Every now and again, i hear from someone asking me about the appearance of banding in their images. Most often this appears on a print, but did not appear on the screen image, although when starting with a JPEG original and editing it, banding can appear on the screen as well.
Banding is caused by an out of gamut condition. At a simple level gamut is a range of colours, tints, shades and tones. Most of the time we are dealing with colour images, but you can be out of gamut in a grayscale image as well.
If we see an out of gamut condition, it means that the device is incapable of rendering the particular colour being requested by the image. While this can happen on a screen, particularly with a low bit level graphics adapter, or when editing an 8 bit file and expecting a response that there is no data to provide, it is most often seen on a print.
I will go to my default starting point immediately. If your display is not calibrated, your chances of getting accurate representation of the data in the file is pretty much zero. Close perhaps, but more likely, out, as in way out. So get a colour calibrator and calibrate your display. They are not crazy money and are very easy to use. Editing without colour calibrating is like trying to match a paint chip when you are colour blind.
Ok, back on track.
JPEGS Suck and Blow Simultaneously
In the case of a screen being out of gamut, this happens most often with a JPEG out of camera that you are trying to push too hard in editing. By push too hard, I mean a slider shift in most any setting of more than 20 points, more than 1.5 stops. JPEG is a highly destructive file format. To learn how this is, please search this site for articles about JPEG and RAW. By the way, if the screen is going to be out of gamut, a print, or print file is going to be WAY out of gamut.
So if you don't have to shoot JPEG and do plan on editing, don't shoot JPEG. A major source of out of gamut display issues is solved.
Moving To High Quality Files
Now guess what? Your editing program will not tell you when you are out of gamut unless you specify an output ICC profile. In the Lightroom Develop module when using the Soft Proof function, the histogram changes to offer you out of gamut warnings for screen and paper instead of out black or white clipping warnings. These warnings are incredibly useful and rarely ever used.
Even if you don't print, but are making files for a web based portfolio or for display on web based services, it is worth your time to go activate Soft Proofing to make sure that your edits are not going to be out of gamut for a screen. Simply choose sRGB as your ICC selection as all JPEGs must use the sRGB colour space. Turn on the gamut warning. In Lightroom this defaults to a blue overlay wherever the image is out of gamut for the screen. You'll see an example a bit further on.
If you do print, you can also activate the out of gamut warning for print. For this to be accurate you have to choose the right ICC profile for the paper that you will be using, or if making a print file to send to a lab, choose sRGB. If you are still using the right click Export function to make files for printing, you should stop and do it through the print module. And you should soft proof otherwise the print that you get back may not come back as you expected. Remember that a print is based on the data in the file. The print is correct, according to the file data. If it looks wrong, either the screen was not calibrated when you edited or your editing pushed the image outside the very limited gamut of sRGB.
Lightroom Screen Shot Examples
I immediately know that the amount of editing changes that have been made have rendered this image unusable for a screen file or a print. I can see it right on the soft proof image. Time to go back to the old drawing board on the edits because this one is already a failure.
Now that we can see that this image, that looked just fine UNTIL we activated the Soft Proof function really is not going to cut it as edited for the screen.
Let's see what the print gamut warning might tell us.
Let's find out what would happen if we printed this to a file as a JPEG in the sRGB space
Yikes! This gamut check for an sRGB colour space print from a JPEG file shows us that well over 30% of the image is going to be out of gamut. If I sent this to a lab and got back garbage, that would be entirely my fault.
Let's take a look at a graphic showing the settings and options available in Soft Proofing.
This is actually quite straightforward. All you have to do is turn Soft Proofing on to make this available. The one setting that people get confused about is Intent.
This refers to rendering intent, and for our purposes, the two that Lightroom offers are all that we really need. I default to Relative for my work, but it doesn't hurt to try Perceptual. In a Relative intent, the colours fall where they do accurately. If there are colours that are out of gamut, they get truncated from the file or data stream and the closest in gamut colour is used, typically the closest boundary colour.
Perceptual intent changes the in gamut colours to try to make space to squeeze the out of gamut colours into the available gamut. It may allow you to make something of an image that doesn't work in Relative, but you accept that there is going to be a colour shift to make this happen. It's personal choice, although most pros will try to edit to get Relative to work.
What's This Re-Edit Stuff?
A display is projected, a print is reflected. Therefore a file edited for screen will not look correct unless reedited for print if it is printed. When we activate Soft Proofing and touch a slider, Lightroom asks if we want to make a proof copy. We do. In fact you can do so manually by clicking the Create Proof Copy button in the Soft Proof Dialog shown above.
Now you edit your image again for your output target. You can do this for screen, but I believe that an image is not done until printed so I will speak to that. When you activate Soft Proofing and Simulate Paper and Ink, the image looks duller, drabber and has less contrast. So work with the sliders to get it where you want it to be. Some folks think that you can just increase exposure and contrast and all will be well, but this is misleading. Turn on the gamut warning for your output type, such as print, and edit the image so everything important is in gamut. When we find an image such as the sample that is so far out of gamut, this is usually the result of excessive saturation manipulation, but it could come from adjustment layers boosting brightness or contrast, or aggressive manipulation of hue, saturation and luminance colour sliders. The example image has had the blues and aquas have their saturation increased and their luminance decreased. The reds have also had their saturation pushed. The entire sky was selected and a mask was applied to darken the sky while leaving the rocks alone.
How Did We Get Here?
Typically any time a slider is pushed very hard, the risk of pushing an image out gamut increases, but some sliders like Saturation and Dehaze will push you out of bounds much faster than others. Be gentle in your edits, don't push too hard and you should be fine.
In the End
When we look at the edited image on screen in the Develop Module, it looks ok, as seen in the example, but as soon as we start inspecting the gamut, we can see that this is going to be horrible. Even with the great range of a RAW file, you can push it too hard and have it look ok in the Develop module but reveal itself as being out of gamut when doing gamut checks.
Learning to edit within gamut constraints is a skill that you develop. Be aware that some plugins or luminosity masking tools could produce a nice looking image that turns out to be a gamut disaster, so start with your end in mind and save time and frustration.
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