First select the 'Pen Tool (P)' from the 'Toolbar'...
Then up on the 'Options Bar' choose the 'Paths' option...
...and with the 'Pen Tool' still selected trace the area that you want to mask (ie. protect), in this case its the entire building...

This tool finds pixels of similar value and automatically erases them to transparency on the active layer . This tool also works best when theres a solid color in the background to be removed.
Experimentation with the 'Tolerance' level will ensure best results...

There's a variety of ways of applying a 'Layer Mask' to achieve the knockout effect. (I'll just show the most flexible example)
The most flexible option is the combination of applying the 'Pen Tool (P)' to add anchors points and in combination with the 'Convert Anchor Point Tool'...

This tool finds pixels of similar value and places a selection around them. A selection is formed based on the color of the pixel clicked on, and the amount of 'Tolerance' indicated in the 'Options Bar'...

Here's a powerful trick to quickly add transparency to an indexed composite using features in the 'Save for Web' Control Panel. Below is a simple index (.gif) image I loaded into Photoshop. Instantly, you should notice that the image is locked...

In this installment, we'll look at the Paths palette and take a glance at the Freeform and Magnetic Pen tools.

Photoshop makes working with vector shapes easier than ever before. While not actual vector objects, such as those created in Illustrator, the new Shapes tools are true time savers. Photoshop provides ellipses and circles, rectangles and squares, rounded rectangles, polygons...

Cropping an image, whether to remove unwanted parts of the picture or for size purposes, is common in Photoshop. Last week's column explored the Crop Tool and the Crop command. This week we'll turn to lesser-known ways to crop and look at Perspective Crop...

What better time than Thanksgiving, when the harvest is in, to discuss the crop? Photoshop offers several ways to resize a picture, and we'll start a two-part look with the Crop Tool and Crop command...

One of the biggest challenges facing many Photoshop users is maximizing compression while minimizing file size. This is an especially tough balancing act when working with photographs for the Web. Selective blurring can help...