Config syntax¶
OpenColorIO is primarily controlled by a central configuration file,
usually named config.ocio
. This page will only describe how to
syntactically write this OCIO config - e.g. what transforms are
available, or what sections are optional.
This page alone will not help you to write a useful config file! See the Configurations section for examples of complete, practical configs, and discussion of how they fit within a facilities workflow.
YAML basics¶
This config file is a YAML document, so it is important to have some basic knowledge of this format.
The Wikipedia article on YAML has a good overview.
OCIO configs typically use a small subset of YAML, so looking at existing configs is probably the quickest way to familiarise yourself (just remember the indentation is important!)
Checking for errors¶
Use the ociocheck
command line tool to validate your config. It
will inform you of YAML-syntax errors, but more importantly it
performs various OCIO-specific “sanity checks”.
For more information, see the overview of ociocheck
Config sections¶
ocio_profile_version
¶
Required.
By convention, the profile starts with ocio_profile_version
.
This is an integer, specifying which version of the OCIO config syntax is used.
Currently there is only one OCIO profile version, so the value is
always 1
(one)
ocio_profile_version: 1
search_path
¶
Optional. Default is an empty search path.
search_path
is a colon-separated list of directories. Each
directory is checked in order to locate a file (e.g. a LUT).
This works is very similar to how the UNIX $PATH
env-var works for
executables.
A common directory structure for a config is:
config.ocio
luts/
lg10_to_lnf.spi1d
lg10_to_p3.3dl
For this, we would set search_path
as follows:
search_path: "luts"
In a colorspace definition, we might have a FileTransform which refers
to the LUT lg10_to_lnf.spi1d
. It will look in the luts
directory, relative to the config.ocio
file’s location.
Paths can be relative (to the directory containing config.ocio
),
or absolute (e.g. /mnt/path/to/my/luts
)
Multiple paths can be specified, including a mix of relative and
absolute paths. Each path is separated with a colon :
search_path: "/mnt/path/to/my/luts:luts"
Finally, paths can reference OCIO’s context variables:
search_path: "/shots/show/$SHOT/cc/data:luts"
This allows for some clever setups, for example per-shot LUT’s with fallbacks to a default. For more information, see the examples in Looks
strictparsing
¶
Optional. Valid values are true
and false
. Default is true
(assuming a config is present):
strictparsing: true
OCIO provides a mechanism for applications to extract the colorspace
from a filename (the parseColorSpaceFromString
API method)
So for a file like example_render_v001_lnf.0001.exr
it will
determine the colorspace lnf
(it being the right-most substring
containing a colorspace name)
However, if the colorspace cannot be determined and strictparsing:
true
, it will produce an error.
If the colorspace cannot be determined and strictparsing: false
,
the default role will be used. This allows unhandled images to operate
in “non-color managed” mode.
Application authors should note: when no config is present (e.g. via
$OCIO
), the default internal profile specifies
strictparsing=false
, and the default color space role is
raw
. This means that ANY string passed to OCIO will be parsed as
the default raw
. This is nice because in the absence of a config,
the behavior from your application perspective is that the library
essentially falls back to “non-color managed”.
luma
¶
Deprecated. Optional. Default is the Rec.709 primaries specified by the ASC:
luma: [0.2126, 0.7152, 0.0722]
These are the luminance coefficients, which can be used by OCIO-supporting applications when adjusting saturation (e.g. in an image-viewer when displaying a single channel)
Note
While the API method is not yet officially deprecated, luma
is
a legacy option from Imageworks’ internal, closed-source
predecessor to OCIO.
The luma
value is not respected anywhere within the OCIO
library. Also very few (if any) applications supporting OCIO will
respect the value either.
roles
¶
Required.
A “role” is an alias to a colorspaces, which can be used by applications to perform task-specific color transforms without requiring the user to select a colorspace by name.
For example, the Nuke node OCIOLogConvert: instead of requiring the
user to select the appropriate log colorspace, the node performs a
transform between scene_linear
and compositing_log
, and the
OCIO config specifies the project-appropriate colorspaces. This
simplifies life for artists, as they don’t have to remember which is
the correct log colorspace for the current project - the
OCIOLogConvert always does the correct thing.
A typical role definition looks like this, taken from the spi-vfx example configuration:
roles:
color_picking: cpf
color_timing: lg10
compositing_log: lgf
data: ncf
default: ncf
matte_paint: vd8
reference: lnf
scene_linear: lnf
texture_paint: dt16
All values in this example (such as cpf
, lg10
and ncf
)
refer to colorspaces defined later the config, in the colorspaces
section.
A description of all roles. Note that applications may interpret or use these differently.
color_picking
- colors in a color-selection UI can be displayed in this space, while selecting colors in a different working space (e.g.scene_linear
ortexture_paint
)color_timing
- colorspace used for applying color corrections, e.g. user-specified grade within an image viewer (if the application uses theDisplayTransform::setDisplayCC
API method)compositing_log
- a log colorspace used for certain processing operations (plate resizing, pulling keys, degrain, etc). Used by the OCIOLogConvert Nuke nodedata
- used when writing data outputs such as normals, depth data, and other “non color” data. The colorspace in this role should typically havedata: true
specified, so no color transforms are applieddefault
- whenstrictparsing: false
, this colorspace is used as a fallback. If not defined, thescene_linear
role is usedmatte_paint
- colorspace which matte-paintings are created in (for more information, see the guide on baking ICC profiles for Photoshop, and spi-vfx)reference
- the colorspace against which the other colorspaces are definedscene_linear
- the scene-referred linear-to-light colorspace, often the same as the reference space (see:ref:faq-terminology)texture_paint
- similar tomatte_paint
but for painting textures for 3D objects (see the description of texture painting in SPI’s pipeline)
displays
¶
Required.
This section defines all the display devices which will be used. For example you might have a sRGB display device for artist workstations, a DCIP3 display device for the screening room projector.
Each display device has a number of “views”. These views provide different ways to display the image on the selected display device. Examples of common views are:
- “Film” to emulate the final projected result on the current display
- “Log” to send log-space pixel values directly to the display, resulting in a “lifted” image useful for checking black-levels.
- “Raw” when assigned a colorspace with
raw: yes
set will show the unaltered image, useful for tech-checking images
An example of the displays
section from the spi-vfx config:
displays:
DCIP3:
- !<View> {name: Film, colorspace: p3dci8}
- !<View> {name: Log, colorspace: lg10}
- !<View> {name: Raw, colorspace: nc10}
sRGB:
- !<View> {name: Film, colorspace: srgb8}
- !<View> {name: Log, colorspace: lg10}
- !<View> {name: Raw, colorspace: nc10}
- !<View> {name: Film, colorspace: srgb8}
All the colorspaces (p3dci8
, srgb8
etc) refer to colorspaces
defined later in the config.
Unless the active_displays
and active_views
sections are
defined, the first display and first view will be the default.
active_displays
¶
Optional. Default is for all displays to be visible, and to respect
order of items in displays
section.
You can choose what display devices to make visible in UI’s, and change the order in which they appear.
Given the example displays
block in the previous section - to make
the sRGB device appear first:
active_displays: [sRGB, DCIP3]
To display only the DCIP3
device, simply remove sRGB
:
active_displays: [DCIP3]
The value can be overridden by the OCIO_ACTIVE_DISPLAYS
env-var. This allows you to make the sRGB
the only active display,
like so:
active_displays: [sRGB]
Then on a review machine with a DCI P3 projector, set the following
environment variable, making DCIP3
the only visible display
device:
export OCIO_ACTIVE_DISPLAYS="DCIP3"
Or specify multiple active displays, by separating each with a colon:
export OCIO_ACTIVE_DISPLAYS="DCIP3:sRGB"
active_views
¶
Optional. Default is for all views to be visible, and to respect order of the views under the display.
Works identically to active_displays
, but controls which views are
visible.
Overridden by the OCIO_ACTIVE_VIEWS
env-var:
export OCIO_ACTIVE_VIEWS="Film:Log:Raw"
looks
¶
Optional.
This section defines a list of “looks”. A look is a color transform defined similarly to a colorspace, with a few important differences.
For example, a look could be defined for a “first pass DI beauty grade”, which is used to view shots with a rough approximation of the final grade.
When the look is defined in the config, you must specify a name, the color transform, and the colorspace in which the grade is performed (the “process space”). You can optionally specify an inverse transform for when the look transform is not trivially invertable (e.g. it applies a 3D LUT)
When an application applies a look, OCIO ensures the grade is applied in the correct colorspace (by converting from the input colorspace to the process space, applies the look’s transform, and converts the image to the output colorspace)
Here is a simple looks:
section, which defines two looks:
looks:
- !<Look>
name: beauty
process_space: lnf
transform: !<CDLTransform> {slope: [1, 2, 1]}
- !<Look>
name: neutral
process_space: lg10
transform: !<FileTransform> {src: 'neutral-${SHOT}-${SEQ}.csp', interpolation: linear }
inverse_transform: !<FileTransform> {src: 'neutral-${SHOT}-${SEQ}-reverse.csp', interpolation: linear }
Here, the “beauty” look applies a simple, static ASC CDL grade, making
the image very green (for some artistic reason!). The beauty look is
appied in the scene-linear lnf
colorspace (this colorspace is
defined elsewhere in the config.
Next is a definition for a “neutral” look, which applies a shot-specific CSP LUT, dynamically finding the correct LUT based on the SEQ and SHOT context variables.
For example if SEQ=ab
and SHOT=1234
, this look will search for
a LUT named neutral-ab-1234.csp
in locations specified in
search_path
.
The process_space
here is lg10
. This means when the look is
applied, OCIO will perform the following steps:
- Transform the image from it’s current colorspace to the
lg10
process space - Apply apply the FileTransform (which applies the grade LUT)
- Transform the graded image from the process space to the output colorspace
The “beauty” look specifies the optional inverse_transform
,
because in this example the neutral CSP files contain a 3D LUT. For
many transforms, OCIO will automatically calculate the inverse
transform (as with the “beauty” look), however with a 3D LUT the
inverse transform needs to be defined.
If the look was applied in reverse, and inverse_transform
as not
specified, then OCIO would give a helpful error message. This is
commonly done for non-invertable looks
As in colorspace definitions, the transform can be specified as a
series of transforms using the GroupTransform
, for example:
looks:
- !<Look>
name: beauty
process_space: lnf
transform: !<GroupTransform>
children:
- !<CDLTransform> {slope: [1, 2, 1]}
- !<FileTransform> {src: beauty.spi1d, interpolation: nearest}
colorspaces
¶
Required.
This section is a list of all the colorspaces known to OCIO. A colorspace can be referred to elsewhere within the config (including other colorspace definitions), and are used within OCIO-supporting applications.
to_reference
and from_reference
¶
Here is a example of a very simple colorspaces
section, modified
from the spi-vfx example config:
colorspaces:
- !<ColorSpace>
name: lnf
bitdepth: 32f
description: |
lnf : linear show space
- !<ColorSpace>
name: lg16
bitdepth: 16ui
description: |
lg16 : conversion from film log
to_reference: !<FileTransform> {src: lg16_to_lnf.spi1d, interpolation: nearest}
First the lnf
colorspace (short for linear float) is used as our
reference colorspace. The name can be anything, but the idea of a
reference colorspace is an important convention within OCIO: all
other colorspaces are defined as transforms either to or from this
colorspace.
The lg16
colorspace is a 16-bit log colorspace (see
spi-vfx for an explaination of this colorspace). It has a
name, a bitdepth and a description.
The lg16
colorspace is defined as a transform from lg16
to the
reference colorspace (lnf
). That transform is to apply the LUT
lg16_to_lnf.spi1d
. This LUT has an input of lg16
integers and
outputs linear 32-bit float values
Since the 1D LUT is automatically invertable by OCIO, we can use this
colorspace both to convert lg16
images to lnf
, and lnf
images to lg16
Importantly, because of the reference colorspace concept, we can
convert images from lg16
to the reference colorspace, and then on
to any other colorspace.
Here is another example colorspace, which is defined using
from_reference
.
- !<ColorSpace>
name: srgb8
bitdepth: 8ui
description: |
srgb8 :rgb display space for the srgb standard.
from_reference: !<FileTransform> {src: srgb8.spi3d, interpolation: linear}
We use from_reference
here because we have a LUT which transforms
from the reference colorspace (lnf
in this example) to sRGB.
In this case srgb8.spi3d
is a complex 3D LUT which cannot be
inverted, so it is considered a “display only” colorspace. If we did have a second 3D LUT to apply the inverse transform, we can specify both to_reference
and from_reference
- !<ColorSpace>
name: srgb8
bitdepth: 8ui
description: |
srgb8 :rgb display space for the srgb standard.
from_reference: !<FileTransform> {src: lnf_to_srgb8.spi3d, interpolation: linear}
to_reference: !<FileTransform> {src: srgb8_to_lnf.spi3d, interpolation: linear}
Using multiple transforms¶
The previous example colorspaces all used a single transform each, however it is often useful to use multiple transforms to define a colorspace.
- !<ColorSpace>
name: srgb8
bitdepth: 8ui
description: |
srgb8 :rgb display space for the srgb standard.
from_reference: !<GroupTransform>
children:
- !<ColorSpaceTransform> {src: lnf, dst: lg16}
- !<FileTransform> {src: lg16_to_srgb8.spi3d, interpolation: linear}
Here to get from the reference colorspace, we first use the
ColorSpaceTransform
to convert from lnf
to lg16
, then
apply our 3D LUT on the log-encoded images.
This primarily demonstrates the meta-transform GroupTransform
: a
transform which simply composes two or more transforms together into
one. Anything that accepts a transform like FileTransform
or
CDLTransform
will also accept a GroupTransform
It is also worth noting the ColorSpaceTransform
, which transforms
between lnf
and lg16
colorspaces (which are defined within the
current config).
Example transform steps¶
This section explains how OCIO internally applies all the transforms. It can be skipped over if you understand how the reference colorspace works.
colorspaces:
- !<ColorSpace>
name: lnf
bitdepth: 32f
description: |
lnf : linear show space
- !<ColorSpace>
name: lg16
bitdepth: 16ui
description: |
lg16 : conversion from film log
to_reference: !<FileTransform> {src: lg16.spi1d, interpolation: nearest}
- !<ColorSpace>
name: srgb8
bitdepth: 8ui
description: |
srgb8 :rgb display space for the srgb standard.
from_reference: !<GroupTransform>
children:
- !<ColorSpaceTransform> {src: lnf, dst: lg16}
- !<FileTransform> {src: lg16_to_srgb8.spi3d, interpolation: linear}
To explain how this all ties together to display an image, say we have
an image in the lnf
colorspace (e.g. a linear EXR) and wish to
convert it to srgb8
- the transform steps are:
ColorSpaceTransform
is applied, converting from lnf to lg16- The
FileTransform
is applied, converting from lg16 to srgb8.
A more complex example: we have an image in the lg16
colorspace,
and convert to srgb8
(using the lg16 definition from earlier, or
the spi-vfx config):
First OCIO converts from lg16 to the reference space, using the transform defined in lg16’s to_reference:
FileTransform
applies the lg16.spi1d
With the image now in the reference space, srgb8’s transform is applied:
- ColorSpaceTransform to transform from lnf to lg16
- FileTransform applies the
lg16_to_srgb8.spi3d
LUT.
Note
OCIO has an transform optimizer which removes redundant steps, and combines similar transforms into one operation.
In the previous example, the complete transform chain would be “lg16 -> lnf, lnf -> lg16, lg16 -> srgb8”. However the optimizer will reduce this to “lg16 -> srgb”.
bitdepth
¶
Optional. Default: 32f
Specify an appropriate bitdepth for the colorspace, and applications can use this to automatically output images in the correct bit-depth.
Valid options are:
8ui
10ui
12ui
14ui
16ui
32ui
16f
32f
The number is in bits. ui
stands for unsigned integer. f
stands for floating point.
Example:
- !<ColorSpace>
name: srgb8
bitdepth: 8ui
from_reference: [...]
isdata:
¶
Optional. Default: false. Boolean.
The isdata
key on a colorspace informs OCIO that this colorspace
is used for non-color data channels, such as the “normals” output of a
a multipass 3D render.
Here is example “non-color” colorspace from the spi-vfx config:
- !<ColorSpace>
name: ncf
family: nc
equalitygroup:
bitdepth: 32f
description: |
ncf :nc,Non-color used to store non-color data such as depth or surface normals
isdata: true
allocation: uniform
equalitygroup:
¶
Optional.
If two colorspaces are in the “equality group”, transforms between them are considered non-operations.
You might have multiple colorspaces which are identical, but operate at different bit-depths.
For example, see the lg10
and lg16
colorspaces in the
spi-vfx config. If loading a lg10
image and
converting to lg16
, no transform is required. This is of course
faster, but may cause an unexpected increase in precision (e.g. it skip
potential clamping caused by a LUT)
- !<ColorSpace>
name: lg16
equalitygroup: lg
bitdepth: 16ui
to_reference: !<FileTransform> {src: lg16.spi1d, interpolation: nearest}
- !<ColorSpace>
name: lg10
equalitygroup: lg
bitdepth: 10ui
to_reference: !<FileTransform> {src: lg10.spi1d, interpolation: nearest}
- Do not put different colorspaces in the same equality group. For
- logical grouping of “similar” colorspaces, use the
family
option.
family:
¶
Optional.
Allows for logical grouping of colorspaces within a UI.
For example, a series of “log” colorspaces could be put in one
“family”. Within a UI like the Nuke OCIOColorSpace
node, these
will be grouped together.
- !<ColorSpace>
name: kodaklog
family: log
equalitygroup: kodaklog
[...]
- !<ColorSpace>
name: si2klog
family: log
equalitygroup: si2klog
[...]
- !<ColorSpace>
name: rec709
family: display
equalitygroup: rec709
[...]
Unlike equalitygroup
, the family
has no impact on image
processing.
allocation
and allocationvars
¶
Optional.
These two options are used when OCIO transforms are applied on the GPU.
It is also used to automatically generate a “shaper LUT” when baking LUT’s unless one is explicitly specified (not all output formats utilise this)
For a detailed description, see How to Configure ColorSpace Allocation
Example of a “0-1” colorspace
allocation: uniform
allocationvars: [0.0, 1.0]
allocation: lg2
allocationvars: [-15, 6]
description
¶
Optional.
A human-readable description of the colorspace.
The YAML syntax allows for either single-line descriptions:
- !<ColorSpace>
name: kodaklog
[...]
description: A concise description of the kodaklog colorspace.
Or multiple-lines:
- !<ColorSpace>
name: kodaklog
[...]
description:
This is a multi-line description of the kodaklog colorspace,
to demonstrate the YAML syntax for doing so.
Here is the second line. The first one will be unwrapped into
a single line, as will this one.
It’s common to use literal |
block syntax to preserve all newlines:
- !<ColorSpace>
name: kodaklog
[...]
description: |
This is one line.
This is the second.
Available transforms¶
AllocationTransform
¶
Transforms from reference space to the range specified by the
vars:
Keys:
allocation
vars
direction
ExponentTransform
¶
Raises pixel values to a given power (often referred to as “gamma”)
!<ExponentTransform> {value: [1.8, 1.8, 1.8, 1]}
Keys:
value
direction
GroupTransform
¶
Combines multiple transforms into one.
colorspaces:
- !<ColorSpace>
name: adx10
[...]
to_reference: !<GroupTransform>
children:
- !<FileTransform> {src: adx_adx10_to_cdd.spimtx}
- !<FileTransform> {src: adx_cdd_to_cid.spimtx}
A group transform is accepted anywhere a “regular” transform is.
LookTransform
¶
Applies a named look
TruelightTransform
¶
Applies a transform from a Truelight profile.
Keys:
config_root
profile
camera
input_display
recorder
print
lamp
output_camera
display
cube_input
direction
Note
This transform requires OCIO to be compiled with the Truelight SDK present.