Creating scene-referred images using Photoshop
Introduction
A scene is defined in ISO 22028-1 as follows:
- scene
- spectral radiances of a view of the natural world as measured from a specified vantage point in space and at - a specified time
A scene may correspond to an actual view of the natural world or to a computer-generated virtual scene simulating such a view.
A scene-referred image is an image where the image data is an encoding of the colors of a scene (relative to each other), as opposed to a picture of a scene. In a picture, the colors are typically altered to make them more pleasing to viewers when viewed using some target medium.
The emergence of a wider variety of reproduction media, and the penetration of digital workflows into virtually all aspects of imaging have led to increased interest in the exchange of camera raw and scene-referred images. Such images can subsequently be color rendered to different reproduction media, optimizing the image for each. Camera raw and scene-referred workflows are fundamentally different from output-referred workflows, such as those based on sRGB or Adobe RGB (1998) image exchange.
While the greatest flexibility for subsequent processing comes from exchanging camera raw files, there are workflows where the exchange of scene-referred images is desirable. Scene-referred images from different capture sources can be encoded in the same scene-referred color encoding, facilitating compositing and the bulk application of color rendering. Also, scene-referred files typically include demosaicing, noise reduction, camera blur removal, white balance, and some transform from camera sensor values to colorimetry. Scene-referred exchange allows a photographer to incorporate this processing in the image file without specifying any intended reproduction medium or color rendering.
Recognizing this need, and noting the capability of Adobe Photoshop Camera Raw to produce scene-referred image data, the ICC provides the following procedure for the creation of scene-referred images. Please note that other camera raw processing applications are also capable of producing scene-referred images.
Procedure
When using Adobe Camera Raw (ACR) in Photoshop CS3, the steps to create linear RIMM RGB scene-referred images are as follows:
- Copy the ISO22028-3_RIMM-RGB-exCR.icc and/or linear_RIMM-RGB_v4.icc profile to the profiles folder, e.g. -
Windows: windows/system32/spool/drivers/color
Mac: Library/Application Support/Adobe/Color/Profiles
NOTE 1 The ISO22028-3_RIMM-RGB-exCR.icc profile includes the nonlinear color component transfer function (CCTF) specified in ISO 22028-3. This CCTF
provides for efficient quantization, and should be used whenever RIMM RGB files will be downsampled to 8-bits per channel. The linear_RIMM-RGB_v4.icc
profile contains a linear CCTF. It should be used when a linear scene-referred encoding is desired, but only with 16 (or more) bits per
channel.
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Open the raw file in Photoshop (which will bring up ACR) and adjust the scene analysis controls as desired. Note that the color rendering controls should be left at zero or linear in order to produce a scene-referred image. See Scene analysis and color rendering controls below.
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Set the Workflow Options (at the bottom of the ACR window) Space to ProPhoto RGB and the Depth to 16 Bits/Channel.
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Open the converted (to scene-referred) raw file into Photoshop.
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Select Edit/Convert to Profile and convert to the ISO22028-3_RIMM-RGB-exCR.icc or linear_RIMM-RGB_v4.icc profile using the relative colorimetric rendering intent. This results in the image being tagged with a profile that indicates the image data is scene-referred.
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(optional) Select View/Proof Setup/Custom, and select the Perceptual Rendering Intent and the desired destination encoding profile for the Device to Simulate. Then click OK in the Customize Proof Condition dialog. This step is only needed if there is a desire to preview the output-referred appearance using the perceptual rendering intent.
NOTE 2 Custom proof setups can be saved for future use.
NOTE 3 An ICC v4 profile is recommended for the destination profile, such as the ISO22028-2_ROMM-RGB.icc or sRGB_v4_ICC_preference.icc profiles, although both v2 and v4 profiles can be used as destination profiles.
NOTE 4 The perceptual rendering intent in the ISO22028-3_RIMM-RGB-exCR.icc and linear_RIMM-RGB_v4.icc profile includes the default color rendering tone curve specified in ISO 22028-3. It should provide a reasonable color rendering for most scenes but will generally not be optimal for any given scene. Scene-specific color rendering (e.g. using the ACR color rendering controls) is typically required to produce the most preferred results.
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(optional) Select View/Proof Colors to see a preview of the output-referred appearance.
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(optional) Select Image/Adjustments/Exposure and adjust the Exposure only to produce the preferred lightness on the output-referred preview.
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Save the image, embedding the ISO22028-3_RIMM-RGB-exCR.icc or linear_RIMM-RGB_v4.icc profile as appropriate.
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To convert the scene-referred image to output-referred in the future, there are two options:
a. Convert to the desired working space using the relative colorimetric rendering intent and perform the color rendering manually. In this case the starting point will be the scene-referred colorimetry estimates.
b. Convert to an output-referred color encoding using the perceptual rendering intent, using for example the ISO22028-2_ROMM-RGB.icc, AdobeRGB1998.icc, or sRGB_v4_ICC_preference.icc profiles and edit the result as desired.
NOTE 5 White balance changes are best performed by going back and starting with the camera raw file.
NOTE 6 If it is desired to convert from one scene-referred encoding to another, a colorimetric rendering intent should be used.
PROFILES
Terms of use
To anyone who acknowledges that the profiles below are provided "AS IS" WITH NO EXPRESS OR IMPLIED WARRANTY, permission to use, copy and distribute this file for any purpose is hereby granted without fee, provided that the file is not changed including the ICC copyright notice tag, and that the name of ICC shall not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. ICC makes no representations about the suitability of this software for any purpose.
By downloading these files you accept the Terms of Use.
Overview of scene analysis and color rendering controls in Adobe Photoshop CS3 Camera Raw
Basic:
White Balance - Temperature and Tint (scene analysis sliders)
The white balance is controlled by the Temperature and Tint sliders, and the matrix applied is created by interpolating two matrices in the DNG file based on the reciprocal of the color temperature.
Exposure (scene analysis slider)
The Exposure slider performs a linear scaling of the camera exposure values and is used to lower the clipping level from the camera clip for the white balance. Generally the ACR Exposure slider should be left at zero as there is no benefit to lowering the clip level before the scene-referred image is imported into Photoshop. Once in Photoshop, the Image>Adjustments>Exposure slider can be used to adjust the image lightness interactively while viewing an output-referred preview.
Recovery (scene analysis slider)
The Recovery slider increases the clipping level from the default camera clip for the white balance in the case where some camera RGB channels are clipped but others aren't. The recovery slider sets a new higher clip limit where the clipped camera channel(s) is (are) estimated from the unclipped channels. This won't recover true scene colors but generally produces acceptable results when highlights are relatively achromatic.
Fill Light (can be considered either a scene analysis or a color rendering slider)
The Fill Light slider increases the gains of the channels in darker regions of the image to simulate the addition of fill light.
The Blacks, Brightness, Contrast, Clarity, Vibrance and Saturation sliders are color rendering sliders and should be set to zero in the creation of scene-referred images.
Tone Curve:
The application of a tone curve is part of color rendering and both the Parametric and Point tone curve panes should be set to linear to produce scene-referred image data.
Detail:
The goal of sharpening and noise reduction in scene analysis should be to present the scene as accurately as possible. Some sharpening and noise reduction may be appropriate, but care should be taken not to overdo.
HSL/Grayscale:
These sliders are used to adjust a LUT that is applied after the scene analysis color matrix. They can be used in the scene analysis case to tweak the results of the matrix using the additional degrees of freedom in the LUT, or in the color rendering case to tweak the desired look.
Split Toning:
These sliders are used to undo channel crossovers in the scene analysis case, or to introduce them as part of the look in the color rendering case.
Lens Correction:
These sliders are fairly self-explanatory, and are part of scene analysis.
Camera Calibration:
This is where the scene analysis color matrix can be adjusted using intuitive sliders. The Camera Profile refers to the DNG metadata and ACR processing model, not to an ICC profile. The Shadows Tint slider allows the shadows tint to be adjusted separately from the highlight tint, which is controlled by the White Balance. (It is not clear why there is no Shadows Temperature slider.) As the controls in this pane affect the scene analysis color matrix, they are generally not part of the color rendering although they may become part of the color rendering if they are used to go beyond accurate scene analysis to produce a desired look.