Resolution, Resizing and Resampling

Computer graphics fall into two main categories: bitmap and vector images. Let's review the difference between the two. Understanding this will help as you create and edit digital images.

Bitmap and vector review


Photoshop and other paint and image-editing programs generate bitmap images, also called raster images. Bitmap images use a grid (the bitmap or raster) of small squares known as pixels to represent images. Each pixel is assigned a specific location and color value. For example, a bicycle tire in a bitmap image is made up of a mosaic of pixels in that location. When working with bitmap images, you edit pixels rather than objects or shapes.


Rasterizing is a process of converting vector images to bitmap images.


A bitmap image is resolution-dependent meaning that it contains a fixed number of pixels. Bitmap images are the best choice for representing subtle gradations of shades and color — for example, in photographs or painted images — to represent its image data. As a result, a bitmap image can lose detail and appear jagged if viewed at a high magnification on-screen or printed at too low a resolution.

High resolution images offer you the best quality for printing photos. Examine a photo's image size for its dimensions and resolution before you place it in your layout and enlarge it. (To find this information in Photoshop, go to Image Menu > Image Size.) See below for a comparision of a low resoluion and a high resolution image enlarge four times (400%).

Low Resolution

72 dpi enlarged 400%

High Resolution

300 dpi enlarged 400%

Drawing programs such as Adobe Illustrator create vector graphics, made of lines and curves defined by mathematical objects called vectors. Vectors describe graphics according to their geometric characteristics. For example, a bicycle tire in a vector graphic is made up of a mathematical definition of a circle drawn with a certain radius, set at a specific location, and filled with a specific color. You can move, resize, or change the color of the tire without losing the quality of the graphic.


A vector graphic is resolution-independent -- that is, it can be scaled to any size and printed on any output device at any resolution without losing its detail or clarity. As a result, vector graphics are the best choice for type (especially small type) and bold graphics that must retain crisp lines when scaled to various sizes (logos, for example). Because computer monitors represent images by displaying them on a grid, both vector and bitmap images are displayed as pixels on-screen.


Resolution, Resizing and Resampling

• Resizing does not change the file size, but changes the relationship of the file dimensions and dpi. You can resize the image to specified pixel dimensions or to a percentage of the original size.

• Resampling changes the pixel dimensions (changed the numbers of pixels in the file) and thus the file size -- see below.

Resizing


Start with a 1x1-inch file with a resolution of 300 dpi (Fig. A). Its file size is 264k.

Fig. A

Increasing the width and height dimensions but not resampling leave the file and data at its original size of 264k (Fig. B).

Fig. B

Resampling


Changing the pixel dimensions (and therefore display size) of an image is referred to as resampling.


When you downsample (or decrease the number of pixels), information is deleted from the image. You can also upsample, or increase the number of pixels, but it doesn't necessarily improve the quality of the image. When Photoshop upsamples it interpolates new pixels based on existing data.


Downsampling


Start with that same 1x1-inch file with a resolution of 300 dpi at 264k (see Fig. A above). Constraining proportions and resampling the image to 72 dpi will decrease the file size to 15.2k (Fig. C). A lot of file information was lost in this process.

Fig. C

Upsampling
When you resample up (or increase the number of pixels by either changing dimensions or resolution), new pixel information is added based on color values of existing pixels. Changing pixel dimensions  affects the size of an image and also can affect its quality.


Begin with a 1.5x1.5 inch file with a resolution of 72 dpi.

Resample (upsample) the image to 6x6 inches.

 Left, upsampled image; right, detail of upsampled image showing image quality.

Take note of the increase of file size, but not necessarily of image quality. Photoshop interpolates (inserts) new pixels for the increased size (or resolution) based on existing data. However, the image quality may not be adequate since Photoshop is essentially guessing at the new data.



To change dimensions of an image


1. In Photoshop, resize an image by choosing Image menu > Image Size.


2. To maintain the current proportions of pixel width to pixel height, select Constrain Proportions. Leave Resample Image box unchecked. Constrain the image proportions by selecting:

• Width, to constrain proportions using the new width value.
• Height, to constrain proportions using the new height value.
• Percent, to constrain proportions while resizing the image by a percent value.

3. Click OK.


Read more about pixel dimensions and resolution from the experts at Adobe: http://livedocs.adobe.com/en_US/Photoshop/10.0/help.html?content=WS75D24624-A761-40b6-832E-8AB0E2383C90.html

Exerpted and adapted from Adobe Photoshop Online Help.