidence angles (10, 20, 30, 40, 50 and 60°)                 Father   Children triangles      Figure 8: Subdivision of the triangles                                                          Figure 9: Hemispherical subdivision at level 0, 1 and 2 

Wavelet-Based Processing of Angular Measurements: Application to Realistic Display Aspect Simulation

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Wavelet-Based Processing of Angular Measurements: Application to Realistic Display Aspect Simulation

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