forked from bartvdbraak/blender
455 lines
19 KiB
C
455 lines
19 KiB
C
/*
|
|
This source is published under the following 3-clause BSD license.
|
|
|
|
Copyright (c) 2012 - 2013, Lukas Hosek and Alexander Wilkie
|
|
All rights reserved.
|
|
|
|
Redistribution and use in source and binary forms, with or without
|
|
modification, are permitted provided that the following conditions are met:
|
|
|
|
* Redistributions of source code must retain the above copyright
|
|
notice, this list of conditions and the following disclaimer.
|
|
* Redistributions in binary form must reproduce the above copyright
|
|
notice, this list of conditions and the following disclaimer in the
|
|
documentation and/or other materials provided with the distribution.
|
|
* None of the names of the contributors may be used to endorse or promote
|
|
products derived from this software without specific prior written
|
|
permission.
|
|
|
|
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
|
|
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
|
|
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
|
|
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY
|
|
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
|
|
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
|
|
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
|
|
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
|
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
|
|
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
*/
|
|
|
|
|
|
/* ============================================================================
|
|
|
|
This file is part of a sample implementation of the analytical skylight and
|
|
solar radiance models presented in the SIGGRAPH 2012 paper
|
|
|
|
|
|
"An Analytic Model for Full Spectral Sky-Dome Radiance"
|
|
|
|
and the 2013 IEEE CG&A paper
|
|
|
|
"Adding a Solar Radiance Function to the Hosek Skylight Model"
|
|
|
|
both by
|
|
|
|
Lukas Hosek and Alexander Wilkie
|
|
Charles University in Prague, Czech Republic
|
|
|
|
|
|
Version: 1.4a, February 22nd, 2013
|
|
|
|
Version history:
|
|
|
|
1.4a February 22nd, 2013
|
|
Removed unnecessary and counter-intuitive solar radius parameters
|
|
from the interface of the colourspace sky dome initialisation functions.
|
|
|
|
1.4 February 11th, 2013
|
|
Fixed a bug which caused the relative brightness of the solar disc
|
|
and the sky dome to be off by a factor of about 6. The sun was too
|
|
bright: this affected both normal and alien sun scenarios. The
|
|
coefficients of the solar radiance function were changed to fix this.
|
|
|
|
1.3 January 21st, 2013 (not released to the public)
|
|
Added support for solar discs that are not exactly the same size as
|
|
the terrestrial sun. Also added support for suns with a different
|
|
emission spectrum ("Alien World" functionality).
|
|
|
|
1.2a December 18th, 2012
|
|
Fixed a mistake and some inaccuracies in the solar radiance function
|
|
explanations found in ArHosekSkyModel.h. The actual source code is
|
|
unchanged compared to version 1.2.
|
|
|
|
1.2 December 17th, 2012
|
|
Native RGB data and a solar radiance function that matches the turbidity
|
|
conditions were added.
|
|
|
|
1.1 September 2012
|
|
The coefficients of the spectral model are now scaled so that the output
|
|
is given in physical units: W / (m^-2 * sr * nm). Also, the output of the
|
|
XYZ model is now no longer scaled to the range [0...1]. Instead, it is
|
|
the result of a simple conversion from spectral data via the CIE 2 degree
|
|
standard observer matching functions. Therefore, after multiplication
|
|
with 683 lm / W, the Y channel now corresponds to luminance in lm.
|
|
|
|
1.0 May 11th, 2012
|
|
Initial release.
|
|
|
|
|
|
Please visit http://cgg.mff.cuni.cz/projects/SkylightModelling/ to check if
|
|
an updated version of this code has been published!
|
|
|
|
============================================================================ */
|
|
|
|
|
|
/*
|
|
|
|
This code is taken from ART, a rendering research system written in a
|
|
mix of C99 / Objective C. Since ART is not a small system and is intended to
|
|
be inter-operable with other libraries, and since C does not have namespaces,
|
|
the structures and functions in ART all have to have somewhat wordy
|
|
canonical names that begin with Ar.../ar..., like those seen in this example.
|
|
|
|
Usage information:
|
|
==================
|
|
|
|
|
|
Model initialisation
|
|
--------------------
|
|
|
|
A separate ArHosekSkyModelState has to be maintained for each spectral
|
|
band you want to use the model for. So in a renderer with 'num_channels'
|
|
bands, you would need something like
|
|
|
|
ArHosekSkyModelState * skymodel_state[num_channels];
|
|
|
|
You then have to allocate and initialise these states. In the following code
|
|
snippet, we assume that 'albedo' is defined as
|
|
|
|
double albedo[num_channels];
|
|
|
|
with a ground albedo value between [0,1] for each channel. The solar elevation
|
|
is given in radians.
|
|
|
|
for ( unsigned int i = 0; i < num_channels; i++ )
|
|
skymodel_state[i] =
|
|
arhosekskymodelstate_alloc_init(
|
|
turbidity,
|
|
albedo[i],
|
|
solarElevation
|
|
);
|
|
|
|
Note that starting with version 1.3, there is also a second initialisation
|
|
function which generates skydome states for different solar emission spectra
|
|
and solar radii: 'arhosekskymodelstate_alienworld_alloc_init()'.
|
|
|
|
See the notes about the "Alien World" functionality provided further down for a
|
|
discussion of the usefulness and limits of that second initalisation function.
|
|
Sky model states that have been initialized with either function behave in a
|
|
completely identical fashion during use and cleanup.
|
|
|
|
Using the model to generate skydome samples
|
|
-------------------------------------------
|
|
|
|
Generating a skydome radiance spectrum "skydome_result" for a given location
|
|
on the skydome determined via the angles theta and gamma works as follows:
|
|
|
|
double skydome_result[num_channels];
|
|
|
|
for ( unsigned int i = 0; i < num_channels; i++ )
|
|
skydome_result[i] =
|
|
arhosekskymodel_radiance(
|
|
skymodel_state[i],
|
|
theta,
|
|
gamma,
|
|
channel_center[i]
|
|
);
|
|
|
|
The variable "channel_center" is assumed to hold the channel center wavelengths
|
|
for each of the num_channels samples of the spectrum we are building.
|
|
|
|
|
|
Cleanup after use
|
|
-----------------
|
|
|
|
After rendering is complete, the content of the sky model states should be
|
|
disposed of via
|
|
|
|
for ( unsigned int i = 0; i < num_channels; i++ )
|
|
arhosekskymodelstate_free( skymodel_state[i] );
|
|
|
|
|
|
CIE XYZ Version of the Model
|
|
----------------------------
|
|
|
|
Usage of the CIE XYZ version of the model is exactly the same, except that
|
|
num_channels is of course always 3, and that ArHosekTristimSkyModelState and
|
|
arhosek_tristim_skymodel_radiance() have to be used instead of their spectral
|
|
counterparts.
|
|
|
|
RGB Version of the Model
|
|
------------------------
|
|
|
|
The RGB version uses sRGB primaries with a linear gamma ramp. The same set of
|
|
functions as with the XYZ data is used, except the model is initialized
|
|
by calling arhosek_rgb_skymodelstate_alloc_init.
|
|
|
|
Solar Radiance Function
|
|
-----------------------
|
|
|
|
For each position on the solar disc, this function returns the entire radiance
|
|
one sees - direct emission, as well as in-scattered light in the area of the
|
|
solar disc. The latter is important for low solar elevations - nice images of
|
|
the setting sun would not be possible without this. This is also the reason why
|
|
this function, just like the regular sky dome model evaluation function, needs
|
|
access to the sky dome data structures, as these provide information on
|
|
in-scattered radiance.
|
|
|
|
CAVEAT #1: in this release, this function is only provided in spectral form!
|
|
RGB/XYZ versions to follow at a later date.
|
|
|
|
CAVEAT #2: (fixed from release 1.3 onwards)
|
|
|
|
CAVEAT #3: limb darkening renders the brightness of the solar disc
|
|
inhomogeneous even for high solar elevations - only taking a single
|
|
sample at the centre of the sun will yield an incorrect power
|
|
estimate for the solar disc! Always take multiple random samples
|
|
across the entire solar disc to estimate its power!
|
|
|
|
CAVEAT #4: in this version, the limb darkening calculations still use a fairly
|
|
computationally expensive 5th order polynomial that was directly
|
|
taken from astronomical literature. For the purposes of Computer
|
|
Graphics, this is needlessly accurate, though, and will be replaced
|
|
by a cheaper approximation in a future release.
|
|
|
|
"Alien World" functionality
|
|
---------------------------
|
|
|
|
The Hosek sky model can be used to roughly (!) predict the appearance of
|
|
outdoor scenes on earth-like planets, i.e. planets of a similar size and
|
|
atmospheric make-up. Since the spectral version of our model predicts sky dome
|
|
luminance patterns and solar radiance independently for each waveband, and
|
|
since the intensity of each waveband is solely dependent on the input radiance
|
|
from the star that the world in question is orbiting, it is trivial to re-scale
|
|
the wavebands to match a different star radiance.
|
|
|
|
At least in theory, the spectral version of the model has always been capable
|
|
of this sort of thing, and the actual sky dome and solar radiance models were
|
|
actually not altered at all in this release. All we did was to add some support
|
|
functionality for doing this more easily with the existing data and functions,
|
|
and to add some explanations.
|
|
|
|
Just use 'arhosekskymodelstate_alienworld_alloc_init()' to initialise the sky
|
|
model states (you will have to provide values for star temperature and solar
|
|
intensity compared to the terrestrial sun), and do everything else as you
|
|
did before.
|
|
|
|
CAVEAT #1: we assume the emission of the star that illuminates the alien world
|
|
to be a perfect blackbody emission spectrum. This is never entirely
|
|
realistic - real star emission spectra are considerably more complex
|
|
than this, mainly due to absorption effects in the outer layers of
|
|
stars. However, blackbody spectra are a reasonable first assumption
|
|
in a usage scenario like this, where 100% accuracy is simply not
|
|
necessary: for rendering purposes, there are likely no visible
|
|
differences between a highly accurate solution based on a more
|
|
involved simulation, and this approximation.
|
|
|
|
CAVEAT #2: we always use limb darkening data from our own sun to provide this
|
|
"appearance feature", even for suns of strongly different
|
|
temperature. Which is presumably not very realistic, but (as with
|
|
the unaltered blackbody spectrum from caveat #1) probably not a bad
|
|
first guess, either. If you need more accuracy than we provide here,
|
|
please make inquiries with a friendly astro-physicst of your choice.
|
|
|
|
CAVEAT #3: you have to provide a value for the solar intensity of the star
|
|
which illuminates the alien world. For this, please bear in mind
|
|
that there is very likely a comparatively tight range of absolute
|
|
solar irradiance values for which an earth-like planet with an
|
|
atmosphere like the one we assume in our model can exist in the
|
|
first place!
|
|
|
|
Too much irradiance, and the atmosphere probably boils off into
|
|
space, too little, it freezes. Which means that stars of
|
|
considerably different emission colour than our sun will have to be
|
|
fairly different in size from it, to still provide a reasonable and
|
|
inhabitable amount of irradiance. Red stars will need to be much
|
|
larger than our sun, while white or blue stars will have to be
|
|
comparatively tiny. The initialisation function handles this and
|
|
computes a plausible solar radius for a given emission spectrum. In
|
|
terms of absolute radiometric values, you should probably not stray
|
|
all too far from a solar intensity value of 1.0.
|
|
|
|
CAVEAT #4: although we now support different solar radii for the actual solar
|
|
disc, the sky dome luminance patterns are *not* parameterised by
|
|
this value - i.e. the patterns stay exactly the same for different
|
|
solar radii! Which is of course not correct. But in our experience,
|
|
solar discs up to several degrees in diameter (! - our own sun is
|
|
half a degree across) do not cause the luminance patterns on the sky
|
|
to change perceptibly. The reason we know this is that we initially
|
|
used unrealistically large suns in our brute force path tracer, in
|
|
order to improve convergence speeds (which in the beginning were
|
|
abysmal). Later, we managed to do the reference renderings much
|
|
faster even with realistically small suns, and found that there was
|
|
no real difference in skydome appearance anyway.
|
|
Conclusion: changing the solar radius should not be over-done, so
|
|
close orbits around red supergiants are a no-no. But for the
|
|
purposes of getting a fairly credible first impression of what an
|
|
alien world with a reasonably sized sun would look like, what we are
|
|
doing here is probably still o.k.
|
|
|
|
HINT #1: if you want to model the sky of an earth-like planet that orbits
|
|
a binary star, just super-impose two of these models with solar
|
|
intensity of ~0.5 each, and closely spaced solar positions. Light is
|
|
additive, after all. Tattooine, here we come... :-)
|
|
|
|
P.S. according to Star Wars canon, Tattooine orbits a binary
|
|
that is made up of a G and K class star, respectively.
|
|
So ~5500K and ~4200K should be good first guesses for their
|
|
temperature. Just in case you were wondering, after reading the
|
|
previous paragraph.
|
|
*/
|
|
|
|
CCL_NAMESPACE_BEGIN
|
|
|
|
#ifndef _SKY_MODEL_H_
|
|
#define _SKY_MODEL_H_
|
|
|
|
typedef double ArHosekSkyModelConfiguration[9];
|
|
|
|
|
|
// Spectral version of the model
|
|
|
|
/* ----------------------------------------------------------------------------
|
|
|
|
ArHosekSkyModelState struct
|
|
---------------------------
|
|
|
|
This struct holds the pre-computation data for one particular albedo value.
|
|
Most fields are self-explanatory, but users should never directly
|
|
manipulate any of them anyway. The only consistent way to manipulate such
|
|
structs is via the functions 'arhosekskymodelstate_alloc_init' and
|
|
'arhosekskymodelstate_free'.
|
|
|
|
'emission_correction_factor_sky'
|
|
'emission_correction_factor_sun'
|
|
|
|
The original model coefficients were fitted against the emission of
|
|
our local sun. If a different solar emission is desired (i.e. if the
|
|
model is being used to predict skydome appearance for an earth-like
|
|
planet that orbits a different star), these correction factors, which
|
|
are determined during the alloc_init step, are applied to each waveband
|
|
separately (they default to 1.0 in normal usage). This is the simplest
|
|
way to retrofit this sort of capability to the existing model. The
|
|
different factors for sky and sun are needed since the solar disc may
|
|
be of a different size compared to the terrestrial sun.
|
|
|
|
---------------------------------------------------------------------------- */
|
|
|
|
typedef struct ArHosekSkyModelState
|
|
{
|
|
ArHosekSkyModelConfiguration configs[11];
|
|
double radiances[11];
|
|
double turbidity;
|
|
double solar_radius;
|
|
double emission_correction_factor_sky[11];
|
|
double emission_correction_factor_sun[11];
|
|
double albedo;
|
|
double elevation;
|
|
}
|
|
ArHosekSkyModelState;
|
|
|
|
/* ----------------------------------------------------------------------------
|
|
|
|
arhosekskymodelstate_alloc_init() function
|
|
------------------------------------------
|
|
|
|
Initialises an ArHosekSkyModelState struct for a terrestrial setting.
|
|
|
|
---------------------------------------------------------------------------- */
|
|
|
|
ArHosekSkyModelState * arhosekskymodelstate_alloc_init(
|
|
const double solar_elevation,
|
|
const double atmospheric_turbidity,
|
|
const double ground_albedo
|
|
);
|
|
|
|
|
|
/* ----------------------------------------------------------------------------
|
|
|
|
arhosekskymodelstate_alienworld_alloc_init() function
|
|
-----------------------------------------------------
|
|
|
|
Initialises an ArHosekSkyModelState struct for an "alien world" setting
|
|
with a sun of a surface temperature given in 'kelvin'. The parameter
|
|
'solar_intensity' controls the overall brightness of the sky, relative
|
|
to the solar irradiance on Earth. A value of 1.0 yields a sky dome that
|
|
is, on average over the wavelenghts covered in the model (!), as bright
|
|
as the terrestrial sky in radiometric terms.
|
|
|
|
Which means that the solar radius has to be adjusted, since the
|
|
emissivity of a solar surface with a given temperature is more or less
|
|
fixed. So hotter suns have to be smaller to be equally bright as the
|
|
terrestrial sun, while cooler suns have to be larger. Note that there are
|
|
limits to the validity of the luminance patterns of the underlying model:
|
|
see the discussion above for more on this. In particular, an alien sun with
|
|
a surface temperature of only 2000 Kelvin has to be very large if it is
|
|
to be as bright as the terrestrial sun - so large that the luminance
|
|
patterns are no longer a really good fit in that case.
|
|
|
|
If you need information about the solar radius that the model computes
|
|
for a given temperature (say, for light source sampling purposes), you
|
|
have to query the 'solar_radius' variable of the sky model state returned
|
|
*after* running this function.
|
|
|
|
---------------------------------------------------------------------------- */
|
|
|
|
ArHosekSkyModelState * arhosekskymodelstate_alienworld_alloc_init(
|
|
const double solar_elevation,
|
|
const double solar_intensity,
|
|
const double solar_surface_temperature_kelvin,
|
|
const double atmospheric_turbidity,
|
|
const double ground_albedo
|
|
);
|
|
|
|
void arhosekskymodelstate_free(
|
|
ArHosekSkyModelState * state
|
|
);
|
|
|
|
double arhosekskymodel_radiance(
|
|
ArHosekSkyModelState * state,
|
|
double theta,
|
|
double gamma,
|
|
double wavelength
|
|
);
|
|
|
|
// CIE XYZ and RGB versions
|
|
|
|
|
|
ArHosekSkyModelState * arhosek_xyz_skymodelstate_alloc_init(
|
|
const double turbidity,
|
|
const double albedo,
|
|
const double elevation
|
|
);
|
|
|
|
|
|
ArHosekSkyModelState * arhosek_rgb_skymodelstate_alloc_init(
|
|
const double turbidity,
|
|
const double albedo,
|
|
const double elevation
|
|
);
|
|
|
|
|
|
double arhosek_tristim_skymodel_radiance(
|
|
ArHosekSkyModelState * state,
|
|
double theta,
|
|
double gamma,
|
|
int channel
|
|
);
|
|
|
|
// Delivers the complete function: sky + sun, including limb darkening.
|
|
// Please read the above description before using this - there are several
|
|
// caveats!
|
|
|
|
double arhosekskymodel_solar_radiance(
|
|
ArHosekSkyModelState * state,
|
|
double theta,
|
|
double gamma,
|
|
double wavelength
|
|
);
|
|
|
|
|
|
#endif // _SKY_MODEL_H_
|
|
|
|
CCL_NAMESPACE_END
|