Publications

Combining Global and Local Virtual Lights for Detailed Glossy Illumination
Tomáš Davidovič, Jaroslav Křivánek, Miloš Hašan, Philipp Slusallek, Kavita Bala
SIGGRAPH Asia 2010
Accurately rendering glossy materials in design applications, where
previewing and interactivity are important, remains a major challenge. While many fast global
illumination solutions have been proposed, all of them work under limiting assumptions on the
materials and lighting in the scene. In the presence of
many glossy (directionally scattering) materials,
fast solutions either fail or degenerate
to inefficient, bruteforce simulations of the underlying light transport. In particular,
manylight algorithms are able to provide fast
approximations by clamping elements of the light transport matrix,
but they eliminate the part of the transport that contributes to accurate glossy appearance. In
this paper we introduce a solution that
separately solves for the global (lowrank, dense) and local (highrank, sparse) illumination
components. For the lowrank component we introduce visibility clustering and approximation, while
for the highrank component we introduce a local light technique
to correct for the missing illumination. Compared to competing
techniques we achieve superior gloss rendering in minutes, making
our technique suitable for applications such as industrial design and
architecture, where material appearance is critical.
[PDF]
[Supplemental PDF]
[PPTX]


Physical Reproduction of Materials with Specified Subsurface Scattering
Miloš Hašan, Martin Fuchs, Wojciech Matusik, Hanspeter Pfister, Szymon Rusinkiewicz
SIGGRAPH 2010
We investigate a complete pipeline for measuring, modeling, and fabricating
objects with specified subsurface scattering behaviors.
The process starts with measuring the scattering properties of
a given set of base materials, determining their radial
reflection and transmission profiles.
We describe a mathematical model that predicts the profiles of
different stackings of base materials, at arbitrary thicknesses.
In an inverse process, we can then specify a desired reflection
profile and compute a layered composite material that best
approximates it. Our algorithm efficiently searches the space of possible
combinations of base materials, pruning unsatisfactory states imposed by
physical constraints.
We validate our process by producing both homogeneous and heterogeneous composites fabricated
using a multimaterial 3D printer. We demonstrate reproductions that have
scattering properties approximating complex materials.
[PDF]
[slides]


Virtual Spherical Lights for ManyLight Rendering of Glossy Scenes
Miloš Hašan, Jaroslav Křivánek, Bruce Walter, Kavita Bala
SIGGRAPH Asia 2009
In this paper, we aim to lift the accuracy limitations of manylight algorithms by introducing a new
light type, the virtual spherical light (VSL). The illumination contribution of a VSL is computed over
a nonzero solid angle, thus eliminating the illumination
spikes that virtual point lights used in traditional manylight methods are notorious for. The VSL
enables application of manylight
approaches in scenes with glossy materials and complex illumination that could previously be rendered
only by much slower algorithms. By combining VSLs with the matrix rowcolumn sampling
algorithm, we achieve highquality images in one to four minutes,
even in scenes where path tracing or photon mapping take hours to
converge.
[PDF]
[PPTX]
[HLSL shader]


Automatic Bounding of Programmable Shaders for Efficient Global Illumination
Edgar VelázquezArmendáriz, Shuang Zhao, Miloš Hašan, Bruce Walter, Kavita Bala
SIGGRAPH Asia 2009
This paper describes a technique to automatically adapt programmable shaders for use in
physicallybased rendering algorithms. Programmable shading provides great flexibility and power
for creating rich local material detail, but only allows the material
to be queried in one limited way: point sampling. Physicallybased
rendering algorithms simulate the complex global ﬂow of light
through an environment but rely on higher level information about
the material properties, such as importance sampling and bounding,
to intelligently solve high dimensional rendering integrals.
We propose using a compiler to automatically generate interval
versions of programmable shaders that can be used to provide the
higher level query functions needed by physicallybased rendering
without the need for user intervention or expertise. We demonstrate
the use of programmable shaders in two such algorithms, multidimensional lightcuts and photon mapping,
for a wide range of scenes
including complex geometry, materials and lighting.
[PDF]


Tensor Clustering for Rendering ManyLight Animations
Miloš Hašan, Edgar VelázquezArmendáriz, Fabio Pellacini, Kavita Bala
EGSR 2008
Rendering animations of scenes with deformable objects,
camera motion, and complex illumination, including
indirect lighting and arbitrary shading, is a longstanding challenge.
Prior work has shown that complex lighting
can be accurately approximated by a large collection of point lights.
In this formulation, rendering of animation
sequences becomes the problem of efficiently shading many surface samples
from many lights across several
frames. This paper presents a tensor formulation of the animated manylight
problem, where each element of the
tensor expresses the contribution of one light to one pixel in one frame.
We sparsely sample rows and columns
of the tensor, and introduce a clustering algorithm to select a small number
of representative lights to efficiently
approximate the animation. Our algorithm achieves efficiency by reusing
representatives across frames, while
minimizing temporal flicker. We demonstrate our algorithm in a variety of
scenes that include deformable objects,
complex illumination and arbitrary shading and show that a surprisingly
small number of representative lights is
sufficient for high quality rendering. We believe out algorithm will
find practical use in applications that require
fast previews of complex animations.
[PDF]
[PPT]
[video]
[comparison video]
[diff video]


Matrix RowColumn Sampling for the ManyLight Problem
Miloš Hašan, Fabio Pellacini, Kavita Bala
SIGGRAPH 2007
Rendering complex scenes with indirect illumination, high dynamic
range environment lighting, and many direct light sources remains
a challenging problem. Prior work has shown that all these effects
can be approximated by many point lights. This paper presents
a scalable solution to the manylight problem suitable for a GPU
implementation. We view the problem as a large matrix of samplelight interactions;
the ideal final image is the sum of the matrix
columns. We propose an algorithm for approximating this sum by
sampling entire rows and columns of the matrix on the GPU using
shadow mapping. The key observation is that the inherent structure of the transfer matrix can be
revealed by sampling just a small
number of rows and columns. Our prototype implementation can
compute the light transfer within a few seconds for scenes with
indirect and environment illumination,
area lights, complex geometry
and arbitrary shaders. We believe this approach can be very useful
for rapid previewing in applications like cinematic and architectural
lighting design.
[PDF]
[PPT]


DirecttoIndirect Transfer for Cinematic Relighting
Miloš Hašan, Fabio Pellacini, Kavita Bala
SIGGRAPH 2006
This paper presents an interactive GPUbased system for cinematic
relighting with multiplebounce indirect illumination from a fixed
viewpoint. We use a deep framebuffer containing a set of view
samples, whose indirect illumination is recomputed from the direct
illumination on a large set of gather samples, distributed around the
scene. This directtoindirect transfer is a linear transform which is
particularly large, given the size of the view and gather sets. This
makes it hard to precompute, store and multiply with. We address
this problem by representing the transform as a set of sparse matrices
encoded in wavelet space. A hierarchical construction is used
to impose a wavelet basis on the unstructured gather cloud, and an
imagebased approach is used to map the sparse matrix computations to the GPU.
We precompute the transfer matrices using a hierarchical algorithm and a variation
of photon mapping in less than
three hours on one processor. We achieve highquality indirect illumination at
1020 frames per second for complex scenes with over
2 million polygons, with diffuse and glossy materials, and arbitrary
direct lighting models (expressed using shaders). We compute perpixel indirect
illumination without the need of irradiance caching
or other subsampling techniques.
[PDF]
[PPT]
[video]

Ph.D. Thesis

Matrix Sampling for Global Illumination
Miloš Hašan (advised by Kavita Bala)
Cornell, August 2009
Global illumination is the problem of rendering images by simulating the light transport
in a scene, also considering the interreflection of light between surfaces. One general
approach to global illumination that gained popularity during the last decade is the
manylight formulation, whose idea is to
approximate global illumination by many automatically generated virtual point lights. In
this thesis, we address two fundamental issues that arise with the manylight formulation:
scalability and generality. We present a new view of the manylight approach, by treating
it as a large matrix of lightsurface contributions. Our insight is that there is usually
a significant amount of structure and redundancy in the matrix; this suggests that only a
tiny subset of the elements might be needed for accurate reconstruction. First, we present
a scalable rendering algorithm that exploits this insight by sampling a small subset of matrix
rows and columns to reconstruct the image. This algorithm is very flexible in terms of the material
and light types it can handle, and achieves highquality rendering of complex scenes in several seconds
on consumerlevel graphics hardware. Furthermore, we extend this approach to render whole
animations, by considering a 3D tensor of lightsurface contributions over time. This allows
us to further decrease the necessary number of samples by exploiting temporal coherence. We
also address a longstanding limitation of all previous manylight approaches that leads to
fundamentally incorrect results in scenes with glossy materials, by introducing a new virtual
light type that does not have this limitation. Finally, we describe an algorithm that computes
a waveletcompressed approximation to the lighting matrix, which allows for interactive light
placement in a scene with global illumination.
[link]

Other

Interactive Cinematic Relighting with Global Illumination
Fabio Pellacini, Miloš Hašan, Kavita Bala
Chapter 9, GPU Gems 3


Volume Rendering of Dosimetric Distribution and Biological Response
from 3D/4D Treatment and Delivery
Miloš Hašan, Hanspeter Pfister, George Chen, John Wolfgang
Appears in the 2010 Annual Meeting of the American Association of Physics in Medicine (AAPM)


Interactive 4D Visualization of Radiological Path Length Variation
for Proton Treatment Port Selection
Miloš Hašan, Hanspeter Pfister, George Chen, John Wolfgang
Poster in the 2010 Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO)


Realtime Hardwareaccelerated Relighting with Approximate Indirect Illumination
Miloš Hašan, Fabio Pellacini, Kavita Bala
Cornell CIS Technical Report TR20051999
[PDF]


An Efficient Frep Visualization Framework
Miloš Hašan, Alexander Pasko, Andrej Ferko
Undergraduate thesis, Comenius University
[available by email]

