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 6 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 6 that
ties directly to the main theme.
The Perspective Crop is a new technique in
Photoshop 6. 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.