It's all about the DPI!

In my last blog installment I discussed the subject of "vector graphics". Vector graphics are an industry standard format for printing and sign making BUT not the ONLY industry standard. There are other file formats that are also acceptable that may be more familiar to you. These fall into the category of "bitmapped or raster graphics" . You'll recognized them more by the name of their file extensions...TIF, JPG, GIF, PNG and BMP (there are many other lesser known ones).


Bitmapped graphics are fundamentally different than their vector counterparts even though they serve the same purpose. Not to go into a full recap, but as discussed in the last blog, we learned that vectors are drawn images created by using a "pen tool" or "shape tool" (of some sort) that generates lines and points. These shapes can be resized indefinitely without losing quality. If you could observe the enormous amount of programming code being executed by a computer when drawing vector images, you would see that It's a very mathematical operation.


A bitmapped image is different in the sense that it is an image made up of solely pixels. These are what images such as digital photographs (or internet images) are comprised of (Fig 1.)

The quality of a "bitmap" or "raster" is described in terms of resolution measurements of DPI or PPI. So let's take a moment to discuss DPI and PPI. Most likely you've heard of one or the other terms.


DPI (dots per inch) which is the more familiar acronym and it refers to the number of printed dots that make up one inch of a printed image. With DPI you're likely to hear numbers such as 150, 200, 300 and even 600 DPI. The industry standard is 300 DPI, although there's nothing saying you can go higher, but the human eye isn't going to notice the difference above 300 DPI. It will just result in a needlessly larger file size. The lowest resolution I have used in professional printing is 150 DPI and the only time I do that is when I'm going to be dealing with a very large image measured in square feet!... and want the file size to be smaller.


PPI (pixels per inch) refers to the number of pixels in within one inch of an image displayed on a computer monitor. Measurements mostly associated with PPI are 72 PPI and 96 PPI and are the standard resolution for internet or power point graphics. This is not to say that these resolutions can't be printed, it's just that they are most likely (not always) to be fuzzy or grainy if you do.


The DPI and PPI thing can get confusing because most of the time you'll hear DPI as the blanket term for both ideas. However, the good thing is that 1 DPI = 1 PPI, 2 DPI = 2 PPI and so on. If you are working in a program like Adobe Photoshop you're probably going to set your workspace up in terms of DPI so just roll with it!


In terms of scalability, with bitmapped graphics there are practical limitations. That's not to say that you can't resize them but unlike vector images, they will lose quality if they are stretched larger. However, if your image is anywhere between 200 and 600 DPI and is already a few inches large in terms of length and width, then usually you can stretch them larger (within reason) without losing noticeable clarity. This is not case with images at 72 and 96 DPI. Some people misunderstand and will convert a 72 dpi image to a 300 dpi image thinking it will increase quality but really all they will end with is a fuzzy 300 DPI image. You can shrink a 300 dpi image down to 72 dpi and maintain a good image but not so much going the other way.


Now lets talk about formats. The term "Bitmapped" or "Raster" is the overall category name of a number of computer generated image formats. As I mentioned above, the more familiar ones are JPG, PNG, GIF, TIF, and BMP. These file extensions are actually acronyms but we don't need to get into that here. These file types are usually what are "EXPORTED" or "SAVED AS" from the main program they are created and/or manipulated in. For example, with Photoshop I can create a graphic and save it as a PSD file and it will be considered a bitmap image but it will maintain all the special properties, effects and multiple layers used to create the image and most likely will be a large file in size (megabytes).


However when I export (or "save as") that artwork from Photoshop as one of the formats I mentioned above, the image will be flattened for all intents and purposes into a single layered image. It will usually result in a smaller file size and have more compatibility with most anything else you are using it for. These files can be used for either printing or digital display, but some of them are better suited for one or the other purpose. I usually prefer printing bitmap files exported in TIF format. I can use JPG's as long as they are 200-300 dpi.


For the internet or digital presentation JPG's, PNG's and GIF's are the standard at either 72 or 96 dpi (or PPI). PNG's are used A LOT in this case because of their transparent background properties.


Most every graphic oriented application will allow for exporting to at least one of these formats. The most popular application is Photoshop but there are others such as the GIMP, and Corel Painter. Vector images can also be exported to these formats as well but that's another blog.


The strange thing is that for as long as this blog is, there is so much more that could be said about this subject but I think by now you get the idea.


For my next blog I want to explain PDF's and EPS and how these file formats are related to both bitmapped and vector graphics, but until then...thanks for reading!

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