Creating scene-referred images using Photoshop CS3Introduction
A scene is defined in ISO 22028-1 as follows:
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.
When using Adobe Camera Raw (ACR) in Photoshop CS3, the steps to create linear RIMM RGB scene-referred images are as follows:
1. Copy the ISO22028-3_RIMM-RGB-exCR.icc and/or linear_RIMM-RGB_v4.icc profile to the profiles folder, e.g. -
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.
2. 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.
4. Set the Workflow Options (at the bottom of the ACR window) Space to ProPhoto RGB and the Depth to 16 Bits/Channel.
5. Open the converted (to scene-referred) raw file into Photoshop.
6. 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.
7. (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.
8. (optional) Select View/Proof Colors to see a preview of the output-referred appearance.
9. (optional) Select Image/Adjustments/Exposure and adjust the Exposure only to produce the preferred lightness on the output-referred preview.
11. 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.
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.