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colour mgt, calibration, profiling, sRGB, web viewing
There's a lot of confusion about colour management, so I'll try to clarify how it works a bit, with particular relevance for web viewing..

RGB colour mode:.

RGB is a colour mode, not a colour space. The mode is about the format of the numbers that make up the image and how they are interpreted to form an image. Other modes include Lab, CMYK and greyscale (grayscale). Most of us mainly use RGB images, so I'll stick with RGB here..

Colour space:.

In an RGB image, each pixel is defined by a set of three numbers that tell us the red, green and blue value for that pixel. The RGB value alone doesn't tell us what the colour should look like in the real world. The RGB values need to exist in a colour space that defines what real world colour each combination of RGB values represents. The colour space is like a map from raw RGB values in the image to some standard colour. Examples of colour space are sRGB, Adobe RGB (aRGB) and ProPhoto RGB (PPRGB). For each colour space, an RGB value means a particular real world colour.

That means that an RGB value that means a certain colour in the sRGB colour space will mean a different colour in the aRGB and PPRGB colour space and will be more extreme in those wider spaces, eg a brighter/more saturated colour..

Profiles:.

A display device has a certain response to RGB values being fed to it. The device profile is essentially a mapping of the RGB values in to the real world colours being output for those RGB values..

Calibration:.

Calibrating your monitor (or video system as a whole) is about making the right settings for your monitor/video system to behave in some kind of standard way. They include adjusting things like brightness, contrast (if available), RGB colour channel gains and possibly gamma settings. It is common to calibrate to set the white point and gamma for sRGB, ie a white point of 6500K and gamma of 2.2. Brightness is often set too high, especially on LCD monitors, which leads to prints looking much darker than screen display..

Profiling:.

Profiling is not the same as calibrating. Profiling your monitor/video system means creating a map of RGB values in to real world colours out. The best results come from using a hardware device to measure the monitor output in conjunction with suitable profiling software. An example of a profiling system is the Spyder 2. The result of profiling is a profile file that contains both gamma information, ie input/output brightness information, and colour mapping information..

Windows and gamma from profile:.

In Windows systems, a utility may run at startup to take the gamma information from the monitor profile, ie the brightness response information, and load it to the video card lookup table. In theory then, a properly calibrated and profiled system will display sRGB images at the right brightness for the sRGB colour space even if the display application is not colour managed. Non-colour managed applications include most Windows web browsers and simple image viewers. However, colours are not going to be accurate sRGB colours in those non-colour managed applications..

Web viewing and sRGB:.

There is a common misconception that non-colour managed applications like Windows web browsers interpret images as being sRGB colour space images and then display them as sRGb images. This simply isn't true. The truth is that non-colour managed applications don't manage colour at all. They don't make any assumption about colour space. All they do is take the RGB values of the image and send them to the computer's video system for display. Any adjustments you've made to your monitor's brightness, etc and any adjustments made through the system gamma loaded at Windows startup will influence the image being displayed, but the colours will not be adjusted in any other way and what gets displayed is not sRGB, but just the system's native response to RGB values.



SRGB and typical video response:.

The good news is that a typical computer video system native response is close enough to sRGB for sRGb images to look ok when viewed in non-colour managed applications. In other words, sending RGB values from sRGb images straight to the computer's video system without colour management generally produces a fair approximation to an sRGb image for general viewing purposes..

The right colour space for web images:.

As the majority of web browsers are not colour managed, it makes sense to use the sRGB colour space when creating images for web viewing so that most people get to see a reasonable representation of your image, even though the colours won't be accurate..

Remember, the web browser is not assuming a colour space or applying sRGB colour management. sRGB images just happen to look ok (generally) when displayed without colour management. Regardless of how good it looks, an sRGB image being displayed in a non-colour managed application is not being displayed acurately as an sRGB image. It's just one of those compromises that we have to live with, at least until colour management becomes more widespread in viewing applications..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comments (7)

Another area of confusion is in assigning colour space vs converting to a colour space..

Colour spaces:.

For an RGB image in a particular colour space, each RGB combination means a certain real world colour. Colour managed applications understand colour spaces and therefore know what real world colour each RGB value in an RGB image is meant to be..

Tagged or untagged with colour space:.

Each RGB image file can be tagged with a colour space, or have no colour space associatd with it and be untagged..

Tagged images:.

Tagged images can have a colour space that is either assigned or embedded. Just being assigned means that the file includes a statement of the colour space, but not a copy of the colour space. Embedding the colour space means a copy of the colour space is included. In Photoshop, you can only embed the colour space at the time of saving the file, which involves including a copy of the colour space..

Untagged images:.

If a file is untagged, a colour space can be assumed, eg sRGB, or can be ignored so that the image is not colour managed. An untagged sRGB image (ie a bunch of RGB values that will display correctly if interpreted by a colour managed app as being sRGB values, but with no information in the file to say that it's an sRGB image file) will display in a non-colour managed web browser exactly the same as a tagged one. Either way, the RGB values are displayed without being changed according to some colour space for display, so the display is not an accurate sRGB display, just an approximation that depends on how closely the video system aproximates an sRGB response..

Assigning a colour space:.

Assigning a colour space to an image includes a reference to the colour space in the image file to indicate that the RGB values in the file should be interpreted according to that colour space for determining the real world colours those RGB values represent. If the source file is an sRGb image and the Adobe RGB colour space is assigned, the RGB values remain unchanged and the resulting image in a colour managed application will look strange, with brighter colours. This is because the RGB values now represent a wider range of colours to cover the wider aRGB colour space..

Converting to a colour space:.

On the other hand, converting to a colour space is a different from simply assignign a colour space. When you convert an image from one colour space to another, you change the RGB values so that the new values mean the same real world colours in the new colour space. It's safest if you embed the colour space in the image. When you convert from a wider colour space, eg aRGB, to a narrower colour space, eg sRGB, there is some adjustment of some of the colours according to the rendering intent you choose (or the application chooses)..

Rendering Intents:.

The most common rendering intents used when converting from one colour space to another are:- perceptual, and- relative colorimetric (usually with black point compensation)..

Perceptual seeks to retain an image that looks pretty muchy like the original by retaining the relative differences between the colours..

RC (with BPC) seeks to preserve the more saturated colours better, but the result may look less like the original..

Choice of rendering intent often comes down to personal preference. I generally prefer perceptual, but it depends on the image..

Tagging for web viewing:.

If you're going to be displaying the image on the web and therefore using sRGB as the colour space, you can safely save the sRGB image without any colour space information and just display that on the web to keep the file size smaller. As I explained in my original post, the resulting display will usually be good enough for general viewing purposes. If it isn't, there's nothing you can sensibly do about it for thehttp://www audience, and there's nothing you can do for your own web viewing beyond calibrating and profiling to at least get the black point, gamma (brightness) part of the display and white point (brightness and colour of white) ok..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #1

Macs have color management in some applications. In particular, the desktop and Safari, Mac's browser, have color management. Thus images with embedded profiles will display properly in Safari assuming the display is calibrated/profiled. All of the images on my site below are sRGB and have embedded profiles. Their appearance on other Mac systems with Safari will be close to what I see.Leonhttp://homepage.mac.com/leonwittwer/landscapes.htm..

Comment #2

Leon Wittwer wrote:.

Macs have color management in some applications. In particular, thedesktop and Safari, Mac's browser, have color management. Thusimages with embedded profiles will display properly in Safariassuming the display is calibrated/profiled. All of the images on mysite below are sRGB and have embedded profiles. Their appearance onother Mac systems with Safari will be close to what I see.Leonhttp://homepage.mac.com/leonwittwer/landscapes.htm.

Hi Leon. Thanks for your comments..

As you no doubt realised, I was aiming the information more at the largest population of web users, which means Windows + Internet Explorer. A secondary target would include Mac users who want most people to see their images accurately when browsing the web, but may not understand the limitations, although I guess Mac users are used to hearing about Windows limitations, right? :^).

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #3

It would be nice if Microsoft would fix some of these things. A photographer friend of mine is sticking with XP for a while because he says that Vista is a step backwards in color management. Something about calibration data in the LUT being changed by the OS without notice.Leonhttp://homepage.mac.com/leonwittwer/landscapes.htm..

Comment #4

Leon Wittwer wrote:.

It would be nice if Microsoft would fix some of these things. Aphotographer friend of mine is sticking with XP for a while becausehe says that Vista is a step backwards in color management.Something about calibration data in the LUT being changed by the OSwithout notice..

That doesn't sound promising. I'm about to 'upgrade' from 32 bit Win XP Pro to 64 bit Vista so mt computer can access all of it's RAM. I wasn't aware of that issue. Hopefully reprofiling will overcome that issue, but from what you've said it doesn't sound like it will be that simple..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #5

That's a great discussion of this subject, and clearly, it's a needed discussion..

I hope a lot of people will read that and be helped by it..

It's a shame to hear about the issues with Vista, but I have read other articles and posts complaining of the somewhat bizarre things that Vista seems to do with color..

Jim H...

Comment #6

Hi Jim, Leaon and I had some exchanges and he put me onto Joe Kurkjian for information about Vista colour management issues. As a result, I started another thread in the PC Tools forum to ask about a reported issue with calibration information loaded to the video card LUT under Vista, but being dumped by Vista when it dims the screen to ask for user authorisation under it's UAC (User Access Control?) function. Hre's the thread:http://forums.dpreview.com/...forums/read.asp?forum=1004&message=26484497.

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #7

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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