Of all the weird and wonderful Adjustment Layers that Photoshop offers, Channel Mixer is one of the least understood and most powerful. With that power comes responsibility: it's also one of the easiest ways to trash an image. Yet, it can be the only way to rescue a truly broken image. Open an image of your choice, and we'll dig in to see what makes it tick.
To apply any adjustment accurately, these panels are helpful '" but for Channel Mixer they can be essential. From the Window menu, choose Info, then choose Histogram. In the Histogram panel submenu, choose All Channels View, to see the individual R/G/B or C/M/Y/K channels in your image.
(Today, I'd definitely recommend staying with RGB images all the way through the layout process until export time in InDesign, but you might have some older CMYK images you'd like to fix '" as in the example here. Channel Mixer is useful with all color modes.)
The original image, converted from RGB to CMYK and with Auto Levels applied, but a weak K channel.
Click and hold on the Eyedropper tool, and you'll see the Color Sampler tool. Offering up to four stationary eyedroppers, it lets you sample multiple parts of the image in a consistent way, no matter where your cursor happens to be. Look over your image, and click four times to set down four color samplers in different parts of the image. Pick the brightest spot, the darkest spot, and other parts you might want to keep an eye on. Reposition any samplers in the wrong places by Command-dragging them.
A close-up of a color sampler in the image.
Several ways to apply adjustment layers are available, but the yin-yang-like menu at the bottom of the Layers panel is probably the easiest. Click the icon and choose Channel Mixer'¦ to apply it. So, what does it do?
The yin-yang-black-and-white-cookie-menu in the Layers panel.
While other tools like Levels or Curves can massage data within each channel (i.e. R/G/B/C/M/Y/K etc.) Channel Mixer can take information from one channel and apply it to another. If the K channel in a CMYK image is completely missing some areas of detail, no amount of work with Levels or Curves can bring it back. Channel Mixer, though, can recreate it from data in the other channels.
The initial Channel Mixer panel, with no changes.
Start out by looking at the Adjustments panel, to see the Output drop-down menu. This is the channel you're currently adjusting, and when you begin, the output is set to 100% of that same channel and none of the others '" no change. You can adjust any of the source channels to increase or decrease the level of the current output channel. Herein lies the danger '" it's very easy to clip the image data.
If these color samplers hit 0% or 100% K, that's too much.
As you adjust the input channels up or down, watch the Info panel to make sure that shadows and highlights aren't clipping. It's difficult to see it happen simply by watching the image, which is why you should watch your color samplers (to check before/after values) and Histogram (to check for clipping).
The all-channels view is always handy when correcting images.
This example CMYK image was created by a simple conversion from a flat, uncorrected RGB image, and it's missing detail in the K channel '" large areas of nothing in the sky at the top.
The K channel, before.
By choosing K as the output channel, you can add information from C, M and Y.
The Channel Mixer panel after some tweaking.
The K channel is boosted all over, manufacturing new data in the sky which can't be created any other way.
The K channel, after, with manufactured image data.
How do you protect the rest of the image? Easy. Use the Layer Mask that comes with each Adjustment Layer, and paint black over the parts you don't want to affect. If you only want to adjust a small part of the image, use Edit > Fill to fill everything with Black, then paint white over the parts you do want to change.
This mask has had some minor work to reduce the effect in the deep shadows.
The Channel Mixer is a very effective rescue tool when used correctly. While Levels and Curves are easier to manage and harder to use wrongly, they just can't do what the Channel Mixer can. If you need to manufacture something from nothing, it's the tool for you.