Apple Numbers '09 - Manual (Page 199)

Chapter 9 Working with Shapes, Graphics, and Other Objects 199
3 Click the color you want to make transparent, and then drag slowly over it.
Dragging selects the contiguous
area that uses colors similar to
the color you click.
As you drag, the selection grows to include the contiguous area that uses similar
colors. You can control how much of the image is selected by dragging less or more.
If you hold down the Option key while you drag, all instances of the color that you’re
dragging over will be removed from the image.
4 Repeat step 3 as many times as you like.
Dragging more here...
...selects more of
the image.
You can restore the parts removed from the image at any time. To revert to the
original image, choose Format > Remove Instant Alpha. To restore parts of the image
removed using Instant Alpha, choose Edit > Undo Instant Alpha until the parts have
been restored.
Changing an Image’s Brightness, Contrast, and Other Settings
You can change the brightness, contrast, and other settings of images to improve their
quality or to create interesting effects. Adjustments you make don’t affect the original
image; they affect only the image’s appearance in Numbers.
To adjust an image:
1 Select the image.