|FlashMob: Near-Instant Capture of High-Resolution Facial Geometry and Reflectance
|SIGGRAPH 2015 Technical Talks / ICT Technical Report #ICT-TR-01-2015
|Paul Graham Graham Fyffe Borom Tunwattanapong Abhijeet Ghosh*
|USC Institute for Creative Technologies Imperial College London*|
Figure 1: (a) Multi-view images shot under rapidly varying flash directions. (b) Refined geometry (c) Diffuse/specular maps (d) Rendering
Modeling realistic human characters is frequently done using 3D
recordings of the shape and appearance of real people across a
set of different facial expressions [Pighin et al. 1998; Alexander
et al. 2010] to build blendshape facial models. Believable characters
which cross the "Uncanny Valley" require high-quality geometry,
texture maps, reflectance properties, and surface detail at
the level of skin pores and fine wrinkles. Unfortunately, there
has not yet been a technique for recording such datasets which
is near-instantaneous and relatively low-cost. While some facial
capture techniques are instantaneous and inexpensive [Beeler
et al. 2010; Bradley et al. 2010], these do not generally provide
lighting-independent texture maps, specular reflectance information,
or high-resolution surface normal detail for relighting. In
contrast, techniques which use multiple photographs from spherical
lighting setups [Weyrich et al. 2006; Ghosh et al. 2011] do capture
such reflectance properties, but this comes at the expense of longer
capture times and complicated custom equipment.
In this paper, we present a near-instant facial capture technique
which records high-quality facial geometry and reflectance using
commodity hardware. We use a 24-camera DSLR photogrammetry
setup similar to common commercial systems1 and use six ring
flash units to light the face. However, instead of the usual process
of firing all the flashes and cameras at once, each flash is fired sequentially
with a subset of the cameras, with the exposures packed
milliseconds apart for a total capture time of 66ms, which is faster
than the blink reflex [Bixler et al. 1967]. This arrangement produces
24 independent specular reflection angles evenly distributed
across the face, allowing a shape-from-specularity approach to obtain
high-frequency surface detail. However, unlike other shapefrom-
specularity techniques, our images are not taken from the
same viewpoint. Hence, we compute an initial estimate of the facial
geometry using passive stereo, and then refine the geometry
using separated diffuse and specular photometric detail. The resulting
system produces accurate, high-resolution facial geometry and
reflectance with near-instant capture in a relatively low-cost setup.
The principal contributions of this work are:
- A near-instantaneous photometric capture setup for measuring the geometry and diffuse and specular reflectance of faces.
- A camera-flash arrangement pattern which produces evenlydistributed specular reflections over the face with a single photo per camera and fewer lighting conditions than cameras.
- A novel per-pixel separation of diffuse and specular reflectance using multiview color-space analysis and novel photometric estimation of specular surface normals for geometry refinement.
Hardware Setup and Capture Process:
Our capture setup is designed to record accurate 3D geometry with
both diffuse and specular reflectance information per pixel while
minimizing cost and complexity and maximizing the speed of capture.
In all, we use 24 entry-level DSLR cameras and a set of six
ring flashes arranged on a gantry seen in Fig. 2.
Camera and Flash Arrangement The capture rig consists of
24 Canon EOS 600D entry-level consumer DSLR cameras, which
record RAW mode digital images at 5202 x 3565 pixel resolution.
Using consumer cameras instead of machine vision video cameras
dramatically reduces cost, as machine vision cameras of this
resolution are very expensive and require high-bandwidth connections
to dedicated capture computers. But to keep the capture nearinstantaneous,
we can only capture a single image with each camera,
as these entry-level cameras require at least 1/4 second before
taking a second photograph.
Since our processing algorithm determines fine-scale surface detail
from specular reflections, we wish to observe a specular highlight
from the majority of the surface orientations of the face. We tabulated
the surface orientations for four scanned facial models and
found, not surprisingly, that over 90% of the orientations fell between
±90° horizontally and ±45° vertically of straight forward
(Fig. 3). Thus, we arrange the flashes and cameras to create specular
highlights for an even distribution of normal directions within
this space as seen in Fig. 4.
Figure 2: Facial capture setup, consisting of 24 entry-level DSLR
cameras and six diffused ring flashes, all one meter from the face.
A set of images taken with this arrangement can be seen in Fig. 1
Figure 3: Surface normal distributions for four faces, covering
ears, forehead, and the front of the neck. The extents of the dotted
rectangles are ±90° horizontally by ±45° vertically, each containing
more than 90% of the normals.
One way to achieve this distribution
would be to place a ring flash on the lens of every camera and position
the cameras over the ideal distribution of angles. Then, if each
camera fires with its own ring flash, a specular highlight will be observed
back in the direction of each camera. However, this requires
shooting each camera with its own flash in succession, lengthening
the capture process and requiring many flash units. Instead, we
leverage the fact that position of a specular highlight depends not
just on the lighting direction but also on the viewing direction, so
that multiple cameras fired at once with a flash see different specular
highlights according to the half-angles between the flash and
the cameras. Using this fact, we arrange the 24 cameras and six
diffused Sigma EM-140 ring flashes as seen in Fig. 5 to observe
24 specular highlights evenly distributed across the face. The colors
indicate which cameras (solid circles) fire with which of the six
flashes (dotted circles) to create observations of the specular highlights
on surfaces (solid discs). For example, six cameras to the
subject's left shoot with the "red" flash, four cameras shoot with
the "green" flash, and a single camera shoots when the purple flash
fires. In this arrangement, most of the cameras are not immediately
adjacent to the flash they fire with, but they create
along a half-angle which does point toward a camera which
is adjacent to the flash as shown in Fig. 6. The pattern of specular
reflection angles observed can be seen on a blue plastic ball in Fig.
4. While the flashes themselves release their light in less than 1ms,
the camera shutters can only synchronize to 1/200th of a second
(5ms). When multiple cameras are fired along with a flash, a time
window of 15ms is required since there is some variability in when
the cameras take a photograph. In all, with the six flashes, four of
which fire with multiple cameras, a total recording time of 66ms
(1/15th sec) is achieved as in Fig. 5(b). By design, this is a shorter
interval than the human blink reflex.
|Implementation Details The one custom component in our system
is a USB-programmable 80MHz Microchip PIC32 microcontroller
for triggering the cameras via the remote shutter release
input. The flashes are set to manual mode, full power, and are
triggered by their corresponding cameras via the "hot shoe". The
camera centers lie on a 1m radius sphere, framing the face using
inexpensive Canon EF 50mm f/1.8 II lenses. A checkerboard calibration
object is used to focus the cameras and to geometrically
calibrate the camera's intrinsic, extrinsic, and distortion parameters,
with reprojection errors of below a pixel. We also photograph
an X-Rite ColorChecker Passport to calibrate the flash color and
intensity. With the flash illumination, we can achieve a deep depth
of field at an aperture of f/16 with the camera at its minimal gain
of ISO 100 to provide well-focused images with minimal noise.
While the cameras have built-in flashes, these could not used due to
an Electronic Through-The-Lens (ETTL) metering process involving
short bursts of light before the main flash. Our ring flashes are
brighter and their locations are easily derived from the camera calibrations.
By design, there is no flash in the subject's line of sight,
and subjects reported no discomfort from the capture process.
Figure 4: (Left) 24 images shot with the apparatus of a shiny blue
plastic ball. (Right) All 24 images added together after being reprojected
onto the ball's spherical shape as seen from the front, showing
24 evenly-spaced specular reflections from the six flash lighting
conditions. The colored lines indicate which images correspond to
Figure 5: (a) Location of the flashes (dotted circles), cameras
(solid circles), and associated specular highlight half-angles (filled
dots). The subject faces the center. The colors are for illustration;
all flashes are the same white color. (b) The firing sequence for the
flashes (dotted lines) and camera exposures (solid strips).
Alternate Designs We considered other design elements for
the system including cross- and parallel-polarized lights and
flashes, polarizing beamsplitters for diffuse/specular separation,
camera/flash arrangements exploiting Helmholtz reciprocity for
stereo correspondence, or a floodlit lighting
condition with diffuse light from everywhere as employed in passive
capture systems. While these techniques offer specific advantages
for reflectance component separation, robust stereo correspondence,
and/or deriving a diffuse albedo map (from flood lit
illumination) respectively, we did not use them since each would
either require adding additional cameras and/or lights to the system
for reflectance acquisition, or not achieve reflectance separation/
estimation when employing flood lighting for acquisition.
Figure 6: Interleaved cameras and highlights: a subset of four
images taken with the apparatus. The first and third cameras fire
with the "red" flash, producing specular highlights at surface normals
pointing toward the first and second cameras. Likewise, the
second and fourth cameras fire with the "green" flash, producing
highlights at surface normals pointing toward the third and fourth
cameras. Left-to-right, the highlights progress across the face.
We employed our system to acquire a variety of subjects in differing
facial expressions. In addition to Figure 1, Figure 9 shows
the high-resolution geometry and several renderings under novel
viewpoint and lighting conditions using our method. The recovered
reflectance maps for one of the faces are shown in Fig. 10. Our
acquisition system produces geometric quality which is competitive
with more complex systems and reflectance maps not available
from single-shot methods.
2015 ICT Tech Report :