Photoshop as a Vector Program
Photoshop is mainly regarded as a pixelbased graphics program, but it does have the capability to be a combined vector and pixel editing program because it also contains a number of vector-based features that can be used to generate images 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 and these are all resolution-independent. These ‘vector’ elements can be scaled up in size in Photoshop without any loss of detail, just as you can do with an Illustrator graphic.
Pixels versus vectors
Digital photographs are constructed with pixels and as such are resolution-dependent. You can scale up a pixel image, but as you do so, the finite information can only be stretched so far before the underlying pixel structure becomes apparent. Objects created in a program like Adobe Illustrator are defined mathematically, so if you draw a rectangle, the proportions of the rectangle edges, the relative placement on the page and the color it is filled with can all be described using mathematical expressions.
An object defined using vectors can be output at any resolution. It does not matter if the image is displayed on a computer screen, or as a huge poster, it will always be rendered with the same amount of detail.
Before proceeding further let me help clarify a few of the confusing terms used and their correct usage when describing resolution.
ppi: pixels per inch. This describes the digital, pixel resolution of an image. But you will notice the term ‘dpi’ is often inappropriately used to describe the digital resolution of film scanners. This is an incorrect use of the term ‘dpi’ because input devices like scanners produce pixels. They don’t produce dots. Only printers can do that! However, it’s become quite common now for scanner manufacturers to use the term ‘dpi’ when they really mean ppi and 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 always refers to the input resolution as being in pixels per inch or pixels per centimeter. So if you have an image that has been captured on a digital camera scanned from a photograph, or displayed in Photoshop, it is always made up of pixels. The pixel resolution (ppi) is the number of pixels per inch in the input digital image.
lpi: lines per inch. This is the number of halftone lines or ‘cells’ in an inch, also described as the screen ruling. The origins of this term go back way before the days of digital desktop publishing. To produce a halftone plate, the film exposure was made through a finely etched criss-cross screen of evenly spaced lines on a glass plate. When a continuous tone photographic image was exposed this way, dark areas formed heavy halftone dots and the light areas formed smaller dots, which when viewed from a normal distance gave the impression of a continuous tone image on the page. The line screen resolution (lpi) is therefore the frequency of halftone dots or cells per inch.
dpi: dots per inch. This refers to the resolution of a printing device. An output device such as an imagesetter is able to produce tiny 100% black dots at a specified resolution. Let’s say we have an imagesetter capable of printing at a resolution of 2450 dots per inch and the printer wished to use a screen ruling of 150 lines per inch. If you divide the dpi of 2450 by the lpi of 150, you get a figure of 16. Within a matrix of 16 × 16 printer dots, an imagesetter can generate a halftone dot varying in size from 0 to 255, which is 256 print dots. It is this variation in halftone cell size (constructed from the combined smaller dots) which gives the impression of tonal shading when viewed from a distance. Desktop inkjet printers correctly use the term dpi to describe the resolution of the printer head. Inkjet printers produce an output made up of small dots at resolutions of between 360 and 2880 dots per inch. The inkjet output is not the same as the reprographic process – the screening method used is quite different.
You can see from these descriptions where the term ‘lines per inch’ originated. In today’s digital world of imagesetters, the defi nition 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 defi nitive 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.
Photoshop is mainly regarded as a pixelbased graphics program, but it does have the capability to be a combined vector and pixel editing program because it also contains a number of vector-based features that can be used to generate images 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 and these are all resolution-independent. These ‘vector’ elements can be scaled up in size in Photoshop without any loss of detail, just as you can do with an Illustrator graphic.
Pixels versus vectors
Digital photographs are constructed with pixels and as such are resolution-dependent. You can scale up a pixel image, but as you do so, the finite information can only be stretched so far before the underlying pixel structure becomes apparent. Objects created in a program like Adobe Illustrator are defined mathematically, so if you draw a rectangle, the proportions of the rectangle edges, the relative placement on the page and the color it is filled with can all be described using mathematical expressions.
An object defined using vectors can be output at any resolution. It does not matter if the image is displayed on a computer screen, or as a huge poster, it will always be rendered with the same amount of detail.
Before proceeding further let me help clarify a few of the confusing terms used and their correct usage when describing resolution.
ppi: pixels per inch. This describes the digital, pixel resolution of an image. But you will notice the term ‘dpi’ is often inappropriately used to describe the digital resolution of film scanners. This is an incorrect use of the term ‘dpi’ because input devices like scanners produce pixels. They don’t produce dots. Only printers can do that! However, it’s become quite common now for scanner manufacturers to use the term ‘dpi’ when they really mean ppi and 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 always refers to the input resolution as being in pixels per inch or pixels per centimeter. So if you have an image that has been captured on a digital camera scanned from a photograph, or displayed in Photoshop, it is always made up of pixels. The pixel resolution (ppi) is the number of pixels per inch in the input digital image.
lpi: lines per inch. This is the number of halftone lines or ‘cells’ in an inch, also described as the screen ruling. The origins of this term go back way before the days of digital desktop publishing. To produce a halftone plate, the film exposure was made through a finely etched criss-cross screen of evenly spaced lines on a glass plate. When a continuous tone photographic image was exposed this way, dark areas formed heavy halftone dots and the light areas formed smaller dots, which when viewed from a normal distance gave the impression of a continuous tone image on the page. The line screen resolution (lpi) is therefore the frequency of halftone dots or cells per inch.
dpi: dots per inch. This refers to the resolution of a printing device. An output device such as an imagesetter is able to produce tiny 100% black dots at a specified resolution. Let’s say we have an imagesetter capable of printing at a resolution of 2450 dots per inch and the printer wished to use a screen ruling of 150 lines per inch. If you divide the dpi of 2450 by the lpi of 150, you get a figure of 16. Within a matrix of 16 × 16 printer dots, an imagesetter can generate a halftone dot varying in size from 0 to 255, which is 256 print dots. It is this variation in halftone cell size (constructed from the combined smaller dots) which gives the impression of tonal shading when viewed from a distance. Desktop inkjet printers correctly use the term dpi to describe the resolution of the printer head. Inkjet printers produce an output made up of small dots at resolutions of between 360 and 2880 dots per inch. The inkjet output is not the same as the reprographic process – the screening method used is quite different.
You can see from these descriptions where the term ‘lines per inch’ originated. In today’s digital world of imagesetters, the defi nition 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 defi nitive 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.
Pixels Versus Vectors
![Pixels Versus Vectors]()
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