Image editing with Picture Window Pro:
Contrast masking
by Norman Koren

Site map/guide to tutorials
Contact | News

Making fine prints in your digital darkroom
Understanding image sharpness and MTF
Image galleries / How to purchase prints
Photographic technique
Image editing with Picture Window Pro

A simplified zone system
Canon FS4000US 4000 dpi scanner
Epson 2450 flatbed scanner
Digital vs. film | Canon EOS-10D
Building simple web pages
updated January 28, 2004
Color Management by X-Rite  Canon  Epson  HP  i1  Eye-One - Densitometers - Spectrophotometers - Eizo - Canon Large Format
 
Help support this site by linking to these merchants
or

In Association with Amazon.com
for books & merchandise

for all your photographic needs



View image galleries

Google

Search WWW Search www.normankoren.com
Table of contents

for Image editing with
Picture Window Pro

Introduction
Making masks
Tinting and hand coloring B&W images
Example: Sunset, Providence
Contrast masking
Introduction | With Photoshop
With Picture Window Pro | Create mask
Histograms | Blur mask | Transform areas



for the Making Fine
Prints series
Getting started | Light & color
Pixels, images, & files | Scanners
Digital cameras | Printers | Papers and inks
Monitor calibration and gamma
Printer calibration | Scanning | Basic image editing
Black & White | Matting and framing
Tonal quality and dynamic range in digital cameras

Color Management: Introduction | Implementation
Profiles with MonacoEZcolor | Evaluating profiles

.
This page illustrates contrast masking-- a technique adapted from the conventional darkroom for balancing shadow and highlight regions of an image while maintaining good contrast in each.

Introduction: Contrast masking the old way

Color transparencies (slides) look beautiful when projected, but their high contrast can make them difficult to print. Shadow and highlight detail is often lost on standard photographic papers. Detail can be maintained by printing on low contrast papers, but the resulting prints can be quite dull. Contrast masking was developed to deal with this issue; with a contrast mask you can make prints with excellent detail and contrast in both shadows and highlights.

In the wet darkroom contrast masking was difficult and tedious. It is described in a page from The Light Room. The essential process is as follows.

The contrast mask tends to leave an image unchanged near mask boundaries. It lightens shadow regions and darkens highlight regions away from boundaries. Its effect depends on the mask exposure, which determines boundary locations, its development, which determines the degree of contrast reduction, and the amount of blur. It isn't easy to control with traditional techniques..

Contrast masks can make an image appear sharper because then enhance contrast at edges relative to the image as a whole. They are the origin of the "Unsharp Mask."

Even though Kodak has discontinued its Pro Masking film, you can still do contrast masking in the traditional way with a kit from RADEKA photography. Their page has some impressive B&W examples.

But contrast masking is ever so much nicer and easier when done digitally. And contrast masking isn't just for color slides-- it can benefit images from any source-- black & white or color; film or digital.

Contrast masking with Photoshop

In December 2000 Richard Pahl of PCPhoto published an article on contrast masking with Photoshop. His technique has been adapted by Michael Reichmann of  Luminous-Landscape.com and Uwe Steinmueller of Outbackphoto.com. with superb results. Ake Vinberg and Nick Thomas have also written well-illustrated articles.

The technique is as follows.

  1. Click on Layer, Duplicate Layer or Right-click on the image name in the Layers palette and select Duplicate Layer. Give the new layer a distinct name like Contrast mask.

  2. .
  3. Select the new layer and click on Image, Adjust, Desaturate to make it into a grayscale image.

  4. .
  5. Click on Image, Adjust, Invert to create a negative of the grayscale image.

  6. .
  7. In the Layers palette, set the box on the upper left (Blending mode in Layer, Layer style, Blending Options) to Overlay. Set Opacity to about 80%. You can fine tune it later. (Michael and Uwe differ in the order and details of this step.)

  8. .
  9. Click on Filter, Blur, Gaussian Blur to blur the mask layer. Adjust Radiusfor the desired effect, which will be visible. A good starting point is 1% of the average image dimension in pixels (e.g., 25 for a 2000x3000 pixel image). Values between 10 and 100 are typical.
The full technique, with illustrations and additional detail, is presented in the pages by Richard Pahl, Michael Reichmann, and Uwe Steinmueller. The results are beautiful. To me, this technique seems a bit convoluted, worthy of Rube Goldberg, though it's no more complex than the technique I present below. Its main problem is that it is quite restricted. As you'll see, my approach is far more flexible.
.
 Contrast masking and visual adaptation
When your eye wanders around a natural scene it constantly adapts to changes in brightness and contrast. But when you look at a print your eye adapts very little; mother nature's visual cues are absent. Prints have a more restricted tonal range than natural scenes: about 100:1 at the most. This discrepancy must be dealt with in making prints. Contrast maskingis one of the most effective means.

Contrast masking simulates the eye's adaptation in a natural scene: it lightens dark areas and increases their contrast; it darkens light areas and enhances detail. Overall contrast is boosted near boundaries: dark areas appear darker and light areas appear lighter than they do at a distance from the boundaries. Boundaries appear natural if an appropriate mask blur radius is chosen. This is a matter of experience. The wonderful thing about digital editing is that you acquire experience very quickly.

Contrast masking with Picture Window Pro

Original image: tones as scannedThe new technique is similar to the old in that you create a mask-- a blurred monochrome image based on the tones of the original image. But the process for creating it is far more flexible. The mask is also used differently. Instead of combining it with the original image (which you can do with Picture Window Pro's Composite transformation), you use it to select portions of the image to be adjusted with transformations. This gives you immense power and control.

The first step in making a contrast mask is to take a close look at the image and decide what you want to accomplish.

We use the image of bristlecone pines on the Arapahoe Glacier trail as an example. I often return to this spot, at timberline in the Indian Peaks Wilderness, an hour's drive and a 90 minute hike (less if you're young and fit; I'm 59) from Boulder. The light is always changing and often magical-- enough to lighten the burden of schlepping heavy camera gear at 11,000 feet (3300 meters). I like the composition-- I got a dynamic element that I don't often get, but it is difficult to print. The sky is very light, and the foreground is rather flat and dark. The goal is to bring the foreground to life by boosting its contrast, along with highlights and color saturation, and to darken the sky. The foreground will be mostly unmasked and the sky mostly masked. (Which portion of the image is masked and which is unmasked is entirely arbitrary.) As you'll see, masks don't have to be perfect. The Color Curves transformation overcomes small mask imperfections.

In the past I tried to make a rather sharp mask to delineate the foreground and the sky. It was tricky, especially around the snow patchesm and it required a lot of manual touchup. It's much easier with the contrast masking technique-- I didn't do any manual touchup. And the results are more pleasing.

1. Create a mask based on image brightness.

If you are not familiar with making masks in Picture Window Pro, look at the tutorial, Making and using masks.

To create the mask, select the input image and click the mask icon or Mask, New. Select the Brightness Curve tool , then click on the Show Histograms icon. Since I want to adjust the foreground and sky separately I add two control points by shift-clicking on the bar below the upper histogram. I move the arrows as indicated, then click Apply to see the results. If I'm not satisfied, I click None, adjust the control points, then click Apply again. An opaque mask overlay is illustrated. I often switch between mask overlay displays: Clear (none), Semitransparent, Opaque, or Mask only.

Creating the contrast mask: first step
This is the appropriate time to apply manual adjustments if they're needed; I didn't apply any for this image.

You will need to blur the mask. There are three approaches.

  1. Blur the mask using Blur, Apply, then OK to save it.

  2. .
  3. Save the unblurred mask by clicking OK and blur it later with the Blur transformation, as illustrated in step 2. This approach allows you to keep the original sharp mask in case you decide to use a different blur radius.

  4. .
  5. Take advantage of a nifty feature of PW Pro's mask transformation: You can combine steps 2 and 3 by performing a transformation while the Mask dialog is active, before you press OK. Blur the mask using Blurwith an estimated Radius, then click Apply. Make sure the image is selected, then select and perform the transformation, in this case, Color Curves, as you normally would. The effects of both the mask and transformation shows up immediately in the Preview window, which you can enlarge as needed. You can switch between the transformation and mask dialog boxes, making changes in each until you get it right. The Undo function of the Mask dialog box is particularly valuable at this time. If the radius isn't optimum, click Undo and perform a Blur with a different Radius. This is a very powerful technique. Unfortunately, your can do this on only one transformation. You'll have to save the mask and use it as indicated in step 4.
Using curves is a powerful technique for creating masks. If you keep the mask linear-- if you don't add any control points-- the results, apart from the blurred mask area, won't be different from using Brightness Curve or Color Curves without a mask.
.
 Histograms and curve transformations
The Brightness Curve tool in the Mask transformation (above) and the Brightness Curve and Color Curves transformations (below) allow you to adjust curves aided by histograms-- charts that display the distribution of tones (also hue, saturation, or RGB colors for Color Curves). The x-axis of the histogram represents values 0-255 (minimum to maximum). I prefer to use them with the Show Histograms diaplay. In this display, the upper and lower histograms represent the distributions before and after the transformation, respectively. The control points (the ends of control lines) control the mapping. Control points are added by shift-clicking on the bar below upper histogram; they can be deleted by control-clicking. Once added, they can be moved freely along either bar.

The selection of control points is a matter of judgment and experience. In the mask building step (above), you use them to select which areas will be masked fully, partly, or not at all. In the tonal adjustments (below), you use them to adjust the tones in the region selected by the mask. In both cases, carefully observe the Preview image to see the effects. If Auto is checked (Default Preview is set to Auto in Preferences), the Preview window is updated whenever an adjustment is altered. You'll gain experience quickly. I almost invariably make sure some portion of the shadow region contains pure black (level 0). The histogram is invaluable for finding the black points of the input and output images. I use the Preview image for most other aspects of tonal adjustments.

To learn more, read Jonathan Sachs' tutorial, Using curves and histograms.

2. Blur the mask.

Select the (unshapened) mask and click Transformation, Blur. In Method: select Gaussian. Leave Threshold at 100%. Select an appropriate Radius. Typical values are 1 to 2% of the of the average image dimension in pixels. I chose a Radius of 40 for the 2998x2000 pixel input image (shown greatly reduced, of course). See the box below for more on selecting Radius. The Mask transformation Blur tool is identical to Gaussian Blur with Threshold = 100%.
Blurring the mask
Click OK. The blurred mask, shown in the Preview window on the right, above, is the contrast mask.
 
 Blur Radius: a key decision
Blur Radius strongly affects the appearance of the final image. Contrast masking typically enhances contrast near boundaries-- dark areas are darker and light areas are lighter than they are at a distance from the boundaries. Radius determines that distance. Ultimately, your choice will be based on your own feeling, experience, and aesthetic judgment. There is no hard and fixed rule. The choice is affected by the initial boundary contrast, boundary sharpness, the degree of manipulation required, and, to a lesser degree, the size of the final print.

The only rule is that you must observe the final image with clarity and mindfulness-- especially the boundaries. If they looks artificial or displeases you in any way, don't be afraid to go back and create a new contrast mask with a different blur Radius. Eschew laziness. You can use the following trick to get the Blur Radius just right.

3. Transform the foreground using the mask.

In both classic contrast masking and the Photoshop approach the mask is combined with the image. The result can be pleasing, but it's not the best. The mask tends to obscure the image and limit adjustment flexibility. It is far better to use it to control the portion of an image that receives a transformation. Several transformations, including Levels and Color, Brightness Curve, and Brightness can do the job, but Color Curves is the most powerful. It allows us to attain our goal precisely: to boost foreground contrast, along with highlights and saturation.

Select the input image and click on Transformation, Color, Curves... Select the mask in the window to the right of Amount. The Amount sliders will be in their default positions: white (the masked region) will be 100% and black (the unmasked region) will be 0%. Since the foreground is unmasked (dark in the mask), reverse them. Add control points by shift-clicking on the bar below the upper histogram, and adjust them for the desired appearance in the Preview window, shown on the right in the illustration below. I chose two control points for this image-- the minimum that produced the effect I wanted. I chose HSV color space because it tends to increase saturation as the image is lightened. I also increased Saturation slightly (in the S window, not shown, to the right of Color Space).

Using Color Curves with the mask to bring up the foreground
The gaps in the lower histograms, above and below, are the result of the 24-bit math used for the preview calculation. If you are editing a 48-bit image, the gaps will be gone after the transformation is complete; the transformation itself is performed with 48-bit precision. That's why I recommend editing in 48-bits.

If you combined steps 2 and 3 by applying the Color Curves transformation with the Mask transformation active, you must now save the mask by clicking OK in the Mask transformation dialog box.

4. Transform the sky using the mask.

The procedure for adjusting the sky is similar to the procedure for the foreground, but reversed.

Select the input image and click on Transformation, Color, Curves... Select the mask in the window to the right of Amount. Leave the Amount sliders in their default positions: 100% for white (the masked region) and 0% for black (the unmasked region). Add two control points by shift-clicking on the bar below the upper histogram and adjust them for the desired effect. I chose HSL color space because it tends to increase saturation as the image is darkened.

Using color curves with the mask to darken the sky

4A. Alternative approach to enhancing the sky

Use Filter instead of Color Curves. Select the mask as above. Use the original image in the Filter box instead of a pure color (the normal approach). Keep Method on Additive (the default). This may be suggested somewhere in the PWP manual. This approach effectively doubles image contrast and saturation (less if you turn down the Amount slider).
Thanks to Ted Kostek for the suggestion. He states, "I have a few photos where the impact on the sky was beathtaking.  All of a sudden my puffball clouds became full of subtle detail and the saturation in the blue areas jumped as well."

The result (almost final)

Here is the result of the transformations; image editing is almost complete. To complete it I would make small adjustments to the color balance of the foreground and the sky, and I would sharpen the foreground using Unsharp Mask. That's it. The contrast mask did an amazing job of balancing this difficult-to-print image-- it didn't require any manual manipulation-- small area adjustments with tools or with manually-drawn masks. That is rather unusual. I'll probably find a few small details to touch up.
 
(Almost) final image-- after contrast masking

Contrast masking is a rather simple process if you are familiar with masks and transformations in Picture Window Pro. It works best with images that have significant dark areas, such as shaded foregrounds, and significant light areas, such as skies, with different illumination from the dark areas. Just create a mask based on image brightness, blur the edges, and use it to control transformations that adjust brightness and contrast in dark and light regions (Color Curves is the most powerful for this purpose). The results of applying a contrast mask can be impressive: it often does such a good job that little or no additional manual adjustment is required.

 .


Images and text copyright © 2000-2014 by Norman Koren.
Norman Koren lives in Boulder, Colorado, founded Imatest LLC in 2004, previously worked on magnetic recording technology. He has been involved with photography since 1964.