Image Resolution – PPI, DPI, LPI and SPI

Unlike the human eye, which captures an image in a continuous, permanent way, graphic environments have tools capable of reaching the image detail, establishing the distinction that separates digitally generated images from mechanically reproduced ones, resulting in two distinct units – the pixel and the point, respectively. The key issue in this distinction is that the notion of point is systematically shared across different sources, justifying a clarifying approach to the four resolution measurements relevant to the screen printing context – PPI, DPI, LPI, and SPI.

PPI – PPI stands for Pixels Per Inch (also called PPP in interfaces translated into Portuguese). The term PPI refers to the image resolution in its digital mode, on a monitor. When editing a photograph in any graphic editing program, the true unit of resolution is PPI – and not DPI, as generically referred to in interfaces.

DPI – DPI refers to Dots Per Inch (also known as PPI in interfaces translated into Portuguese). In this case, it refers to the unit of measurement for converting a digital image into a printed image on a physical substrate – hence the change from pixel to point, because printers use points to reproduce a single pixel of the image. The concept of a pixel is much more complex and interesting than generalized descriptions. A pixel is not a simple linear unit corresponding to that tiny square of the image when enlarged to its maximum. The generic graphic representation of the square on the screen is nothing more than a model adopted by the creators of graphics cards as a way to simplify the visual representation of the pixel, which, in reality, has no defined shape and somehow had to be represented. The true notion of a pixel may be one of the reasons why software programmers choose to replace PPI with DPI, since a pixel corresponds to a sample of a given point in the image, and in color images this pixel can even contain three samples – one for each primary color (p. 298) – as if it were a kind of three sub-pixels. The fact is that the amorphous and somewhat abstract nature of the pixel has given rise to context dependence, attracting several other synonyms besides the point and even beyond a concept of resolution.

LPI – LPI refers to Lines Per Inch (also called LPP in interfaces translated into Portuguese). This represents another measurement related to printing and is used exclusively in professional processes, created for halftone techniques, especially applied to screen printing, letterpress, lithography, offset, pad printing and laser printing (among others).

SPI – This other resolution reference, designated Samples Per Inch (SPI), is also discussed. This measurement is exclusively linked to image scanners because a scanner is a specific input device that captures images differently from a photographic camera. SPI translates the optical resolution of the scanner and is related to the color depth of the digitized object, which is translated into bits (pages 298 to 301). The quality of the scan is defined by the number of color samples that the optical part of the scanner is able to capture for each linear inch. This capacity varies from scanner to scanner, according to the type and range, and therefore, many budget scanners advertise that they capture millions of colors per inch when, in fact, they do not have real optical capacity, but only a mode capable of simulating millions of colors – the so-called interpolated resolution, which uses algorithms to artificially insert additional pixels – and which, obviously, does not translate the real colors of an image.

This article is just a small compilation of samples from an e-book sold on this website, but its first edition is only available in Portuguese.

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