Adobe Photoshop CS5 for Photographers - Martin Evening [131]
Figure 5.1 Digital images are made up of a mosaic of pixels. This means that a pixel-based digital image will always have a fixed resolution and is said to be ‘resolution- dependent’. If you enlarge such an image beyond the size at which it is meant to be printed, the pixel structure will soon become apparent, as can be seen here in the left-hand close- up view. Suppose though the above picture originated not as a photograph, but was drawn as an illustration using a program like Adobe Illustrator. If a picture is constructed using vector paths, it will be resolution-independent. The mathematical numbers used to describe the path outlines shown in the example on the right can then be scaled to reproduce at any size: from a postage stamp to a billboard poster. As you can see in the comparison shown here, the pixel image starts to break up as soon as it is magnified, whereas the outlines in the vector-drawn image will reproduce perfectly smoothly at any size.‘Stalkers’ by The Wrong Size. Photograph: © Eric Richmond.
Photoshop as a vector program
Photoshop is mainly regarded as a pixel-based graphics program, but it has the capability to be a combined pixel and vector editor because it does also contain a number of vector-based features that can be used to generate things such as custom shapes and layer clipping paths. This raises some interesting possibilities, because you can create various graphical elements like type, shape layers and layer clipping paths in Photoshop which are all resolution- independent. These ‘vector’ elements can be scaled up in size in Photoshop without any loss of detail, just as you can with an Illustrator graphic.
Confusing terminology
You can see from this description where the term ‘lines per inch’ originated. In today's digital world of imagesetters, the definition is somewhat archaic, but is nonetheless commonly used. You may hear people refer to the halftone output as ‘dpi’ instead of ‘lpi’, as in the number of ‘halftone’ dots per inch, and the imagesetter resolution referred to as having so many ‘spi’, or ‘spots per inch’. Whatever the terminology I think we can all logically agree on the correct use of the term ‘pixels per inch’, but I am afraid there is no clear definitive answer to the mixed use of the terms ‘dpi’, ‘lpi’ and ‘spi’. It is an example of how the two separate disciplines of traditional repro and those who developed the digital technology chose to apply different meanings to these same terms.
Image resolution terminology
Before I proceed any further let me help explain a few of the terms that are used when describing image resolution, and clarify their correct usage.
ppi: pixels per inch
The term ‘pixels per inch (ppi)’ should be used to describe the pixel resolution of an image, although the term ‘dpi’ is also often used to inappropriately describe the digital resolution of an image. It is incorrect to use the term ‘dpi’ because input devices like scanners and cameras don't produce dots, they produce pixels. Only printers can produce dots! However, it's become commonplace for scanner manufacturers and other software programs to use the term ‘dpi’ when what they really mean is ‘ppi’. Unfortunately this has only added to the confusion, because you often hear people describing the resolution of an image as having so many ‘dpi’, but if you look carefully, Photoshop and the accompanying user guide always refers to the input resolution as being in ‘pixels per inch’. So if you have an