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Chapter 4. PIXEL BASICS > Changing dimensions and resolution

Changing dimensions and resolution

To change an image's pixel dimensions for on-screen output:

1.
Choose Image menu > Image Size.

2.
Make sure the Resample Image box is checked .



3.
To preserve the image's width-to-height ratio, leave the Constrain Proportions box checked.

4.
Set the Resolution to 72 ppi.

5.
Enter new values in the Pixel Dimensions: Width and/or Height fields.

6.
Click OK.

To change an image's dimensions for print output:

1.
Choose Image menu > Image Size.

2.
To preserve the image's width-to-height ratio, check the Constrain Proportions box . To modify the image's width independently of its height, uncheck the Constrain Proportions box.

3.
Optional: To preserve the image's resolution, check the Resample Image box and choose Nearest Neighbor, Bilinear, or Bicubic as the interpolation method. Bicubic causes the least degradation in image quality.



Previewing the print

To see the image size relative to the paper size, press and hold on the status bar at the bottom of the image window or use File menu > Print Options. To display the image on screen at the size it will print, choose View menu > Print Size.

Note At 100% view on a Macintosh monitor, the on-screen display size will match the print size only if the image resolution is the same as the monitor resolution (72 ppi). On a Windows monitor (96 ppi) at 100% view, a Mac-generated image will look slightly smaller than actual size. What if you're in a cross-platform workgroup? You could set the Windows monitor screen area and the Mac's monitor resolution to a similar pixel dimension.


4.
Choose a unit of measure from the pop-up menu next to the Print Size: Width and Height fields.

5.
Enter new numbers in the Width and/or Height fields. The Resolution will change if the Resample Image box is unchecked.

6.
Click OK.

Tip

To restore the original Image Size dialog box settings, Alt-click/Option-click Reset.


Tip

File menu > Print Options can also be used to resize and rescale print dimensions, but it won't preserve the image resolution.


Cash in on a high resolution

An image contains a given number of pixels after scanning, and its print dimensions and resolution are interdependent. If an image's resolution or dimensions are changed with the Resample Image box unchecked in Image menu > Image Size, the file's total pixel count is preserved. Increasing an image's pixels per inch resolution will shrink its print (physical) dimensions; lowering an image's pixels per inch resolution will enlarge its print dimensions.

If your file has a higher resolution than needed (more than twice the screen frequency), you can allocate the extra resolution to the print size dimensions by unchecking the Resample Image box (the width, height, and resolution are now interdependent), and then lowering the resolution to twice the screen frequency. The width and height values will automatically increase, and the file storage size and pixel dimensions will remain constant—no pixels will be added or deleted from the image.

If you must further enlarge the image's dimensions, click in the Width field, check the Resample Image box, and enter a new Width value. The Height will change proportionately, and the file storage size and pixel dimensions will increase, but you'll be resampling, so after clicking OK, apply the Unsharp Mask filter to resharpen (see pages 74–75).


Note If you increase an image's resolution (resample up) with the Resample Image box checked, pixels will be added and the image's file storage size will increase, but its sharpness will diminish. If you decrease an image's resolution (downsample), information will be deleted from the file, and it can only be retrieved using the History palette before the image is closed. Blurriness caused by resampling may only be evident when the image is printed; it may not be discernible on screen. That's why it's always best to scan or create an image at the proper resolution. Follow the instructions on page 74 to resharpen a resampled image. (See also pages 47 and 50.)

To change an image's resolution:

1.
Choose Image menu > Image Size.

2.
To preserve the image's dimensions (Width and Height), check the Resample Image box .



or

To preserve the image's total pixel count, uncheck Resample Image. The Width and Height dimensions will change to preserve the current pixel count.

3.
Enter a number in the Resolution field.

4.
Click OK.

Tip

The History Brush won't work on a resampled image. You will only be able to set the source for the History Brush from the current state forward.


The Fit Image command has no effect on an image's resolution—it only changes its physical dimensions. Use it to make an image smaller.

To resize an image to fit a specific width or height:

1.
Chose File menu > Automate > Fit Image .



2.
Enter a Width or Height value in pixels. The other field will automatically adjust after you click OK, so the width-to-height ratio will stay the same.

3.
Click OK.

The Resize Image command duplicates an image and resizes the duplicate automatically. All you have to do is respond to a sequence of dialog boxes—Photoshop will figure out the math for you.

To resize an image automatically:

1.
Choose Help menu > Resize Image.

2.
Click Print or Online , then click Next.



3.
Enter the desired output size , then click Next. If you chose Online in the previous step, click Finish now. For print output, follow the remaining steps.



4.
Click or enter the lpi as per your print shop's instructions , then click Next.



5.
Move the Quality slider , then note the final image size in the Results box. If there's a message below the Results box, read that as well. If you want to proceed, click Next.



6.
Click Finish , and then save the resized image. (Or click Cancel.)



If you change an image's dimensions or resolution with the Resample Image box checked, convert it to CMYK Color mode, or transform it, blurring may occur due to the resampling process. Despite its name, the Unsharp Mask filter has a focusing effect. It increases contrast between adjacent pixels that already have some contrast. You can specify the amount of contrast to be added (Amount), the number of surrounding pixels that will be modified around each pixel that requires more contrast (Radius), and determine which pixels the filter effects or ignores by specifying the minimum degree of existing contrast (Threshold).

Note The Unsharp Mask effect may be more noticeable on screen than on high-resolution print output.

To apply the Unsharp Mask filter:

1.
Choose Filter menu > Sharpen > Unsharp Mask .



2.
Choose an Amount for the percentage increase in contrast between pixels . Use a low setting (below 50) for figures or natural objects; use a higher setting if the image contains sharp-edged objects. Too high a setting will produce obvious halos around high contrast areas. The larger the image, the less sharpening may be required. For a high-resolution image, use an Amount between 150 and 200%.



3.
To choose an appropriate Radius value, which is a little trickier, you need to factor in the final size, the resolution, and the subject matter of the image. Choose a Radius value (0.1–250) for the number of pixels surrounding high contrast edges that will be modified. Try between 1 and 2 pixels. A higher value could produce too much contrast in areas that are already high contrast.

The higher the resolution of the image, the more pixels there are on the border between high contrast areas, and thus the higher the Radius setting is required. Try a high Radius setting for a low contrast image, and a lower Radius setting for an intricate, high contrast image.

Note The higher the Radius setting, the lower the Amount setting can be, and vice versa.

4.
Choose a Threshold value (0–255) for the minimum amount of contrast an area must have before it will be modified. At a Threshold of 0, the filter will be applied to the entire image. A Threshold value above 0 will cause sharpening along already high-contrast edges, less so in low contrast areas. If you raise the Threshold, you can then increase the Amount and Radius values to sharpen the edges without over-sharpening areas that don't require it. To prevent noise from distorting skin tones, specify a Threshold between 8 and 20.





5.
Click OK.

Tip

To soften a grainy scan, apply the Filter menu > Blur > Gaussian Blur filter at a low setting (below 1) and then apply the Filter menu > Sharpen > Sharpen Edges once or twice afterward to resharpen.




Tip

To avoid waiting for the full screen Unsharp Mask preview on a large image, first get close to the desired settings using just the preview window with Preview unchecked, then check the Preview box to preview the results on the full screen, and finally, readjust the settings, if needed.


Tip

Try applying the Unsharp Mask filter to one or two individual color channels (for example, just the Red or Green channel in an RGB image). If you sharpen two separate channels, use the same Radius value for both. You can also convert an image to Lab Color mode then apply the filter to the L channel to sharpen luminosity without affecting color pixels.


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