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Title: July 2008: Innovations in Camera Profiling Post by: admin on May 27, 2009, 03:10:59 PM Innovations in Camera Profiling
Background In January 2007, I demonstrated a method for creating an ICC profile for your digital camera using a standard IT8 target. The article covered how to set up and shoot the target and how to process the photo and create a profile using my Profile Prism software. While the process was relatively simple, camera profiling has always come with some limitations and tradeoffs. The biggest problem with camera profiling is being able to create a profile that works for all photos. Because things like exposure, lighting, and white balance are always relative, creating an ICC profile from a shot of a reference target can always be problematic as the camera often uses different tone curves for different subjects and lighting. In addition, almost all cameras and developing software tend to "enhance" the tone curves to produce more vibrant photos because "linear" tone curves can look a bit dull. The question I often got from people trying to create camera profiles was, "How can I create a profile that corrects color problems without modifying the tone curves or making the image look dull." Until recently, this was not possible or at least not easily achieved because an ICC profile will always try to correct all aspects of color: tonality (brightness), hue, and saturation. With the recent release of Profile Prism v6.5, it is now possible to create hue correcting profiles that correct problems such as reds looking too orange, blues looking too purple, undersaturation of yellow, and so on without changing the contrast chosen by the camera or developing software. Let's take a look at how this is done.
The problem A device (camera) ICC profile is a file that describes how to accurately reproduce color for that device. Unfortunately "accurate" profiles are rarely what people want or need because they produce linear tonality in photos which can look dull: like there is a fog over the photo compared to what we are used to seeing. In other words, we are used to seeing the linear/accurate result that has been modified to add a little "pop" to the photo. This usually entails making the shadows a little darker and the highlights a little brighter. This is done automatically by your camera or raw software and is often not optional. If you notice some minor shifts in color when using your particular camera and you want to create a profile to keep that red sweater from turning orange, you might think an ICC profile is the best way to do this without having to edit the photo manually each time or eyeballing corrections using color channel sliders. You'd be right, except when you create that profile, it'll not only correct the red/orange shift but will also "undo" the tonality adjustments that make your images pop. That's the nature of an ICC profile and ICC profiling tools: they try to be all things at once, describing luminance, hue, and saturation accurately rather than how you may want to see it. Another issue with creating camera profiles is that it has traditionally been difficult to impossible to create a good camera profile for JPEG images straight from the camera. While creating profiles for raw developing tools worked reasonably well, how do you create a profile that corrects color issues in a JPEG that came from your camera: a JPEG that has already been "profiled" once to a color space such as sRGB or Adobe RGB but one that may have a few hue/saturation mistakes in certain areas? Fortunately a solution now exists that can address both problems, allowing you to create camera "calibration profiles" for any raw developing tool, any camera, and any in-camera JPEG without changing brightness and contrast.
The solution The solution to correcting color without dulling your images lies with the profiling tool. It must be able to discern the underlying tone curve along with the "enhancements" made to that tone curve in order to reproduce the intended contrast. Doing so will allow color (hue) corrections without changing overall contrast or brightness. Most cameras and raw developing tools allow the photographer to select tonality settings such as "neutral" or "vivid" and those selections allow you to make a decision about contrast. The biggest complaint and one that has traditionally been impossible to correct in camera (or in raw developing software) is one of hue shifts where colors look shifted in hue or under/over saturated. Let's take a look at how to create one of these calibration profiles using Profile Prism v6.5:
That's it! By choosing "Gamma Match (Auto)" in Profile Prism's "Tone Reprod. Curve", you are telling it to discover the intended/underlying gamma curve so that it can reproduce the same brightness and contrast, correcting only errors in hue and saturation. This method should work in the majority of situations. The manual gamma match options such as Gamma Match (2.2) or Gamma Match (1.8) only need to be used if the resulting profile appears to make images look too dull or too contrasty. In those situations, Profile Prism may not have been able to automatically detect the proper curve due to the camera or raw developing software manipulating the curves too much. In all situations, manually selecting either Gamma Match (2.2) or Gamma Match (1.8) will solve the problem and restore the original brightness/contrast.
Camera and raw software settings The beauty of the gamma match camera profiling options is the fact that they can be used to create non-tonality-modifying calibration profiles without trial and error modification of color channels. Due to the fact that they correct only color shifts and saturation problems, they can be used on any type of photo from your camera whether JPEG or raw. But what about camera settings or raw developing tool settings? What should you use? The answer is simple. Again, because these profiles are only correcting (presumably small) shifts in color and saturation, you would use whatever method you normally use to capture photos and then create a profile for those developed photos. For example, if you normally shoot in JPEG mode and you have your camera set to Adobe RGB color space, keep doing the same: take your shot of the IT8 target and then develop the profile based on that Adobe RGB JPEG from the camera. The resulting profile is then assigned to the image. The assigned profile overrides the initial Adobe RGB color space and assigns a profile that describes color more accurately than Adobe RGB. By assigning the profile and using color management aware software (like PhotoShop or Qimage), your corrections are automatic because the software you are using will see and utilize the new (corrective) profile. This is the preferred method since there is no second/additional profile conversion. Your calibration profile in this case is doing nothing more than modifying how to interpret the RGB values in the photo. If you are creating photos to be viewed in non color managed software such as photos that will be viewed on the web or via email, you'll want to convert from the camera profile to a standard color space like sRGB rather than just assigning the camera profile. Whether you choose to assign your camera profile or convert from that profile to a standard color space, the profile should correct all color issues without affecting brightness and contrast. When creating profiles for photos processed in raw developing tools, the same rule applies. You can keep all your raw development settings in place and create a profile to assign after the photos have been developed. Raw tools give you one additional option, however, in that some raw developing tools allow you to turn off color management and create a profile based on the raw data. Most tools offer the ability to set your color management or camera profile to "none" or "embed camera profile". This effectively turns off all color manipulation while only applying a tone curve (gamma). This method is even better because you can profile the photo before any changes have been made to hue or saturation. Raw developing tools that allow you to turn off color management usually offer a way to activate the new profile within the software so see the program help for your raw developing tool for more info. Of course, if you use more than one raw developing tool, you must develop separate ICC profiles for each raw developing program as they all produce color slightly differently.
Summary Camera profiling has always been hit or miss due to the fact that exposure, lighting, and other factors are not constant from shot to shot. As a result, camera profiles often cause unwanted changes in brightness or contrast as the profile tries to "correct" for the preferred tone curve of the camera or raw developing tool. I've found that people are almost always happy with brightness and contrast but often want to make subtle changes to color in order to correct issues with saturation or color shifting. Because existing profiling tools are designed to correct all aspects of color including brightness and contrast, people often find that ICC profiles cause unwanted changes in brightness and contrast in addition to correcting hue and saturation issues. This has forced most people to create manual color "calibrations" or macros by using generic color charts (often with only a few colors), eyeballing differences, and changing color channel sliders to compensate. With Profile Prism v6.5, it is now possible to create hue/saturation correcting profiles that do not alter brightness or contrast. Such profiles can be described as color calibration profiles and as far as I know, no other tool currently has the capability to create profiles that correct hue and saturation while leaving brightness and contrast untouched. Being able to create calibration ICC profiles has some significant advantages over creating color calibration routines or macros:
Mike Chaney |