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Changing image size and resolution

photoshop

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Changing image size and resolution




Once you have scanned or imported an image you may want to adjust its size In Photoshop the Image Size command lets you adjust the pixel dimensions print dimen- sions and resolution of an image in ImageReady you can only adjust the pixel dimen- sions of an image.

For assistance with resizing and resampling images in Photoshop choose Help > Resize Image. This interactive wizard helps you scale your images for print or online

media.

Keep in mind that bitmap and vector data can produce different results when you resize an image Bitmap data is resolution-dependent therefore changing the pixel dimensions of a bitmap image can cause a loss in image quality and sharpness In contrast vector data is resolution-independent you can resize it without losing its crisp edges.

Displaying image size information

You can display information about the current image size using the information box at the bottom of the application window ( Windows) or the document window (Mac OS).

(See “Displaying file and image information on section 48

To display the current image size:

Do one of the following:

(Photoshop) Press Alt ( Windows) or Option (Mac OS) position the pointer over the file information box and hold down the mouse button. The box displays the width and height of the image (both in pixels and in the unit of measurement currently selected for the rulers) the number of channels and the image resolution.

(ImageReady) Click an image information box and select Image Dimensions from the pop-up menu. The box displays the width and height of the image in pixels.

 
About resampling

 
Resampling refers to changing the pixel dimensions (and therefore display size) of an image When you downsample (or decrease the number of pixels) information is deleted from the image. When you resample up (or increase the number of pixels) new pixels are added based on color values of existing pixels. You specify an interpolation method to determine how pixels are added or deleted (See “Choosing an interpolation method on section 66

CA  B

Resampling examples:

A Downsampled B Original C Resampled up (Selected pixels displayed for each image)

Keep in mind that resampling can result in poorer image quality For example when you resample an image to larger pixel dimensions the image will lose some detail and sharpness Applying the Unsharp Mask filter to a resampled image can help refocus the image s details (See Sharpening images on section 155

You can avoid the need for resampling by scanning or creating the image at a high enough resolution If you want to preview the effects of changing pixel dimensions on- screen or print proofs at different resolutions resample a duplicate of your file. Choosing an interpolation method

When an image is resampled an interpolation method is used to assign color values to any new pixels it creates based on the color values of existing pixels in the image. The more sophisticated the method the more quality and detail from the original image are preserved.

The General Preferences dialog box lets you specify a default interpolation method to use whenever images are resampled with the Image Size or transformation commands. The Image Size command also lets you specify an interpolation method other than the default. To specify the default interpolation method:

Do one of the following:

In Windows or Mac OS 9.x choose Edit > Preferences > General.

(Photoshop) In Mac OS X choose Photoshop > Preferences > General.



(ImageReady) In Mac OS X choose ImageReady > Preferences > General.

For Interpolation choose one of the following options:

Nearest Neighbor (Jagged) for the fast but less precise method. This method is recom- mended for use with illustrations containing non-anti-aliased edges to preserve hard edges and produce a smaller file However this method can result in jagged effects, which become apparent when distorting or scaling an image or performing multiple manipulations on a selection.

(Photoshop) Bilinear for a medium-quality method.

Bicubic (Smooth) for the slow but more precise method resulting in the smoothest tonal gradations.

Changing the pixel dimensions of an image

When preparing images for online distribution it s useful to specify image size in terms of the pixel dimensions Keep in mind that changing pixel dimensions affects not only the size of an image on-screen but also its image quality and its printed characteristics— either its printed dimensions or its image resolution (See About image size and resolution on section 62

To change the pixel dimensions of an image (Photoshop):

Choose Image > Image Size.

Make sure that Resample Image is selected and choose an interpolation method.

(See “Choosing an interpolation method on section 66

To maintain the current proportions of pixel width to pixel height select Constrain Proportions. This option automatically updates the width as you change the height, and vice versa.

Under Pixel Dimensions enter values for Width and Height. To enter values as percentages of the current dimensions choose Percent as the unit of measurement. The new file size for the image appears at the top of the Image Size dialog box with the old file size in parentheses.

For best results in producing a smaller image downsample and apply the Unsharp

Mask filter. To produce a larger image rescan the image at a higher resolution.

To change the pixel dimensions of an image (ImageReady):

Choose Image > Image Size.

To maintain the current proportions of pixel width to pixel height select Constrain

Proportions.

Under New Size enter values for Width Height or Percent. The New Size text field displays the new file size for the image.

Select a resampling method from the Quality pop-up menu.

For information on setting action options see “Recording image size options

(ImageReady) on section 489

Changing the print dimensions and resolution of an image

(Photoshop)

When creating an image for print media it s useful to specify image size in terms of the printed dimensions and the image resolution. These two measurements referred to as the document size, determine the total pixel count and therefore the file size of the image; document size also determines the base size at which an image is placed into another application. You can further manipulate the scale of the printed image using the Print with Preview command however changes you make using the Print with Preview command affect only the printed image not the document size of the image file (See Positioning and scaling images on section 472

If you turn on resampling for the image you can change print dimensions and resolution independently (and change the total number of pixels in the image) If you turn resam- pling off you can change either the dimensions or the resolution—Photoshop adjusts the other value automatically to preserve the total pixel count For the highest print quality, it s generally best to change the dimensions and resolution first without resampling. Then resample only as necessary.



To change the print dimensions and resolution of an image:

Choose Image > Image Size.

Change the print dimensions image resolution or both:

To change only the print dimensions or only the resolution and adjust the total number of pixels in the image proportionately make sure that Resample Image is selected. Then choose an interpolation method (See “Choosing an interpolation method on section 66

To change the print dimensions and resolution without changing the total number of pixels in the image deselect Resample Image.

To maintain the current proportions of image width to image height select Constrain Proportions. This option automatically updates the width as you change the height and vice versa.

Under Document Size enter new values for the height and width If desired choose a new unit of measurement Note that for Width the Columns option uses the width and gutter sizes specified in the Units & Rulers preferences For more information see Using columns (Photoshop) on section 44

For Resolution enter a new value If desired choose a new unit of measurement.

To return to the original values displayed in the Image Size dialog box hold down

Alt ( Windows) or Option (Mac OS) and click Reset.

To view the print size on-screen:

Do one of the following:

Choose View > Print Size.

Select the hand tool or zoom tool and click Print Size in the options bar.

The magnification of the image is adjusted to display its approximate printed size as specified in the Document Size section of the Image Size dialog box Keep in mind that the size and resolution of your monitor affect the on-screen print size.

Determining a recommended resolution for an image

(Photoshop)

If you plan to print your image using a halftone screen the range of suitable image resolu- tions depends on the screen frequency of your output device. You can have Photoshop determine a recommended resolution for your image based on your device s screen frequency (See About image size and resolution on section 62

Note: If your image resolution is more than 2.5 times the screen ruling an alert message appears when you try to print the image. This means that the image resolution is higher than necessary for the printer Save a copy of the file and then reduce the resolution.

To determine a suggested resolution for an image:

Choose Image > Image Size.

Click Auto.

For Screen enter the screen frequency for the output device If desired choose a new unit of measurement Note that the screen value is used only to calculate the image resolution not to set the screen for printing.

Important: To specify the halftone screen ruling for printing, you must use the Halftone Screens dialog box accessible through the Print with Preview command (See Selecting halftone screen attributes on section 475

For Quality select an option:

Draft to produce a resolution the same as the screen frequency (no lower than 72 pixels per inch).

Good to produce a resolution 1.5 times the screen frequency.

Best to produce a resolution 2 times the screen frequency.






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