The virtual ruler of PixelZoomer is activated by creating a selection: in the center of the upper toolbar you can read the width and height of the rectangle selection in pixels. A selection is also possible when the screenshot is enlarged and therefore it's easy to do pixel perfect measurements. A once created selection can also be moved across the window border – the visible image area is moved by the program automatically. You can also create square regions by holding down the SHIFT key on your keyboard.
Finally, you can move around the selection in real-time: by pressing the SPACE key and moving the mouse simultaneously, you can change the position of the current selection. So you don't have to recreate a selection if you missed a pixel edge.
Together with the integrated screen magnifier, the selection tool represents the core feature of PixelZoomer. With these features you can quickly measure distances or even control visual effects. Especially centered elements can be realized differently with CSS. That's why a final check is more important so that you can ensure an element is really centered.