First published 25 January 2008; updated 25 May 2008; reformatted and shortened August 2012.
Many photographers claim that they want their photographs to look as realistic as possible. Taken literally, this implies a scene-referred image (explained in more detail below). Unfortunately, scene-referred images tend to look washed out and dark on displays and print, due to differences in contrast range and brightness. Hence, scene-referred images are commonly dismissed as no more than a technical curiosity.
In this essay I will argue that whilst scene-referred images are not very suitable as a photographic end product, they can fulfill a valuable role as an intermediate stage in the image processing workflow. Amongst other benefits, scene-referred intermediate images provide an effective way to 'insulate' the photographer and his/her style from the constant changes in digital camera equipment. Also, the use of scene-referred images suggests a natural workflow for HDR imaging and panorama stitching. Below, I describe the proposed workflow and a detailed list of potential advantages.
A typical outdoor scene encompasses a large range of brightness and color. The contrast range can easily reach 10,000:1 and some of the colors can be extremely saturated. That alone means that we cannot hope to reproduce this information on a typical print with a contrast range of approximately 200:1 and a much more limited color palette.
Traditionally, most photography has been destined for a specific output medium (print or projection). Because of this, photographers are used to target their work directly to this output medium and make the necessary adjustments at an early stage in their workflow. For example, increasing the development time of a negative film would boost the contrast of the prints. The digital equivalent of this process is the use of output-referred images: images that are processed from the beginning with a specific output in mind.
Contrast this with scene-referred images. A scene-referred image reflects the relative brightness differences and colors as accurately as possible, even if they cannot be reproduced on the targeted output medium. For most photographers, scene-referred images have been nothing more than a curiosity. The reason for this is that most scene-referred images simply cannot be represented accurately on a monitor. Because an 18% brightness level is considered medium grey, the monitor only has approximately 2.5 stops between grey and white. Many scene-referred images require more highlight headroom to be viewed properly. To illustrate this point, it may be worth pointing out that high dynamic range (HDR) images are also scene-referred, at least before they are tone-mapped to an output-referred image.
For more information on scene-referred images, see this white paper by the ICC. For those interested for a more thorough discussion, I highly recommend the following Adobe white paper by Karl Lang: Rendering the Print: the Art of Photography.
By now, it will have become clear that a scene-referred image is not desirable as a final product. However, it is nevertheless the most accurate representation of the colors and brightness distribution in the original scene. This property makes it attractive as an intermediate point in the workflow, for reasons I will describe below.
The scene-referred image is neither what comes out of the camera nor is it the final product. Instead, it can be used to naturally divide the image processing workflow into two distinct stages. The first stage is what I will call the scene reconstruction stage. This step is concerned with constructing a scene-referred image from the unprocessed data. Effectively, this removes the 'fingerprint' of the camera as much as possible. This is followed by a second stage: the creative processing. The following image illustrates the proposed workflow.
|The scene-referred workflow and its two stages: scene reconstruction and creative processing. Also listed are the typical tasks one would perform in each stage.|
Splitting the workflow into a scene reconstruction and a creative processing step offers a number of advantages:
Note that this proposal has analogies with the three-step sharpening process that is steadily gaining ground. However, the scene-referred workflow makes no distinction between the creative and output stages, because the properties of the output medium also define the boundaries for creative processing. In the specific case of sharpening the two are more or less independent, allowing one to separate the two steps.
The scene reconstruction stage is the first step in the proposed scene-referred workflow. It's purpose is to remove the 'fingerprint' that the camera has left on the image. Obviously, there are limits to this process. For example, one cannot recreate detail that has not been resolved in the first place, or restore color nuances that have not been not registered. The output of this stage, the scene-referred image, should ideally contain all the information that is needed to recreate the same visual stimulus of the original scene - assuming a perfect output medium.
The process of undoing the distortions the camera has imposed on the captured scene naturally suggests a number of processing steps. Most of them are the usual controls that you will find in a RAW converter. Note, however, that tonal corrections are absent at this stage. Also, some operations that are usually done in third-party software, such as HDR merging and panorama stitching, are listed as part of this processing step, because they assist in the recreation of the original scene.
Note that these are all technical processes that benefit from a scientific/engineering approach. In addition, these steps can all be performed with relatively little manual intervention. The scene reconstruction step is therefore well suited to batch processing by specialized software.
The creative processing stage is concerned with transforming the accurate-but-bland scene-referred image into an appealing final image. Creativity is often bounded by the limits of the output medium, so the properties of the output medium (dimensions, contrast range, color gamut, viewing surround, etc.) generally have to be taken into account. For obvious reasons, there is no well-defined list for the things that should be done in the creative processing stage, but the following list of steps would be a good start.
The above may seem like an academic discussion on the benefits of an imagined process. To a certain extent that is true. I think that the two-stage process provides a good conceptual framework for developers and users that like to know the detailed properties of their tools. However, there are some real world applications where a scene-referred workflow makes sense right now. Here is a short listing of the cases in which I personally use this workflow:
This brings us to the problem of obtaining a scene-referred image in the first place. Most RAW processing software available today aims to be an all-in-one solution that takes you from the RAW file to the print. These programs mix and entangle elements from the scene reconstruction and creative processing steps described above. However, there are three RAW converters that I know of in which all of the creative processing steps can be bypassed in order to generate a scene-referred image. These three programs are: Adobe Camera Raw (used in Photoshop, Photoshop elements and Lightroom), LightZone, dcraw and UFRaw (a front end to dcraw). Below, I will show the necessary settings for Adobe Camera Raw, Lightroom and LightZone. I have no first hand experience with dcraw or UFRaw.
I have provided a number of arguments for the adoption of scene-referred images as an intermediate step in the digital imaging workflow. Such a workflow allows for a clean division of the image processing steps into a scene reconstruction stage and a creative processing stage. Besides its appeal as a conceptual tool, this approach has several real world advantages, for example in HDR creation and panorama stitching. Basic tools that make such a two-stage approach possible are currently available.
Due to limited software support, a scene-referred workflow is at the moment mostly relegated to low-volume specialist and evaluation uses. However, this should be sufficient to get a feeling for the possibilities and advantages. Native support for a scene-referred workflow in future software could make these advantages available to a broader audience.