49 lines
2.9 KiB
TeX
49 lines
2.9 KiB
TeX
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\section{Miscellaneous topics}
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\label{sec:miscellaneous}
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\subsection{A word about color spaces}
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\label{sec:colorspaces}
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When using one of the downloadable release builds of Mitsuba, or a version
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that was compiled with the default settings, the renderer internally
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operates in \emph{RGB mode}: all computations are performed using a representation
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that is based on the three colors red, green, and blue.
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More specifically, these are the intensities of the red, green, and blue primaries
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defined by the sRGB standard (ITU-R Rec. BT. 709-3 primaries with a D65 white point).
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Mitsuba transparently converts all input data (e.g. textures) into this space before
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rendering. This is an intuitive default which yields fast computations and
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satisfactory results for most applications.
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Low dynamic range images exported using the \pluginref{ldrfilm} will be stored in a
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sRGB-compatible format that accounts for the custom gamma curves mandated by this
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standard. They should display as intended across a wide range of display devices.
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When saving high dynamic range output (e.g. OpenEXR, RGBE, or PFM), the computed radiance
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values are exported in a linear form (i.e. without having the sRGB gamma curve applied to it),
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which is the most common way of storing high dynamic range data.
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It is important to keep in mind that other applications may not support this
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``linearized sRGB'' space---in particular, the Mac OS preview currently
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does not display images with this encoding correctly.
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\subsubsection{Spectral mode}
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Some predictive rendering applications will require a more realistic space for
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interreflection computations. In such cases, Mitsuba can be switched to \emph{spectral mode}.
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This can be done by compiling it with the \code{SPECTRUM\_SAMPLES=}$n$ parameter
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(\secref{compiling}), where $n$ is usually between 15 and 30.
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Now, all input parameters are converted into color spectra with the specified
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number of discretizations, and the computation then proceeds using this space.
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The process of writing an output image works differently: when spectral output
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is desired (\pluginref{hdrfilm}, \pluginref{tiledhdrfilm}, and \pluginref{mfilm}
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support this), Mitsuba creates special image files with many color channels (one
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per spectral band).
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Generally, other applications will not be able to display these images. The Mitsuba GUI
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can be used to view them, however (simply drag \& drop an image onto the application).
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It is also possible to write out XYZ tristimulus values, in which case the spectral
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data is convolved with the CIE 1931 color matching curves. This is most useful to users
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who want to do their own color processing in a space with a wide gamut.
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Finally, sRGB output is still possible. However, the color processing used in this case
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is fairly na\"ive: out-of-gamut values are simply clipped. This is something that may
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be improved in the future (e.g. by making use of a color management library like \code{lcms2})
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