COMPUTER GRAPHICS

Image Size

Resolution

IMAGE SIZEThe most important thing to understand about resolution is the relationship between an image’s resolution (DPI) and an image’s print size (the actual width and height).

• Once you scan an image or take a picture with your digital camera, it becomes digitized.

• Made up of hundreds of thousands of pixels.

• Pixels are nothing more than very tiny coloured squares that you can see if you increase the magnification of any image to its maximum

PIXELS IN A BITMAP IMAGE

RESOLUTION

Resolution is the number of pixels in a linear inch—pixels per inch (or PPI), but it is most commonly referred to as dots per inch (DPI).

The more pixels, or “dots,” per inch, the higher your image resolution will be.

There are two settings that affect the quality of the bitmap.

The first is Bit Depth, Colour Depth or Image Mode.

The number of colours that each pixel can represent.

A pixel in a typical 24-bit RGB image can represent one of 256 levels of red, green and blue which gives a possible tonal range of over 16 million colours (256 x 256 x 256).

A pixel in an 8-bit indexed or grayscale image on the other hand represents one of just 256 possible values.

The second is Pixel Dimensions.

The number of pixels in the image both horizontally and vertically.

With a pixel dimension of 300 x 300, for example, an image has a total of 90,000 pixels.

With a doubled pixel dimension of 600 x 600 the image has four times as many pixels (360,000) to play with. And clearly the more pixels, or specific points of information there are in a picture, the more detail that can be represented.

By dividing the number of pixels in the height and in the width of a raster or bitmap by its resolution will determine the physical size of the image—e.g., a 300ppi raster image that is 900 pixels wide and 600 pixels high is 3 inches by 2 inches in size:

900 pixels ÷ 300ppi= 3 inches wide

600 pixels ÷ 300ppi= 2 inches high

An image resolution at 72 dpi is the Web or screen viewing standard.

Different resolution settings have no effect on onscreen size... So if it makes no difference to a Web image's size if it was created at 72 dpi or 144 dpi or 1440 dpi, what function does the resolution setting play?

The answer of course is print. Here there can be no flexibility.

Taken together the pixel dimensions and image mode provide the horizontal, vertical and depth settings that define the level of detail and tonal range in an image and hence its quality (aesthetics aside).

By fixing how many image pixels are output per inch of paper, the image resolution setting fixes the image's printed size.

Resizing an image for print makes no difference whatsoever to the pixel data (as opposed to Resampling).

In other words the resolution setting has no real effect on image quality it is really just a convenience for managing output size.

The resolution setting is also important when you want to change the size of your print.

The resolution setting affects the size of the image rather than its quality

Image quality is determined purely by the pixel information.

The Image File Size considerations relate to quality. If you are creating an image for the Internet would you want a large file (high resolution, lots of pixels per inch) or a smaller file (low resolution, 16 bit)?