PI6 QuickTip #25: Tone Map Color Channels


Color correcting digital photos can be easier than you'd think is you use a tone map. When a photo shows a disproportionate amount of red, blue or green, the solution may be counterintuitive -- to increase the range of the overpowering color.

This digital photo of a mockingbird looks pretty good, but the overarching canopy of green leaves has tinted the light green, which results in a green cast to everything in the photo. The browns in the branches and the bird do not stand out well because the colors are all too green.

too much green

Show at 30% Actual Size

The first step was to choose Format, Tone Map (Ctrl+Shift+T) to access the Tone Map dialog box, and I clicked the Map tab. In the screen shot below, notice that when the Master channel is selected, the Use Complete Range Enhancement is grayed out. That might cause you to think that the full color range is being used.

Master channel Enhancements available

Use Complete Range isn't available, either, in the Red and Blue channels, which is the opposite of what you'd expect, given the excessive amount of green in this image. However, if you select the Green channel and click Enhancements, Use Complete Range does become available, indicating that some improvement is possible in the distribution of colors. 

green channel has complete range available

I selected Use Complete Range and subtle, but needed changes occurred in the photo -- notice that the browns in the branches and the bird show up stronger now, helping them stand out from the leaves. Even the bird's eye shows up better because the improved range of colors improves the contrast in this small area.

tone map applied

Because the image was reduced in size considerably for this QuickTip, some loss of sharpness was unavoidable. To restore detail and brighten the photo a bit, I chose Effect, Blur & Sharpen, Unsharp Mask. I like this command for sharpening images and adding a bit of light to JPG's, which tend to darken when compressed in the Image Optimizer. Instead of selecting from the thumbnails, I clicked the Options button to open a secondary dialog box which permits a little more tweaking.

Unsharp Mask settings

In a smaller image, low Sharpen factor and Aperture radius values are best. The default values were much too high for this photo. After some trial and error I selected a Sharpen factor of 15 and an Aperture radius of 3, which lightens the image just enough to improve contrast without "bleaching out" the whole thing.

Finally, I clicked the Quick Color Control's Contrast key's plus (+) sign 1X to improve the contrast. The final image is not so overpowered by green, is brighter, and has better contrast.

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