Photoshop 5 Colour Handling

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Photoshop 5 colour handling

Tom Arah looks at the potential benefits and potential traps of Photoshop 5's radical new approach to colour management.

Adobe Gamma 

In all the excitement over the latest version of Photoshop much has been made of the new features such as the multiple undo, spot colour support and text editability. However, the most fundamental change is the move to a new colour management system. After all, colour handling is what image editing ultimately boils down to - controlling the colour values of the bitmap pixels. The new system introduced by Adobe offers great potential benefits, but it also opens up some serious potential traps. It's therefore important to understand just what changes are being brought in and why.

Device-Dependence

The vast majority of image editing work within Photoshop is carried out in RGB mode. In this mode the colour of each pixel is determined by three numbers representing separate red, green and blue values. The advantage of RGB is that the same additive colour system underlies human vision, scanners and monitor CRTs. The huge drawback is that the system is "device-dependent". The same RGB values will not necessarily give the same perceived colour, as these depend on the individual colour filters of the scanner and phosphors of the monitor. Crucially, this means that the same image is unlikely to look the same from one monitor to the next. In short, there is no in-built colour accuracy or consistency to RGB.

With earlier versions of Photoshop there are two ways to minimize the problems of device-dependence. The first is to use the Monitor Setup command to calibrate your monitor to make the display as accurate as possible. Essentially this involves finding out the relative input of the red, green and blue values (the phosphors), how they interact (the white point) and their response curve (gamma). Taken together this information provides a profile of how the red, green and blue values are mapped across the full colour space that the monitor is capable of producing. Using this information Photoshop knows what colours you are seeing and can use this as the basis for accurate conversion to CMYK and so on.

This is certainly a step forward, but it doesn't avoid the problem that each monitor is likely to have a different colour space so that the same values on one system will still look completely different on another. For many professional users the only reliable solution is to effectively ignore how the image appears on screen. A sure sign of the pre-press professional is that they work with their eyes as much on the Info Palette as on the screen. Whatever a colour looks like on screen you know that the CMYK or grayscale percentages shown on the Info Palette will make it through to the printing plates and from there onto the page. Likewise, for those working to the Web, the 216 safe browser colours can be specified numerically and, although these will look different on different monitors, at least they won't be dithered.

Profiling

Such a standalone and almost mathematical approach to colour management is clearly not ideal, but it has been made to work and has put Photoshop at the heart of the entire professional imaging industry. With Photoshop 5, however, an entirely new colour management system designed to build the program into a broader cross-application and cross-system workflow has been introduced. The first indication of this is the fact that the internal Monitor Setup command has been replaced with the external Gamma utility. This again allows the monitor to be calibrated and characterized by setting white point, gamma and phosphor information. Now, however, the colour space information is saved as an ICC (International Colour Consortium) profile which can be read by any ICC-supporting application. Most importantly it is read by Windows itself which loads the profile and calibrates the monitor each time the system starts.

The result is much better colour accuracy across the system especially with ICC-supporting applications. However, such calibration on its own can do little to ensure consistency across multiple systems as each monitor's different colour space still means that identical RGB values will produce different results. The new solution is to embed the originating system's colour profile into the image to be read by supporting applications. By mapping the originating colour space to the new colour space a much more accurate and consistent impression of how the image appears onscreen can be maintained throughout the production workflow. This process of embedding profiles into images is called "tagging" and by default Photoshop 5 now tags all supporting file formats including EPS, JPEG, PDF, PSD, and TIFF.

Of course the tagging system only works with supporting applications which are few on the ground at the moment, but with non-supporting applications the profile is simply ignored. However, there are drawbacks. To begin with the profile in each file takes up approximately 2k. This is inconsequential for a print-based multi-megabyte graphic, but would quickly mount up for every image on a web site. More importantly, when the application maps from the originating colour profile to the current profile it actually changes the pixels' RGB values to compensate. When swapping a file back and forth between systems this would mean that colours were always shifting to the lowest-common-denominator which would soon cause unacceptable degradation.

Working Spaces

Again the solution is in-place. Rather than embedding the monitor space profile, Photoshop 5 actually comes with a range of pre-defined "working-space" profiles which are used instead. These working-space profiles - chosen from the new RGB Settings dialog - have been absolutely precisely defined in terms of component phosphors, gamma, white space and expected viewing conditions so much so that they are "colometrically" accurate. This means that each RGB colour can be mapped to a CIE - ie spectrally-defined - colour so tightly that the device-dependent RGB effectively becomes device-independent. Better still, the working-space can have a wider and more uniform gamut than the actual monitors in the workflow so that you can effectively work to a highest-common-denominator without colour-clipping or bias.

The strength of these working-space profiles really comes into its own with devices - scanners, monitors and printers - for which these spaces are "hardware native" as this enables accurate and consistent colour at all stages of the production workflow to be guaranteed. For existing equipment, however, Photoshop 5 can simply map from the optimised working-space to the vagaries of the devices' actual colour spaces. As such, each of the standardised RGB working-spaces can effectively act as an honest broker between the myriad different monitor spaces in the workflow. This move from a monitor-centric to a working-space colour management system is a huge shift which means that, whatever platform or machine you are working on, you can edit your images within the same device-independent colour space. So does this mean that the problems of colour accuracy and consistency are solved once and for all?

Problems

The answer has to be "no". Working-space management is clearly preferable to a free-for-all monitor-to-monitor system where every time an image is opened on a new system its pixel values are altered. However, although the working-space system means that no changes are necessary when the same space is used, conversion is still necessary when moving between different working-spaces. Of course, when a closed workflow is up and running, this isn't likely to occur but it is still a factor that the high-end user needs to know about, especially in an open system where images might be based on different working-spaces.

What turns this from an occasional issue into a serious problem is the fact that the latest Photoshop has completely shifted its own default working-space! In other words, under default settings, exactly the same RGB values will display and print differently under Photoshop 4 and 5. This change has huge repercussions. In particular it means that when any RGB image created with a previous version of Photoshop is opened, it must be converted to the new space. It's worth emphasizing that this isn't changing how the image appears on screen, rather it is exactly the reverse - the image's actual pixel values are being changed to make them appear as they did previously!

The only indication of this transformation is the unobtrusive "Converting Colours." message in the status bar. Most desktop users will remain happily oblivious to the message and the entire process, as the end result is that the image looks unchanged, but for the professional user it can spell serious trouble. The various Photoshop forums and newsgroups are echoing with the howls of anguish as bewildered users find that their finely-tuned RGB source images built on the web-safe colours are no longer safe at all after conversion. In many ways Photoshop's change of default colour space is as fundamental as Microsoft changing its Office format files. The difference in this case is that the latest release can still read and write the older file formats - it just changes the information they contain!

sRGB

Ultimately then, after taking on board the moves to external calibration, profile tagging and working colour spaces it is the choice of the new default working-space that is the most significant change to colour management in Photoshop 5. The default working-space Photoshop has opted for is sRGB (standardised RGB) and the most important change in colour space it involves is the move from the previous 1.8 gamma to a new 2.2 gamma. The gamma controls how the midtones of the red, green and blue components are handled and so the linearity of the colour response curve from 0% to 100%. Effectively changing gamma has a similar effect to shifting the mid-point in Photoshop's Curves dialog.

The change isn't minor. The difference can be seen if you swap between Photoshop 5's new default sRGB space and its previous space - now called "Apple RGB" - with the RGB Setup dialog. When previewing in sRGB, the same image is noticeably darker which means that, when converting, Photoshop must lighten the image to compensate. It's possible to put some figures to this by using Photoshop's new Profile to Profile command. By converting a grayscale calibration image - 100%, 90%, 80% etc - to RGB and then converting between Apple and sRGB spaces and then back to grayscale the newly converted figures read 100%, 87%, 74%, 63%, 53%, 43%, 34%, 26%, 16%, 8%, 0%. Looking at the gaps between these figures and the former equally spaced 10% increments - ie 0%, 3%, 6%, 7%, 7%, 7%, 6%, 5%, 4%, 2%, 0% - shows how the previous linear response curve has been distorted. Those designers used to relying on the feedback from the Info Palette will be driven mad.

The important point is that, because RGB colours appear so different under the sRGB default, when Photoshop 5 opens legacy files for the first time, it must convert the actual pixel values to compensate. The obvious question is "why"? The advantages of a standardised and optimised working-space are clear, but why must it involve such drastic change? Why not use a working-space based on the existing 1.8 gamma so that both appearances onscreen and pixel values remain identical, or at least much more similar? Clearly there must be big advantages to sRGB to justify such disruption, but Adobe says very little on the matter. The only direct comment on sRGB is the help file comment: "This colour space is endorsed by a wide variety of hardware and software manufacturers and is becoming the default colour space for many scanners, low-end printers, and software applications." Hardly a ringing endorsement.

So just what is sRGB?

Fortunately, the original sRGB specification paper is widely available and explains the thinking behind sRGB in more detail. The main argument for the specification comes in the first sentence "Hewlett-Packard and Microsoft propose the addition of support for a standard color space, sRGB, within the Microsoft operating systems, HP products, the Internet, and all other interested vendors." Inevitably any standard with Microsoft's backing has a lot going for it. More practically, with support at operating system level and through hardware-native devices, the potential is there to close the colour management loop and so to offer accurate and consistent colour throughout the production cycle. Throw in the flavour of the decade, the Internet, and you clearly have an attractive recipe.

Reading on, though, soon sets alarm bells ringing. A major concern stressed throughout the paper is the need to ensure "minimum system . and colour management overhead". Quality concerns always come in second-best to processing simplicity. As such, the move to a 2.2 gamma is justified because this is the natural gamma of the electron-gun in the "average" CRT display which therefore cuts out the need for a look-up table to convert values. "This, many readers will recognize, describes in a roundabout way what has been the practice in color television for some 45 years. This proven methodology provides excellent performance where it is needed the most, the fast display of images in CRT monitors." When the viewing environment is discussed, this paradigm shift becomes even more apparent: "The current proposal assumes an encoding ambient luminance level of 64 lux which is more representative of a dim room . (although) the typical office or home viewing environment actually has an ambient luminance level around 200 lux." In other words, sRGB is a standard designed more for watching in your sitting-room than using in your office.

sRGB is effectively the blueprint for colour management for Web TV. The problem of course is that print-based colour makes very different demands to TV-based colour. In particular, sRGB's sacrifice of absolute quality for processing simplicity and the cut-down gamut, which is unable to produce a saturated cyan and so bright printed greens and blues, just isn't acceptable for pre-press work. This isn't really a criticism of sRGB at all. sRGB was never designed with high-quality colour management in mind. In fact the sRGB specification paper makes clear that the whole purpose of sRGB is to fill in the gap where high-end, profile-based colour management isn't practical.

Incredible Defaults

It's not a criticism of sRGB itself then, but it certainly is of Adobe. They seem to have picked as the default colour space profile for the entire high-end print-based imaging industry a system whose whole raison d'etre is to provide "good enough" colour on "average" displays over the Web - and without the need for profiling! If there is one program that isn't about "the fast display of images in CRT monitors", surely it is Photoshop. Inevitably, criticism of the new sRGB default is mounting with various Photoshop gurus recommending restoring the Apple RGB working-space, switching to the SMPTE-240M or ColorMatch options or moving on to their own customised profiles.

With no clear lead from Adobe it's difficult to be sure exactly what the best approach to take is. In the long run, when supporting hardware arrives, the average office user may very well be better off staying permanently within the limitations of sRGB for all their work. sRGB's advantages as an outputting space for online graphics are more compelling - though make sure you save files with the "exclude non-image data" option to avoid embedding the unnecessary 2k colour profile. For pre-press work, though, the format is inadequate so it looks like high-end designers will have to get used to swapping between at least two working-space set-ups.

Practical Steps

After a semi-official nod from Adobe (via a sneak preview of their internal white paper), I think the safest option for quality print work is to use the ColorMatch RGB working space. This has a reasonably wide gamut that nearly encompasses the full CMYK range, is supported natively by Pressview monitors, and is so close to the space used in previous versions of Photoshop that existing RGB values in legacy images are hardly affected by conversion. Converting the grayscale calibration test between Apple RGB and ColorMatch profiles, for example, left the 10% increments completely unaffected. Even so, the last step I would strongly recommend is to go into Profile Setup and change the RGB "Assumed Profile" and the RGB "Profile Mismatch" handling to "Ask When Opening" so that, in future, Photoshop only converts colour values with your knowledge and approval.

When these changes have been made, the new colour management system can finally begin to fulfil its true potential. Adobe must be given full credit for devising a powerful and flexible management architecture that will eventually make accurate and consistent onscreen colour management across widely varying software and hardware a reality. At the same time Adobe must accept full blame both for the bizarre choice of defaults that has turned the system into a minefield, and also for completely failing to provide the open and intelligible advice needed to guide users through the dangers.

July 1998

Tom Arah

Interested (and technically-minded) readers can find more information on colour from Charles Poynton's encyclopaedic http://www.inforamp.net/~poynton/ColorFAQ.html. The sRGB specification is available at (http://www.w3.org/Graphics/Color/sRGB). An enlightening critique of Adobe's colour management by Photoshop gurus Bruce Fraser and David Blatner is available at http://www.pixelboyz.com/ps5/PS5_RGB.PDF. Finally, by the time you read this, the long-awaited, but much-postponed, colour management white paper may also be available on the Adobe web site http://www.adobe.com.

 


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