Perceptual rendering intent use case issues

Perceptual rendering intent use case issues
White Paper #2
Level: Advanced
Date: Jan 2005
Perceptual Rendering Intent Use Case Issues
The perceptual rendering intent is used when a pleasing pictorial color output is
desired. [A colorimetric rendering intent is used when an output is to be color
matched to its source image.] The perceptual rendering intent is most often used
to render photographs of scenes (i.e. views of the three-dimensional world), and
when the objective for a reproduction is to obtain the most attractive result on
some medium that is different from the original (i.e. re-purposing), rather than to
represent the original on the new medium (i.e. as in proofing or re-targeting).
Some level of color consistency is required - for example colors should not
change hue names. However, with perceptual rendering, if the reproduction
medium, for example, allows for greater chroma than the original medium, then
chroma may be increased to produce a more pleasing result. Likewise, if the
reproduction medium has a smaller color gamut than the original medium,
perceptual rendering may alter in-gamut colors to allow for graceful
accommodation of the original color gamut through gamut compression. In
comparison, colorimetric rendering maintains in-gamut colors across media at the
expense of sub-optimal colorfulness on larger gamut reproduction media and
clipping artifacts on smaller gamut reproduction media.
Keep in mind that the perceptual rendering intents in ICC profiles provide one
approach to perceptual color rendering or re-rendering. There are other ways.
Devices such as digital cameras and printers perform embedded (typically
proprietary) perceptual renderings to and from standard color encodings like
sRGB. In certain workflows, abstract ICC profiles can be used in combination
with a colorimetric rendering path through source and destination ICC profiles to
perform color re-rendering from source image colorimetry to destination image
colorimetry directly in the PCS, before transforming to the destination encoding.
Alternatively, a user may apply manual image editing techniques to optimize an
image for a particular output condition. Finally, a color management system
(CMS) may offer color rendering or re-rendering capabilities beyond that built into
any source and destination profiles.
‘Media-relative colorimetric plus black point compensation’ is a simple and widely
used perceptual rendering that uses the media-relative colorimetric rendering
intent in the source and destination ICC profiles, combined with black point
scaling performed by the CMS. Simple media white and black scaling can
accommodate differences in dynamic range between an original and a
reproduction and (to some extent) differences in color gamut size. In cases
where color gamut shapes are roughly similar, and gamut size differences
correlate with white and black point differences, media-relative colorimetric plus
black point compensation may produce excellent perceptual rendering. However,
this approach is not universally available because some CMSs do not support
black point compensation. In other cases, more elaborate perceptual transforms
are required to produce optimal results, especially when the source and
destination media are quite different. The inclusion of an explicit perceptual
rendering intent in ICC profiles enables well-defined, repeatable, and high quality
perceptual rendering across all ICC based color management systems.
Scene to Reproduction
Scene-to-reproduction perceptual rendering is discussed first because such color
rendering must happen first in the capture of natural scenes, and understanding
this transformation is helpful in understanding subsequent transformation
requirements. However, users should be aware that in typical digital camera
workflows scene-to-reproduction perceptual rendering is not accessible to user
control. Virtually all digital cameras perform scene-to-reproduction color
rendering in the camera. The image file output by the camera does not represent
the scene, but rather represents what the camera manufacturer feels will likely be
a pleasing reproduction of the scene. This reproduction typically includes
alterations of the scene colorimetry, including highlight compression, and midtone
contrast and colorfulness enhancements as discussed below.
Likewise, camera raw processing applications typically embed scene-toreproduction color rendering. While it is possible to create true scene-referred
images from camera raw image data, most camera raw processing applications
do not do this. Camera profiling applications include scene-to-PCS color
rendering but may not offer user controls (note that with some camera profiling
applications the scene color analysis accuracy is limited more by the accuracy of
the target based characterization method than by intentional preferential
alterations).
In the future, it is expected that users will have more access to scene-referred
image data, thereby gaining more explicit control over scene-to-reproduction
color rendering. At present, this material is primarily intended as background, and
for custom workflows where special camera modes or processing applications
are used to enable true scene-referred image creation, followed by scene-toreproduction color rendering.
The essential process in any scene to reproduction (scene-referred-toreference-output-referred) perceptual transformation is a coordinated
combination of color appearance adaptation, preference adjustments and gamut
mapping. This perceptual rendering intent color-rendering transformation is used
to map scenes to the fixed range of a reproduction in a pleasing way. When a
source image is scene-referred, the Device-to-PCS perceptual transform
performs a perceptual rendering from the scene to the perceptual intent
reference medium. Note that in an ICC Version 4 compliant [scene-referred] input
profile (e.g., a digital camera input profile), the reference-output-referred-toscene-referred PCS-to-Device perceptual rendering intent transform should invert
(i.e., undo) that profile’s own Device-to-PCS perceptual rendering intent
transform.
Commonly, the color appearance adaptation portion of a perceptual colorrendering transformation includes adaptation from the scene adopted white (both
the chromaticity and luminance) to the adopted white of the reproduction.
Reproduction constraints and color appearance preferences determine the
mapping of the adopted-white-adapted scene colorimetry to produce a pleasing
reproduction. For example, if the scene luminances are much higher than those
of the reproduction in the anticipated viewing conditions, a chroma boost may be
necessary to maintain the appropriate colorfulness. The anticipated surround of
the reproduction can affect the desired contrast, with darker surrounds requiring
higher contrast. Preferences play a significant role in determining this mapping,
as viewers tend to prefer increased colorfulness and contrast in reproductions, to
the extent that the increases do not look unnatural. Ideally, mappings are
determined on a scene and output medium specific basis, implying imagespecific perceptual intents. In production workflows fixed mappings that work
reasonably well for most scenes are often used. These mappings typically boost
the scene gamma and mid-tone contrast. For example, film reproduction systems
have a mid-tone gamma greater than unity (~ 1.2 to 1.6, depending on the
anticipated output medium) combined with highlight and shadow roll-offs. This sshaped mapping allows film systems to accept both low and high dynamic range
scenes, while maintaining preferred mid-tone contrast and colorfulness. Likewise,
video systems have a system gamma of ~1.2 to 1.4 and some highlight
compression (at least in high-end systems).
The preference adjustment portion of a perceptual color-rendering transformation
often includes preferential expansion or compression of the source gamut and
dynamic range to match that of a particular output (visualization) medium. Source
scene gamut expansion and compression may be determined based on the
potential scene extent from a particular digitization source device. Alternatively, in
scene-specific color-rendering cases, each specific source scene gamut extent
may be evaluated and preferentially expanded or compressed to match the
output medium. In some cases, preferential mappings also explicitly consider the
reproduction of memory colors. Following such appearance-preference mapping,
it may be necessary to apply gamut mapping to bring the remapped colors to
within the actual gamut of the destination medium. Ideally the appearancepreference mapping would accomplish this, but practically, a following gamut
mapping operation may be required. Note that the perceptual rendering intent
color rendering provided in Version 4 input profiles targets the ICC perceptual
intent reference medium.
Optimal preference mappings differ for scenes of low, medium, and high dynamic
range, key, and gamut extent. Some scenes have colors out to the spectral locus
(and beyond, after chromatic adaptation) and have very high luminance
(dynamic) ranges, however, many scenes do not. In fact, most scenes have
dynamic ranges (and gamuts) smaller than the 288:1 of the ICC perceptual intent
reference medium. ICC profiles are often used in capture-condition or
visualization-condition (i.e., image state) specific – rather than image specific –
workflows. With these workflows, customizing the choice of rendering intent is
one way to adapt the use of an ICC profile to a particular scene or color object.
It should be noted that the capture digitization of an original (two-dimensional)
artwork or photograph is different from the capture of a scene, which is a view of
the natural (three-dimensional) world. The discussion above relates to the
capture of scenes. The capture of originals, even using a digital camera, falls
under re-targeting or re-purposing as discussed below. Perceptual rendering
intents for scene capture will generally not be appropriate for the capture of twodimensional originals.
Re-targeting and Re-purposing
After data is color rendered to a particular reference-output-referred or actualoutput-referred first visualization condition, i.e., output-referred image state, it
may be necessary to transform the data for a second visualization. For example,
in a typical digital camera workflow, the ‘pleasing reproduction of the scene’
produced by the camera is targeted for viewing on a softcopy display. That
display-referred data may be color re-rendered when a print output is desired.
Two scenarios are defined regarding the color re-rendering transformation. When
the second visualization is intended to represent or match the original first
visualization, this is called re-targeting. Re-targeting is typical for ‘proofing.’ When
the second visualization is independent of (i.e., not constrained by) the first
visualization and can be optimized for the second visualization condition, this is
called re-purposing. Keep in mind that both re-targeting and re-purposing are
intended to operate on source images that are already in a picture-referred image
state (either original- or output-referred, but not scene-referred).
In re-targeting, the Device-to-PCS media-relative colorimetric transform of the
first visualization output or display profile is sequenced with the PCS-to-Device
media-relative colorimetric transform of a second visualization output or display
profile. [Absolute colorimetric intents can be used when the color of the target
substrate from the first visualization is to be carried through to the second
visualization.] No new or revised image state preferential rendering is called for in
re-targeting. The accuracy of the representation through the second visualization
condition will be proportional to the capability of the second visualization
condition to match the first visualization condition (e.g., gamut volume shape,
luminance range, and color differentiation).
In re-purposing, the first concern is to remove the constraints in the color data
that were induced by prior perceptual rendering for a particular visualization
condition (constraints preferentially based on a color aim determined as a
function of prior source and destination image states). It is problematic that the
constraints induced by a first preferential color rendering cannot be determined
by examining color data after it has been so rendered. Color aim preferential
rendering behavior is also not easily determined by examining the perceptual
rendering intent transform of an output profile. Further, preferential capabilities in
a CMS may have contributed to the first visualization, and can be difficult to
extract in preparation for a later visualization.
In support of re-purposing, the ICC Version 4 specification places a new
emphasis on perceptual rendering intent transformations:
• In ICC Version 4 compliant [actual-output-referred] output profiles, the actualoutput-referred-to-reference-output-referred Device-to-PCS perceptual
rendering intent transform should invert (i.e., undo) that profile’s own PCS-toDevice perceptual rendering intent transform, to allow for re-purposing from
the ICC perceptual intent reference medium.
• In ICC Version 4 compliant [original-referred] color space encoding and
scanner input profiles (e.g., an sRGB profile, a document scanner profile), the
Device-to-PCS perceptual rendering intent transform should color re-render
the original to an appropriate ICC perceptual intent reference medium
representation (i.e., transform from the device, or encoding, medium image
state to the ICC perceptual intent reference medium image state).
• In ICC Version 4 compliant [original-referred] color space encoding and
scanner input profiles (e.g., an sRGB profile, a document scanner profile), the
PCS-to-Device perceptual rendering intent transform should color re-render
back to the original (i.e., transform from the ICC perceptual intent reference
medium image state to the device, or encoding, medium image state) to allow
for a new re-purposing directly from the original-referred image state. Note that
in order to provide for lossless round-trip this PCS-to-Device perceptual
rendering intent transform should be an inverse of the Device-to-PCS
perceptual rendering intent transform.
With Version 4 ICC profiles, re-purposing can be accomplished by sequencing
the Device-to-PCS perceptual rendering intent transform of a “source” first
visualization output profile with the PCS-to-Device perceptual rendering intent
transform of a second visualization output profile. The Device-to-PCS perceptual
transform from the source output profile "undoes" the previous perceptual color
re-rendering from the perceptual intent reference medium to the source profile's
actual output medium.
Note that use of the perceptual “undo” is appropriate only if the first visualization
resulted from a perceptual rendering transformation. The rule of thumb is that the
inverse of the rendering intent that was used to produce a particular visualization
should be used to “undo” that visualization. Also note that even with the improved
support in compliant Version 4 ICC profiles, subsequent visualizations can be
constrained by loss of color detail in earlier transformations.
For re-purposing in general, when the destination output-referred image state
gamut and viewing environment condition are “like” that of a source outputreferred image state, then a colorimetric intent, with no preferential adjustment,
may achieve acceptable results. (In fact, if the source and destination media are
similar to the ICC perceptual intent reference medium, there should be little
difference between the colorimetric and perceptual intent transforms.) On the
other hand, when there are significantly different gamut constraints, and/or
viewing environments, then a perceptual rendering intent, with inherent
preference adjustments, can improve results. PDF/X-3 files, containing a fully
populated (complete sets of PCS-to-Device and Device-to-PCS transforms) ICC
output profile that describes the PDF Output Intent, support this type of repurposing.
The goal with version 4 ICC profiles is to enable blind use of perceptual intents
for re-purposing. It is expected that as version 4 profiling tools become more
capable in generating quality perceptual color re-rendering transforms, this goal
will be realized. However, in critical applications with media that are quite
different from the perceptual intent reference medium, sophisticated users may
find that careful, controlled application of colorimetric intents, abstract profiles,
and CMS color rendering can produce better results.
Preserving an Artistic Intent through Multiple Visualizations
Preserving an artistic intent through multiple visualizations can require a
combination of re-targeting and re-purposing approaches. The approach that is
most likely to produce the best results in a particular situation depends on the
similarities of the various actual media to each other, and to the perceptual intent
reference medium. When multiple independently optimized visualizations are
planned in advance, alternative approaches can be considered. If a specific
artistic intent is desired, particular care should be taken with the first visualization.
A large gamut output-referred source image can be obtained by first applying the
appropriate perceptual intent transform to color render scene-referred image data
to the ICC perceptual intent reference medium, and then transforming the
colorimetry of that reference-output-referred first visualization image to an
appropriate storage color encoding such as ROMM/ProPhoto RGB. (Note that for
a color encoding to be appropriate for this use the encoding image state will
match the ICC perceptual intent reference medium image state, and the profile
for that color encoding will have identical perceptual and colorimetric rendering
intents). Alternatively, after using an appropriate perceptual intent transform to
color render scene-referred image data to the perceptual intent reference
medium, a first ‘actual’ visualization can be obtained by using an appropriate
perceptual intent color re-rendering transform to re-render from the perceptual
intent reference medium to the medium of a large gamut output device. Using
such a “superset” first visualization as the source for subsequent visualizations
can improve the optimization for each subsequent visualization, while maintaining
color fidelity with the intended artistic intent.
When a color rendering to a first visualization represents a "master" image,
including the artistic intent of the image creator, subsequent color transformations
should not "undo" the initial perceptual intent color rendering. A subsequent
actual output-referred visualization can be produced via a re-targeting approach
(i.e., using colorimetric transforms) when the actual output medium is “like” the
master image medium. When a subsequent actual output medium is dissimilar to
the master image medium, the approach most likely to produce the best results
depends on the relations of the media to each other, and to the perceptual intent
reference medium.. If the master image is targeted to the perceptual intent
reference medium and an actual output medium is dissimilar to the perceptual
intent reference medium, then the perceptual intent transform of the actual output
destination profile should be used to color re-render from the perceptual intent
reference medium to the actual output medium.
If the master image is targeted to a first actual visualization medium that is
substantially different from the perceptual intent reference medium, and a
subsequent actual output medium is similar to the first visualization medium, then
a Device-to-PCS colorimetric intent transform from the source (first) output profile
should be used to re-encode the master image in ICC PCS. Then the PCS-toDevice colorimetric intent transform of the subsequent actual output destination
profile should be used to create device values for the subsequent actual output
medium.
The case where the first actual visualization medium, the perceptual intent
reference medium, and the subsequent actual output medium are all substantially
different from each other is the most challenging for color management. Ideally,
the first actual visualization medium profile should perform color re-rendering to
the perceptual intent reference medium, followed by color re-rendering from the
perceptual intent reference medium to the subsequent actual output medium.
However, it is possible, perhaps likely, that the input and output profile perceptual
color re-renderings may not be complementary with each other to preserve the
master image artistic intent. In that case using specifically tuned DeviceLink
profiles to transform directly between the first visualization and each subsequent
visualization may produce better results.
Note that when no related artistic intent is required among the multiple
visualizations, then more flexibility in the final output can be obtained by retaining
capture-referred (scene- or original-referred wide gamut RGB) data to use as the
source for each independent visualization color rendering or re-rendering. This
enables maximum flexibility for each visualization. It should be noted that this
approach can produce significantly different versions of the same image, as
scene-to-picture color rendering can be quite aggressive, and involve choices
such as overall lightness, contrast, tone and saturation that go beyond the
optimization of the scene to some output medium.
Additional Rendering Intent sequence examples
Visualization of the ICC perceptual intent reference medium image: When it is
desirable to visualize the perceptual intent reference medium rendition of a color
image directly, a visualization device with capability matching or exceeding the
perceptual intent reference medium is required. Given that, one can use mediarelative colorimetric rendering from the PCS, re-targeting the perceptual intent
reference medium image to the actual output device (after correct perceptual
rendering to the perceptual intent reference medium). Such visualizations should
then be viewed in the reference viewing conditions (ISO 3664 condition P2) to
produce the appropriate appearance.
Image specific preferential color rendering: As discussed above, image specific
profiles and/or rendering intents can be used to obtain optimized preferential
color-renderings from the capture-referred state to the reference-output-referred
ICC perceptual intent reference medium. Use of image specific color-renderings
should consider the need for color appearance compatibility across the various
color objects intended for a particular document.
Color rendering or re-rendering from an ambiguous image state RGB color
encoding: The first decision in this situation is, “Has the color data been
previously color rendered to an output-referred state?” Certain RGB encodings
inherently carry with them a particular image state: sRGB is output-referred for
monitor viewing; ROMM/ProPhoto RGB is output-referred for the ICC perceptual
reference medium print condition. It can be helpful to understand the use case or
workflow that produced the RGB data when inferring the color-rendering image
state condition. Typically, RGB data that is exchanged will have been color
rendered to a first visualization and can be considered output-referred. However,
beyond that it may be difficult to determine whether the RGB data is optimized for
print or monitor viewing. When color re-rendering from an RGB working space,
both the image state of the data, and the medium to which it may have been
previously “color rendered” can affect the outcome of a subsequent color rerendering. Keep in mind that manual adjustments may have been applied to
optimize the data for a particular visualization. Caution is required because
repeating a scene-referred-to-output-referred perceptual rendering intent
transformation (as described above) will degrade image quality, as will applying
an inappropriate color re-rendering transformation.
A source rendering intent can be selected to be appropriate for the image data in
a particular working space. For example, prior to printing typical sRGB image
data, it should be re-purposed from its display-referred state to the referenceoutput-referred image state corresponding to the ICC perceptual intent reference
medium. On the other hand, if a user has been editing Adobe RGB image data to
produce a desired appearance on a print media, a relative colorimetric source
rendering intent may be appropriate when transforming for print.
When selecting the ‘next visualization’ destination rendering intent for a
previously color-rendered (output-referred) RGB encoded image, as above, color
re-rendering from the perceptual intent reference medium to an actual-output
visualization encoding can be media-relative, or absolute colorimetric when the
actual-output visualization gamut extent and tone range are similar to the
reference medium gamut extent and tone range. When the actual output
visualization gamut extent and tone range are significantly different from the
reference medium gamut extent and tone range then perceptual rendering may
provide an improved result.
Color re-rendering of computer generated imagery: Use of the perceptual
rendering intent in reproducing computer generated color infers the computer
display as the ‘original’ capture device. The computer display ‘synthetic original’
(original-referred image state) can be preferentially color re-rendered to the ICC
perceptual intent reference medium using the perceptual rendering intent of a
Version 4 compliant input profile for the computer display. Consideration of the
rendering intent to use from the reference medium to the ‘next visualization’
actual output encoding is similar to that discussed above.
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