Using Masks

Masks are an important tool in Picture Window since they let you localize the effects of transformations to specific parts of an image. Similarly, when used with tools, they can protect some parts of an image from being affected by the tool. Finally, in some specialized cases they are use to identify an object within an image. Regardless of what you are going to use it for, the way you create a mask is the same.

A mask is a black and white image associated with a base image of the same dimensions. You can think of it as a semi-transparent overlay that just covers the base image. Areas where the mask is white are selected while areas where the mask is black are not -- areas where the mask is gray are partially selected. Masks are used to restrict the operation of transformations and tools to the selected areas of the base image, for example to lighten or darken just part of an image.

Masks are created via Amount Controls. The amount control associates a mask with an image and determines how the mask is used.

Step 1 -- Open the image and select a transformation

For example, lets assume you want to lighten one area of an image using the Brightness Curve transformation, so first you need to open the image you want to work on and then select Transformation/Gray/Brightness Curve from the main menu:

Step 2 -- Bring up the Mask Dialog

To create a mask for the transformation, click the white box to the right of the Amount control -- this pops up a menu -- select New Mask... . This in turn brings up a Mask Dialog and also switches you to a split screen display with the input image on the left. The input image will be the base image to which the mask is applied.

Step 3 -- Outline the Area of Interest

The Mask Dialog has a lot of controls, but the idea behind it is fairly simple. You select a masking tool from the tool bar along the bottom of the Mask Dialog and then apply the tool to the input image. You can then refine the mask by applying the same or a different tool continuing until you get the result you want.

Initially, the freehand outline tool at the far left is selected, so let's use that first. Click on the input image and drag to circle the approximate area you want to lighten -- then release the mouse button when you are done. A semi-transparent red overlay will be displayed over the masked area.

If you want, you can modify the mask overlay color and transparency with the mask color and mask transparency tool bars:

Step 4 -- Blur the Edges

Click on the mask blur tool (the blurry disk in the center of the tool bar). This will add a slider and an Apply button to the bottom of the Mask Dialog.

Set a Radius of about 80 and click Apply. This blurs the mask, giving it a soft edge.

If you want more or less blurring, click Undo , select a new Radius and click Apply again.

At this point we are done with the mask for now. If you want, you can reduce screen clutter by closing the Mask Dialog. This also hides the red overlay, letting you more easily compare the input and output images. To get the Mask Dialog back, you can click on the Amount control again and select Re-open Mask Dialog Box from the menu.

Step 5 -- Change the Transformation settings

Until now, we have left the brightness curve at its default setting which makes no changes to the image, so the mask has no effect. Once we change the curve however, the transformation will only be applied to the areas with the red mask overlay. For one way to lighten the image, add a control point in the middle of the curve and drag it upward.

Note how the output image is now brighter than the input image in the masked areas. Also note that a miniature of the mask is display in the Amount control and also in the Image Browser. You can make further adjustments to the curve or mask as necessary. When you are done, click OK to close the Brightness Curve transformation.

See Mask for more detailed descriptions of all the Mask Dialog tools and settings.