Photoshop 7

Colour Settings

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By Ian Lyons

 

 

The eagerly awaited Photoshop 7 is now with us and we find that Adobe has made a few minor but important changes to colour management. This essay will explain these changes and also make suggestions as to how the Photoshop 7 colour management system should be configured, although the reader shouldn't consider my preferences "the best", just a base point from which to begin the process.

 

For ease of reading and to assist with quicker download of screenshots, etc. I have broken the essay down into small sections. You can easily access any section by clicking the appropriate link in the Index shown below. I've also included a printer friendly version (850 Kbytes) in the form of an Adobe Acrobat file. Click the following Icon to download it

 

Index

 

To skip to a specific page simply click the underlined text

 

Page 1 Introduction and  Colour Management Primer
Page 2 and 3 Monitor calibration
Pages 4 and 5 Photoshop colour settings
Page 6  Colour Management Policies, Conversion Options and Advanced Settings
Page 7 Soft Proofing
Page 8 Assign Profile, Convert to Profile and Saving Files

 

Colour Management Primer

Ever since the beginning of colour reproduction, colour management has existed in one form or another. The basic concept underlying Colour Management is to ensure that colour data is processed in a consistent and predictable way throughout the entire imaging workflow. A typical Imaging System will consist a wide range of Input and Output Devices, and each device will reproduce colour differently. This means that a colour represented by one device will rarely if ever match the same colour represented on another device. In other words, colour is Device-dependent. So expanding upon our earlier definition we can say that the purpose of a Colour Management System (CMS) is to maintain the consistent and accurate "appearance" of a colour on different devices (e.g. scanners, monitors, printers, etc.) throughout our imaging workflow.

Components of a Colour Management System

In order that we can achieve the above aims a colour managed system will require three basic components, namely: -

  • A device-independent colour space - this is usually referred to as the Working Space or Reference Colour Space.
  • ICC/ColorSync device profiles for each device (printer, scanner, monitor, digital camera, etc.) that describe the colour characteristics of the specific device.
  • A Colour Matching Module (CMM) that will interpret the information contained within a device profile and carry out the instructions on the way the colour gamut of each device should be treated.

The following diagram demonstrates a typical "colour-managed workflow" and shows the image being passed along the chain - from scanner/digital camera - to - computer - to - monitor - and printer with the ICC profiles ensuring that the colour data from/to each device is correctly described.

 

Image

 

Colour Numbers, their Meaning, and Profiles

A digital image will comprise pixels each of which is represented by a number. This number will describe the location of the pixel within the image and its particular colour value (typically an RGB value). We have already noted that since colour is device-dependent the "appearance" of each coloured pixel will vary for each device. We also noted that this is because each device has its own unique way of translating the raw colour value into visual colour. To minimise the discrepancies that result from the widely differing colour characteristics of each device we use the device profiles to inform the CMM how the colour values produced by that device should actually appear. This may be on our monitor, in print or on film output. In simple terms it is the device profile that conveys the "meaning" of the raw colour numbers associated with each pixel.

Whilst "Consumer "class film and flatbed scanner software applications are now ICC/ColorSync aware they tend to be based upon sRGB, likewise consumer digital cameras. This colour space is not normally regarded as appropriate for high quality image editing, especially when print or film output is required, but is quite often all we can expect to get. We should not confuse matters by assuming that the scanner or digital camera is an sRGB/Adobe RGB device - they most certainly aren't. The fact that the image delivered into Photoshop by the scanner or digital camera application software is already in a device-independent colour space means that it has already undergone a considerable amount of data processing.

Some digital hardware vendors allow scanning/capture of images in any user selected "working space" colour space. Unfortunately, the image is rarely delivered into Photoshop with an embedded ICC/ColorSync profile, which can lead to confusion and error.

Device profiles come in two basic forms, i.e. "Input" and "Output". Input profiles typically describe the colour characteristics of scanners and digital cameras, whereas Output profiles describe devices such as monitors, printers and film recorders. Input profiles are often referred to as one-way since it is only possible to select them as the "Source" meaning we can never convert an image into the colour space of our scanner or digital camera. Output profiles are two-way meaning we can convert "From" or "To" them.

Many "Prosumer" class scanners and digital printers are now supplied with some form of generic or "canned" profiles. Whilst these profiles are useable they are rarely accurate.  For truly accurate colour matching you should seriously consider getting "Customised" profiles for each device and/or media type. These profiles can be created professionally or by buying your own profiling software. Sadly very few, if any digital camera vendors have adopted the "canned" profile approach instead opting process the images into a device-independent colour space as discussed above.

Why bother with Profiles?

Even though colour correction and colour management are not the same thing they are often confused with each other, especially by the novice Photoshop user. The colour characteristics of most imaging devices are such that it is very rare for them to be truly linear (i.e. R=G=B=Neutral). Sometimes this characteristic is referred to as the device "not being well-behaved". Scanners and printers are good examples of "badly behaved" devices. Obviously it would be extremely difficult for a Photoshop user to edit an image where a group of pixels with values of R=G=B=128 (grey) actually "appeared" to be non-neutral. In such circumstances colour correction would be an absolute nightmare. To overcome these discrepancies we usually carry out all our editing in a colour space that is well-behaved. Well behaved colour spaces are more usually referred to as the "working space", and are always characterised by having R=G=B appearing neutral. Without the aid of accurate device profiles the accurate translation of the raw colour data (the numbers) from our scanner/digital camera into the working space will prove very difficult, if not impossible. The translation from the working space into the media specific colour space of a digital printer will prove equally difficult without the aid of media specific printer profiles.

So the benefit offered by colour management is that the process of colour correction can be undertaken by the user in the knowledge that the image displayed on their monitor is an accurate visual representation of the original subject, and that the final print will accurately reflect the colours of the displayed image.

 

What changes have been made to colour management?

 "EXIF" is a familiar term to many digital camera users, but until Photoshop 7 it had little use. EXIF is a standardised format agreed by various digital camera vendors for incorporating data such as the exposure time, aperture, lens, picture resolution, colour space, etc into the image file. However, Photoshop 7 now reads the EXIF information embedded within the JPEG format images captured by digital cameras and can also write new information to the EXIF file. There are a number of quirks in the way Photoshop 7 manages the colour space of these image files, and so those already familiar with Photoshop 6 should be prepared for a few surprises.

The screenshot below is presented for those readers who've never seen EXIF data before. I captured the information file from the new File Browser window but you can also get it by selecting "File Info > EXIF" from the Photoshop File menu.

 

Image

 

So far as colour management and Photoshop 7 are concerned EXIF holds some useful information (circled red in the screenshot), and this information. The first piece of  information is the potential availability of an embedded "Colour profile" that describes the actual colour space of the image. The second is the EXIF "Colour space" to be assumed in the event of no colour profile being embedded. The embedded colour profile can be anything the camera vendor chooses to allow or nothing at all. At time of writing this essay it appears that only Nikon have chosen to allow embedding of colour profiles, and so many users will never see an embedded profile. On the other hand the EXIF colour space appears to be widely used.

It is worth noting that the EXIF colour space recorded within the EXIF information can only be one of two values, i.e. sRGB or Uncalibrated. Generally digital camera manufacturers have set their software to automatically write sRGB as the EXIF colour space. This means that in the absence of an embedded colour profile Photoshop 7 will attempt to open JPEG format digital camera images into the sRGB colour space. If the EXIF colour space has been set to Uncalibrated Photoshop will rightly assume that the image has no predefined colour space information and the image will not be colour managed (the image will be defined as Untagged RGB).

The screenshot shows the EXIF file for a JPEG image captured on a Nikon D1X digital SLR camera. The owner had configured the camera software to output the image in Adobe RGB and as we can see the Nikon software has been very obliging and embedded the Adobe RGB colour profile. Prior to Photoshop 7 this colour profile could not be read and so the image would have opened with the "Missing Profile" dialog flagging a potential problem - not anymore! The image will now open directly into Photoshop or present the user with a "Profile Mismatch" warning! Note: Each of these terms will be discussed in Part 2 of this essay.

Also notice that the screenshot shows the EXIF colour space is set for sRGB - so we have an image with two conflicting sets of colour space information. Thankfully Adobe has coded Photoshop 7 to give priority to the colour profile and when it's not present Photoshop will use the EXIF colour space information.

2 September 2002

It appears that some users were getting confused with the appearance of the Profile Mismatch dialog so in late August Adobe released a Plug-in designed to make Photoshop 7 behave in the same way as earlier versions, and so ignore the presence of the EXIF colour space information. The new plug-in is called Ignore EXIF Colour Space and can be downloaded from:

PC platform - http://www.adobe.com/support/downloads/detail.jsp?ftpID=1881

Mac platform - http://www.adobe.com/support/downloads/detail.jsp?ftpID=1882

 

Some Photoshop Revision!

Photoshop 7 continues to use document/image specific colour settings, which means that the colour space of each document is independent of others that may be open on the Photoshop desktop. As with Photoshop 6, the working space defined in "Colour Settings" only has a bearing on three types of image, viz.: -

  1. new images/documents;

  2. existing images/documents without an embedded profile; and

  3. images/documents with no embedded ICC/ColorSync profile (i.e. "untagged images/documents").

"Image specific" colour means that it's the profile embedded within an image that determines how the image will be displayed (it's appearance) and not the Photoshop working space. With Photoshop 7 we can have multiple images, each in its own unique working space open at the same time and each will be displayed accurately (of course this assumes a well calibrated monitor).

Note that the "Assign Profile", "Convert to Profile" and "Proof Setup" commands have undergone minor alteration. I will discuss each of them in detail later.

Also note that except for the obvious difference in user interfaces; Photoshop 7 for Mac OS9.x, Mac OSX and Windows share a virtually identical feature set. Therefore the Windows based screen shots that follow should be equally useful to Mac users.

 

 

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2002 Ian Lyons. All Rights Reserved