Image editing with Picture Window Pro:
Making and using masks
by Norman Koren

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Table of contents

for Image editing with
Picture Window Pro

Tinting and hand coloring B&W images
Example: Sunset, Providence
Contrast masking
Making masks
Introduction  |  Creating a Mask  |  Using masks
The Mask dialog box  |  Gradient tool
Examples  |  Notes

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 explains how to make and use masks: the key to adjusting selected portions of an image.

Introduction to masking and selective image adjustments

Dodging and burning-- lightening and darkening portions of an image-- are among the most important techniques for turning raw images into a fine prints. It the classic darkroom you dodged and burned by placing a mask-- your hands or a cutout made of opaque cardboard-- between the enlarging lens and the paper. Exposure times were based on experience and guesswork. It was hit and miss, and you couldn't see the results until the print came up in the developing tray.

Image manipulation techniques such as dodging and burning are far more powerful in the digital darkroom. Not only can you precisely control the region and amount of alteration, but you can adjust contrast, color and and sharpness as well. Any adjustment you can make to the entire image can be applied to a selected portion.

Picture Window Pro provides two approaches for adjusting portions of an image. For small areas (typically under 100 pixels) you can use Miscellaneous Tools, which include Lighten, Darken, Increase Saturation, Decrease Saturation, Blur, Sharpen, Speck Removal, Smudge, Red Eye Removal, and Add Noise. These operations are performed with a brush with adjustable Radius, Transparency, and Softness. Each has options for fine control. For example, Lighten and Darken can operate on Highlights, Midtones, Shadows, or All. For tightest control, a high level of Transparency is recommended. You can undo errors with control-Z.

Mask exampleMiscellaneous tools are fast and convenient for touching up small areas, but for moderate to large areas you should use a mask-- a Black & White (grayscale) image with the same pixel dimensions as the image you are editing. Masks provide far more precise control. The strength of a mask is proportional to its grayscale level. It is normally minimum for black and maximum for white, but it can be inverted. Masks can also be used to protect areas from being modified by Clone and Miscellaneous Tools and to identify portions of images to be combined using the Composite transformation.

Making masks quickly is one of the primary skills of the fine digital printmaker. It isn't difficult-- Picture Window Pro's tools for creating and editing masks are powerful and convenient, but it takes practice to use them with speed and confidence. I often make several masks in the course of editing an image. Making a mask can be as simple as doing a freehand drawing and blurring the edges . This is what I do most of the time; it's similar to traditional dodging/burning. But there are several powerful tools for masking areas based on image properties-- great for selecting skies or areas with complex boundaries. There are lots of tricks to master.

Jonathan Sachs has written two excellent, lucid PDF tutorials, Creating and Using Masks, and Dodging and Burning, which I strongly recommend. I unavoidably duplicate some of his material, but I present my own viewpoint here. It never hurts to study a subject from more than one perspective.

Creating a mask with the Mask tool

Although any B&W image with the same pixel dimensions as the input image can be used as a mask, the best way to create one is with the Mask or Gradient tools. The Gradient tool is best for gradual masks with simple geometries; we'll discuss it later. The Mask tool is far more versatile. To use it, select the image you want to mask (the input image), then click on Mask, New or click on the Mask iconMask Tool. This opens the Mask dialog box, illustrated on the right. You can also use Mask to open an existing mask for editing.  Mask dialog box for Freehand Outline, Add
Once the Mask dialog box is open you create a mask with the following steps.
  • Mask portions of the image (with mask mode at its default Add setting) using
    • Freehand outline or Paint -- for drawing with the mouse.
    • Geometrical objects: Rectangle, Oval, Polygon, or Spline.
    • Tools that create a mask based on image properties: Color Range, Brightness Curve, and Paint with Similar Pixels. These are particularly powerful.
  • Adjust the mask by removing excess areas: Use any of the available tools with mask mode set to Subtract. Toggle between Add and Subtract as necessary.
  • Smooth or blur edges as necessary using Feather or Blur. An unsmoothed mask can result in unnatural and unpleasant edges.
  • When the appearance of the mask is correct, press the OK button. The mask becomes an open image with the name Untitled n, where n is an integer. You can name and save it if you wish, and you can always reopen it for editing. Since a mask is an ordinary B&W image, you can edit it with any of Picture Window Pro's tools.
The mask on the right was created using Paint with Similar Pixels (Track): an extremely powerful technique for masking an area based on pixel values. It's not quite complete as shown; it still needs some blurring or feathering.
Mask preview with semitransparent overlay
Mask overlay display:  Semitransparent (default)
Mask preview with opaque overlay
Mask preview with mask-only overlay

The mask is displayed as an overlay on the input image, updated after each step prior to pressing OK. Its appearance is controlled by the mask overlay display commands in the second row of the dialog box. Examples are shown on the right. I usually leave it on the default Semitransparent setting, but I switch to Only when I want to scrutinize the mask for omissions (holes) or excess coverage; I usually enlarge the mask and scroll around it when I'm doing this. I switch to Clear to examine the image behind the mask.

Using masks

Amount slider and mask window from transformation dialog boxTo use a mask with a transformation, select the input image, open the transformation dialog box, then click on the button to the right of the Amount: slider. All available masks (B&W images with the same pixel dimensions as the input image, loaded into Picture Window Pro) are displayed. Select the mask by clicking on it. The Amount: slider splits in two. The white (upper) slider, which defaults to 100%, indicates the strength of the transformation in the masked portion of the image (where the mask is white). The black (lower) slider, which defaults to 0%, indicates the strength of the transformation on the unmasked (black) portion. Gray levels fall between the two. Adjust the sliders as desired. I often invert the mask by setting white to 0 and black to 100%.

Before I complete the transformation by pressing OK, I usually move the active (non-zero) slider back and forth to see the effect of the transformation. If the mask has important boundaries (for example, between foreground and sky), I enlarge the Preview window to make sure there are no unpleasant edge effects.

You can perform a transformation while the Mask is active, before you press OK. Make sure the image is selected, then select and perform the transformation as you normally would. You can switch between the transformation and mask dialog boxes, making changes in each until you get it right. The effects of both dialog boxes appear in the Preview window. The Undo function of the Mask dialog box is particularly valuable at this time. For example, you may need a mask with a blurred boundary, but you may not know how much blur is best. Blur the mask  using an estimated Radius, observe its effect in the Preview window, then click Undo and perform a blur with another Radius. This is a very powerful technique.

To use a mask to protect areas from change by small-area tools (Miscellaneous Tools, Clone, or Paint), select the image, click on Mask, select the desired mask, then open the tool. Use the tool as usual. The masked area is protected against change.

The Mask dialog box

The mask dialog box contains several groups of icons whose functions are summarized in the table below.

The top row contains basic utilities.

Top row
Top row Utilities  
All Mask the entire image.
None Clear the mask.
Invert Invert the mask.
Area... Compute the area of the input image that is currently covered by the mask. Pixels that are partially masked are counted as fractions. I rarely use this function.
Undo Undo the previous operation. Very useful.
Cancel Close the Mask dialog box without completing the mask.
OK Create the mask and close the dialog box.

The second row contains icons that control the function of the Mask drawing tools and the Mask preview display.

Second row
Mask mode
and display
Second row Mask mode: Sets the function of Mask drawing tools.
 Add Mask Add Add the selected area to the existing mask. This is the default when the Mask dialog box is opened.
 Subtract Mask Subtract Remove the selected area from the existing mask.
 Overlap The existing mask remains only where it overlaps selected area. Use this mode to create a mask based on two conditions being met simultaneously (Boolean AND), for example, to create a mask consisting of pixels in a certain region and with colors in a certain range.
 Invert Invert the existing mask in the selected area.
Second row Mask overlay display
 Overlay color The color of the current mask, which is displayed as an overlay on the input image. The default is red. It should contrast with the image. You'll want to change it if the image is predominantly red.
 Clear mask overlay The mask overlay is not visible.
 mask overlay
(Default) Displays the mask overlay as a semitransparent layer whose color is set by Overlay color. Lets you see the input image and mask at the same time. Depending on the colors in the input image, changing the mask overlay color may make the mask easier to see. See illustration, above.
 mask overlay
Displays the mask overlay as an opaque layer whose color is set by Mask overlay color. This makes the mask more visible but obscures the masked parts of the input image.
 Mask only Displays the mask overlay as white on a black background. Hides the input image.

The third row contains the Mask tools. Controls specific to individual tools are displayed below the third row.

Third row
Mask tools
Third row Mask tools: See Mask dialog in Picture Window Help for details.
Freehand Outline Click on the input image, press the mouse button, then draw the outline of the region you want to mask. When you release the mouse button, the starting and end points are connected and the interior of the outlined region becomes the new mask object, which is then combined with the existing mask according to the mask mode setting, Add (the default), Subtract, Overlap, or Invert. To remove an area you accidentally masked, use Subtract. TIP:  It's easier to draw a complex shape with several small Freehand outlines than with one large one.
Rectangle  Click on the input image, press the mouse button, then move the cursor diagonally. When you release the mouse button, a rectangle is outlined. You can click and drag the sides, corners, or interior to reposition it. To create a square region, hold down the Shift key while dragging the sides or corners of the rectangle. When you click Apply, the rectangle is combined with the existing mask according to the mask mode setting.
Oval Click on the input image, press the mouse button, then move the cursor to the diagonally opposite corner of a rectangle that contains the oval. When you release the mouse button, the oval is outlined. You can click and drag the sides, corners, or interior to reposition it. To create a circular region, press the Shift key while dragging the sides or corners of the rectangle. When you click Apply, the oval is combined with the existing mask according to the mask mode setting.
Polygon Click on the input image, press the mouse button, then move the cursor diagonally. When you release the mouse button, a polygon in the shape of a rectangle is outlined. A second, outer rectangle is used to resize the polygon. Shift-click on a side of the inner rectangle to add a new vertex. Control-click to remove one. There must be at least three vertices. Click and drag to reposition a vertex; click and drag the handle in the center to reposition the polygon,. To resize the polygon, click and drag a side or corner of the outer rectangle. When you click Apply, the polygon is combined with the existing mask according to the mask mode setting. Save... and Load... can be used with Polygons.
Spline A spline is a smooth curve that passes through a specified set of control points.

Click on the input image, press the mouse button, then move the cursor diagonally. When you release the mouse button, a rounded shape with four control points is outlined. A second, outer rectangle is used to resize the shape. Shift-click on the inner shape to add a new control point. Control-click to remove one. There must be at least four control points. Click and drag to reposition a control point; click and drag the handle in the center to reposition the shape To resize the shape, click and drag a side or corner of the outer rectangle. When you click Apply, the spline is combined with the existing mask according to the mask mode setting. Save... and Load... can be used with Splines.

TIP:.. The five previous commands (Freehand Outline, Rectangle, Oval, Polygon, and Spline) all draw outlines of shapes. Mask mode controls their effect. Normally, Mask mode is set to AddMask Add to draw a mask. It should be set to SubtractMask Subtract to remove areas of a mask. I typically start a mask with one or more Freehand Outlines or geometric shapes, then I trim any excess. I soften edges with Feather or Blur.
Feather Feather the edges of the existing mask, i.e., soften (blur) them while either shrinking or expanding the mask. The controls are: Feather width: specifies the distance in pixels to extend the mask edges. Positive values expand the mask; negative values shrink it. Falloff: Gradual softens the edge of a mask by causing it to fade gradually. Sudden grows or shrinks a mask by extending it inward or outward by the feather width. Corners: Round extends mask edges in a circle; Square extends mask edges in a square. Click Apply to feather the existing mask. Feather may be applied several times to obtain a more gradual mask than allowed by the maximum radius. Note:Feather can be much slower than Blur for large masks with complex details.
Blur Blur edges of the existing mask. For example, masks created by the outline drawing tools (above) have hard edges that can be softened with Blur. The Radius: setting specifies the amount of blur in pixels. Equivalent to Gaussian Blur. Click Apply to blur the existing mask. Blur may be applied several times to obtain a more gradual mask than allowed by the maximum radius. Useful for Contrast masking.
Color Range Color Range dialog box- lower portionMask regions of the input image based on color properties. You can set Color Space: to RGB, HSV, or HSL. The three color controls are labeled accordingly. For Black & White images there is only a single control for gray level.
(right) the bottom of the dialog box, set for HSL color space
Each color control consists of a color bar on top and a selector bar below. The grayscale values of the selector bars indicate the values of each component to be selected. White = selected; black = deselected; gray = partially selected. Each selector bar has two pairs of sliders, one white and one black. The white sliders adjust the range of selected values. The black sliders control how suddenly the selection falls off. You can adjust the sliders manually or you can adjust them automatically, following the procedure below.

When you click on the image with the left mouse button, short vertical markers between the color and selector bars indicate the values of the image pixel. The color above the marker is the color you clicked on. The color stripes reflect the range of each color component with the others held fixed. For example, the colors in the S bar illustrate the color changes as S (saturation) goes from 0 and maximum, with constant H (hue) and L (lightness).

Clicking Contract shrinks the selected ranges to the values indicated by the three markers. To expand the selected ranges to cover an area of an input image, move the cursor over the area while pressing the shift key and the left mouse button. See TIP, below.

Color Range behaves like a Boolean AND: a pixel has to fall within the white region of all three sliders to be included in the masked region. When you click Apply, the pixels to be masked are computed and combined with the existing mask according to the mask mode setting. If you are not satisfied with the selection, click Undo and continue. Color Range works faster if you first isolate a region with the Freehand Outline (or similar) tool and then switch to Overlap mode.

TIP:.. To mask an area based on its color properties, select Color Range, click anywhere in the area you want to mask, then click on Contract. Move the cursor around the area, keeping the shift key and the left mouse button depressed. Be sure to cover all the representative values. Click Apply to view the mask. You can repeat the process (without Contract) to mask areas you missed; you can also manually adjust the mask sliders. When coverage is satisfactory you may want to blur or feather the mask. Using Feather with a positive pixel setting is a good way to cover noisy pixels you might have missed. Paint with Similar Pixels (below) performs a similar function.
Paint Paint dialog box- lower leftPaint on the mask by clicking the left mouse button and dragging the mouse. Radius, Transparency, and Softness are adjustable (with sliders, not shown). An image of the brush is displayed at the bottom left of the Mask Dialog. Painting adds to or subtracts from the existing mask according to the mask mode setting.

Apply To: controls the way Paint works.

  • All Pixels (default) – Paint operates normally.
  • Pixels in Color Range – Painting is restricted to areas within the range of colors selected by the Color Range tool. To adjust the color range settings, switch to Color Range then switch back to Paint.
  • Similar Pixels (Fixed) – The Paint tool samples the color of a small area of the underlying image at the start of each brush stroke to establish a base color. For the remainder of the brush stroke, only pixels close to the base color are masked. A Threshold slider lets you adjust how closely pixels must match the base color to be masked.
  • Similar Pixels (Track) – Similar to Similar Pixels (Fixed), except that the base color is resampled as you move the cursor. Since the brush can be larger than the sampled area, you can mask one side of an object boundary even if the background color is varying. Very powerful for creating masks based on image area properties.
TIP:.. Paint with Similar Pixels (Track) is very useful  for masking boundaries. Set the brush Radius large (you'll learn to adjust it for the geometry) and the Transparency and Softness to 0. Click the left mouse button and drag the mouse near a boundary. If Threshold is set to an appropriate value, you'll mask the area on one side of the boundary very nicely. If you make an error, set mask mode to Subtract and drag on the other side of the boundary. Use Paint, Freehand Outline, or another tool to complete the mask, then apply Blur or Feather as needed. The result is similar to Color Range, Contract (above), but is frequently more accurate because the selection criteria continuously adapts to the local region. It can be difficult to use near complex boundaries, i.e., trees with lots of branches.
Combine Masks Combine Masks dialog box- lower leftClicking on the small window to the right of Combine Mask: displays a list of mask images that that can be combined with the current mask. When you select one, a thumbnail version of the mask image is displayed in the small window. When you click Apply, the masks are combined according to the current mask mode setting.
Brightness Curve Mask the image based on brightness. The curve control at the bottom, identical to the Brightness Curve... dialog box, lets you select portions of the tonal range to mask. The higher the grayscale value, the stronger the mask. When you click Apply, the selected mask image is combined with the existing mask according to the mask mode setting. Used in Contrast masking.

Creating a mask with the Gradient tool

The Gradient tool can be used to generate gradual grayscale masks with simple geometries. The most useful gradient shapes (set by Type) are Linear (the default) and Oval. Rectangular and Multi-point are sometimes useful.

The best way to illustrate the Gradient tool is to jump right in with an example, using Type: Oval. For most gradients used for masks, keep Amount: at 100.0%, Operation: Fill, and Cycle: 1. Increase Antialias: above 1: only if smoother transitions are required.

Making an oval Gradient
The gradient shape is adjusted with control points, which are overlaid on the input image (top of the illustration, above). Some of the control points may be outside the input image. When this is required, you should reduce the magnification of the input image, then enlarge the window by dragging a side or corner. You can force an oval or rectangular gradient to be circular or square by holding down the shift key while you drag one of the edge control points.

For a linear gradient, the control points are located at the start and end points of the gradient line. The gradient itself is perpendicular to the line connecting the start and end points.

Gradient tones and transitions can be adjusted using the color line control at the bottom of the dialog box (bottom left of the illustration, above). There are at least two control points, at the beginning and end of the line. You can add a control point by shift-clicking on the color control line. You can remove a control point by control-clicking on it. You can move the control point and adjust its grayscale level by double-clicking on the number above the control point. These adjustments give you great flexibility. There is one added control point (2) in the illustration above. Added control points appear as lines or curves in the Preview overlay.

Clicking on the bottom of the color control line brings up a list of options for defining the transition betwen control points. The default is Line (linear transition). Sine can often produce a pleasing result when there are just two control points; the start and end of the gradient are more gradual. Other options include Step Left, Step Center, and Step Right.

The OPT button (to the right of Cancel) lets you select various gradient options:


Simple masks are used in Tinting and hand coloring B&W images. A detailed example of a mask created with Color Range appears in Example: Sunset, Providence. Contrast masking is an exceptionally powerful tool for masking (and taming) contrasty images.

Notes on masks

Since a mask is just an ordinary B&W (grayscale) image whose only requirement is that it have the same pixel dimensions as the input image, you can use a number of transformations in addition to the Mask and Gradient tools to create them. Among them, These can be quick and useful for certain transformations like selective blurring or fancy contrast/saturation control. You can also edit masks with any appropriate transformations: Blur, Sharpen, Levels and Color, Brightness Curve, Brightness, Negative, etc.

If you resize or crop an image and want to continue using a mask, you should resize or crop the mask with the same settings.


Images and text copyright (C) 2000-2013 by Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording technology for high capacity data storage systems until 2001. Since 2003 most of his time has been devoted to the development of Imatest. He has been involved with photography since 1964.