Gamma Correction of OIIO

\apiitem{"oiio:ColorSpace" : string}
	The name of the color space of the color channels. Values incude:
	\qkw{Linear}, \qkw{sRGB}, \qkw{GammaCorrected}, \qkw{AdobeRGB},
	\qkw{Rec709}, and \qkw{KodakLog}.
	\apiend
	\apiitem{"oiio:Gamma" : float}
	If the color space is \qkw{GammaCorrected}, this value is the gamma
	exponent.
	\apiend

The {\cf ImageSpec::extra_attribs} field may store metadata that reveals
	the color space the image file in the \qkw{oiio:ColorSpace}
	attribute, which may take on any of the following values:
	\begin{description}
	\item[\halfspc \rm \qkw{Linear}] indicates that the
	color pixel values are known to be linear.
	\item[\halfspc \rm \qkw{GammaCorrected}] indicates
	that the color pixel values (but not alpha or $z$) have
	already been gamma corrected (raised to the power $1/\gamma$), and
	that the gamma exponent may be found in the \qkw{oiio:Gamma} metadata.
	\item[\halfspc \rm \qkw{sRGB}] indicates that the
	color pixel values are in sRGB color space.
	\item[\halfspc \rm \qkw{AdobeRGB}] indicates that the
	color pixel values are in Adobe RGB color space.
	\item[\halfspc \rm \qkw{Rec709}] indicates that the
	color pixel values are in Rec709 color space.
	\item[\halfspc \rm \qkw{KodakLog}] indicates that the
	color pixel values are in Kodak logarithmic color space.
	\end{description}
	The \ImageInput sets the \qkw{oiio:ColorSpace} metadata in a
	purely advisory capacity --- the {\cf read} will not convert pixel
	values among color spaces. Many image file formats only support
	nonlinear color spaces (for example, JPEG/JFIF dictates use of sRGB).
	So your application should intelligently deal with gamma-corrected and
	sRGB input, at the very least.
	The color space hints only describe color channels. You should assume that
	alpha or depth ($z$) channels (designated by the {\cf alpha_channel} and
	{\cf z_channel} fields, respectively) always represent linear values and
	should never be transformed by your application.

spec.attribute("oiio::ColorSpace", "Linear");

spec.attribute("oiio::ColorSpace", "GammaCorrected");
spec.attribute("oiio::Gamma", (float)gamma);

spec.attribute("oiio::ColorSpace", "sRGB");
spec.attribute("oiio::ColorSpace", "Rec709");

----------------------------------------------------------------------------------------------------------

Textures

Applies an inverse gamma correction to texture lookups. For example, a value of 2.2 will linearize a texture that comes in with a gamma of 2.2. This is done for 8 and 16 bit images.

Arnold scans the contents of a texture (it does not rely on the extension) and anything that is not float is considered to have a gamma baked even if the image has a different color space embedded.

Tx texture format

You can convert your textures to linear space with the make tx tool. This way you can ensure that all of your textures are linear before rendering. For example:

maketx --colorconvert sRGB linear C:\solidangle\mtoadeploy\2014\bin\image.jpg