2017

Color Managing Rendered 3D CG Images
Images rendered from 3D scenes created in an application such as Maya are typically scene-referred linear
RGBA images in a floating-point format such as OpenEXR.
To composite these images in a scene-linear working space such as ACES, you typically need to use one of
the transforms in the primaries/ directory to convert from the native primaries, for example, from the Rec.
709 primaries.
If the linear CG images have incorrectly had a gamma applied but have not actually been tone-mapped,
you can use one of the transforms in the gamma/ directory to remove it first.
The alpha channel represents the pixel coverage and should remain unmodified by any transform.
Color Managing Matte Paintings
All compositing should be done in a scene-linear color space, and this includes composites with painted
matte backgrounds. However, matte paintings can be produced using different techniques, and these affect
how you should process the images.
First, you must ensure that the images are scene-linear:
■ If the image was created using a tone map for display as part of a color-managed workflow, then it is
probably scene-linear already. In addition, the primaries are probably correct for the chosen working
space, but you can change them if they are not.
■ If the image was not created using a tone map for display, then it is probably output-referred with
gamma-encoded values, for example, sRGB. To convert such an image to scene-linear, you must remove
its gamma and apply an inverse tone map. These two operations are often combined in one transform.
Ideally, you should use the inverse of the tone map that will be used for output and display. For example,
if you will be using the ACES RRT tone map, then you can use sRGB_to_ACES from the RRT+ODT/
directory as an inverse tone map. Alternatively, inversePhotoMap_gamma_2.4 from the primaries/
directory is a generic transform that works well in many cases.
■ It is also possible to receive a matte painting as a log-encoded image. In this case, use one of the transforms
in the film/ directory to convert it to scene-linear. See
Color Managing Images from Scanned Film (page
1314) for additional considerations.
Once the image is scene-linear, you can convert the primaries to your working space using one or more of
the transforms in the primaries/ directory. For example, if you converted the image to scene-linear ACES
and you want to work in scene-linear ProPhoto-RIMM, you can apply ACES_to_CIE-XYZ followed by
CIE-XYZ_to_ProPhoto-RIMM.
Color Managing Textures and Maps
3D CG elements should be rendered using a scene-linear working space. However, images that are used for
textures and other maps should be processed in various ways, depending on both the image state and the
type of map.
Material Colors
Maps that are used for material colors should be scene-linear, but restricted to a 0-to-1 range. This is because
they represent the proportion of light that is reflected or transmitted. These include diffuse, specular, and
ambient reflectance colors, as well as transparency color.
1316 | Chapter 25 Colour Management