Recently I've seen many people asking how to avoid channel clipping during editing in Photoshop. After some playing around, I've created an action for warning of impending blowouts. You can download it a little further down this page.
First, let's discuss clipping (particularly highlight clipping) ...
About channel clipping
The Red, Green and Blue channels consist of 256 levels each, on a scale of 0 (dark) to 255 (light). (This is true even in high-bit mode - Photoshop's interface still operates on a 0-255 scale).
A channel is considered "clipped" if there are pixels which have a value of 255.
I want to stress, though, that isolated pixels with 255 channel values are not dangerous - in fact, I would consider an image with a smattering of these pixels to be excellent utilization of tonal range, resulting in outstanding contrast.
Channel clipping becomes a problem where there is an area of clipped pixels. This results in a "blowout" in that part of your image. If one or two channels are clipped (eg Red channel in skin, Red and/or Green channels in a yellow dress, etc), then we see areas of colour but no detail. If all three channels are clipped, then we see flat, featureless white.
Clipping can be used deliberately to great artistic effect. But in most cases we want to avoid it. So we find ourselves performing a delicate balancing act between lovely bright colours, and blowouts.
By the way, I've encounted a strange school of thought that anything above 245 is clipped. This misunderstanding arises when people are advised to keep their highlights at 245. Regardless of your tonal range strategy, you may be assured that 255 is clipped, anything less is absolutely fine.
Clipping warnings in Adobe Camera Raw
ACR has an excellent clipping warning feature - just turn it on and it shows you where the blowouts will be. I'm sure other Raw processing programs offer the same feature.
It's this feature that I wanted to simulate in Photoshop.
Checking for clipping in Photoshop
The first two methods for checking clipping are the Info Palette, and the Histogram.
As you move your mouse over suspicious areas of your image, watch the RGB values in the Info Palette. If you see areas of 255, that's clipping. This method isn't infallible, though - the accuracy will depend on how closely you're zoomed in, and of course you need to know where to look, unless you've got time to run your mouse over every pixel in the image!
The Histogram tells you there's clipping if you see data jammed up at the ends of the graph. This isn't perfect either - you need to view each channel's histogram separately for accuracy, and even when you see evidence that clipping exists, it might not be clear what part of the image is affected.
We want something easier.
The Threshold layer method
The simplest tool Photoshop offers for checking clipping and near-clipping is a Threshold adjustment layer. If you set the Threshold Level to 250, for example, it shows you what pixels are above that level.
BUT ... this only shows you luminosity clipping. So it's quite possible for one channel to be clipped while the others remain low, and the Threshold layer doesn't tell you. It's really only handy for black-and-white images.
We want something more precise.
Clipping warning in Levels and Curves
As you're probably aware, the Levels dialog (and in recent versions of Photoshop, the Curves dialog as well) has an excellent built-in clipping warning. Just hold down the Alt key as you drag the highlight slider, and it shows you exactly what's blown out.
That's great ... for Levels and Curves. But it doesn't help when you're adjusting Hue/Saturation, or doing a defog, or running an action on your image.
We want something more flexible.
My Clipping Warning action
Here it is:
(Mac users, you might have to Option-click the link to make it work.)
It creates two adjustment layer groups, which sit above your image, and are turned off by default.
Simply turn them on to see what's clipped or almost clipped.
1. The action is set up to warn you of any pixels that are within five levels of clipping - ie over 250 in the highlights, and below 5 in the shadows.
2. You can't use both warnings at once - sorry. Turn off the Highlight Warning before turning on the Shadow Warning, and vice versa.
3. This action should work in both Photoshop and Elements.
4. In case it doesn't work for you for some reason, I've also provided a small PSD file with the warning layers already created, if the action won't work for you: DOWNLOAD THE PSD. Simply drag the layers onto your image.
Let's see it in action
I have a photograph with some blues I want to saturate. I run the action to create the Clipping Warnings:
I turn on the Highlight Warning, then make a Hue/Saturation adjustment layer above my image:
+50 is too much saturation - I can see some significant blue channel clipping:
I ease it back to +40, which is perfect - only a few speckles:
If you have a question about this action, or any Photoshop question, please visit me at my "Ask Damien" Facebook page.
All content © Damien Symonds 2014