Full of Crop!

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.

Photoshop added another way to shrink the size of your image, borrowing this one from ImageReady. Little-known, but highly effective, the Trim command is now available under the Image menu, too. The Trim command differs from Crop command in how it determines what should be deleted. With the Crop command, anything outside of a rectangle based upon a selection is eliminated. Trim, on the other hand, uses pixel color and position to determine what goes and what stays. The Trim dialog box is shown in here.

Using the pixel color/characteristic selected at the top, Trim will remove from the image pixels on any or all of the four sides of the canvas. One of the best uses for the Trim command is a scan that picked up a more than was needed, as shown below.

Before doing color correction with adjustments that use a histogram, it's probably a good idea to trim away the areas that aren't necessary for the image. That prevents the Histogram from being skewed by the irrelevant data. For example, in the case of this image, the large areas of gray around the outside substantially change the appearance of the Levels dialog box. The image below shows the histograms from the Levels dialog box for this picture, with (top) and without (bottom) the gray areas surrounding.

Note that the Trim command works with solid color or transparency only. If the fringe area is noisy or gradated, Trim is not an appropriate choice. There's another way to crop an image precisely in Photoshop. The menu command Image> Canvas Size can delete specific areas of the image, with accuracy to a single pixel. The next image shows the dialog box.

The dialog box will open with the dimensions of the current document already input. You can change the width, the height, or both. In addition, the 3x3 grid allows you to specify what section of the image to protect. When the center square is selected (the default) an equal amount of canvas (image) will be selected (or added) on all four sides. If, on the other hand, the upper-left box is selected, any change to the canvas size will take place on the right and bottom. when reducing the canvas size, Photoshop will open a warning box that tells you you're about to lose some image data. Click the Proceed button.

SIX SHOOTERS
PUTTING THINGS IN PERSPECTIVE

For this column, we'll do an extended "Six Shooters" and concentrate on an aspect of Photoshop that ties directly to the main theme.

The Perspective Crop is a new technique in Photoshop . The Crop tool can be used to correct keystoning and off-angle images. (Keystoning is the perspective distortion that comes from photographing a tall object or shooting upward or downward.) One a crop marquee has been dragged out, the Options Bar changes. The second set of crop variables are shown in the next image. (Note that the Options Bar has been chopped and stacked to fit on screen.

Checking the Perspective box allows the corners of the marquee to be repositioned, as we will see. Our test image was take from off center and suffers, to a slight degree, from keystoning.

First we draw our marquee, positioning it virtually anywhere in the image. Then drag the corner handles to make the sides of the marquee align with things in the image that should be vertical. In this case, we'll choose the edges of the pillars, although the bars of the gate are just as tempting. You'll also want to align the top and bottom edges with horizontal lines in the image. We've chosen the edge of the step at the bottom, and the invisible line between the tops of the columns.

Once the edges of the marquee line up properly, use the handles in the center of the sides and the top and bottom to drag the edges out. Using the middle handles allows the marquee to maintain its shape.

You can reposition the center point to where it could actually have been, had the image been photographed the way you want it. In this case, we'll move the center point up a little, but it can't go too high without triggering an error message.

We hit our Return or Enter key and, hopefully, we get what we were looking for ç a nicely-straightened picture.

If we haven't positioned the edges of the crop marquee in what could be a natural rectangle (or what will be one after the crop), we get this error message:

Hitting Cancel returns you to the crop, ready to reposition and try again; hitting Don't Crop cancels the entire operation. If you position the edges along things in the image that should be vertical and horizontal, you should be able to avoid this nasty-gram.