Friday, December 04 2009 @ 10:05 AM PST (Read 1302 times)
PPI vs DPI, A Brief History (and an even shorter future)
Pixels Per Inch (PPI) and Dots Per Inch (DPI) are often misunderstood, used interchangeably, and cause a lot of anxiety. To understand how all this came about requires a little bit of history.
Back a hundred and fifty years ago, before the invention of the telephone, electronic digital communication was the telegraph. The basic telegraph consisted of two wires, a battery pack, a telegraph key to send messages with, and a clicker at the other end to hear the signal. All letters and symbols were made up of a combination of short (dots) and long (dashes) pulses, usually in Morse Code. You've no doubt seen these in western movies.
Jumping past the invention of the telephone, telegraphy was automated by connecting (simply stated) a wired-up typewriter to a another wired-up typewriter so that what someone typed at one end would come out at the other end. This was called teletype. To make them faster these operated on power-off (zeros) and power-on (ones) and went through a number of different codes. One popular code was named after its inventor, Baudot, for which "baud rate" is named. The code that has endured is the ASCII code, which is still widely used today by computers.
Many innovations later, these teletypewriters were better and faster but could still print only the typewriter characters and were limited to fixed spacing. Each character had the same amount of space on the paper and this was referred to as characters-per-inch (CPI) or pitch. Ten and 12 pitch are still commonly used today. At some point someone figured out that they could make pictures (crude as they were) by printing periods (or other characters) in patterns. These periods were called dots and the clarity of the picture was measured in dots-per-inch (DPI).
A few innovations later and printers were built specifically to create pictures with dots. Because they were not printing characters they could be made to print at a higher pitch and 300 DPI was settled on as a standard because it was hard for people to see the dots. We still use 300 DPI for most printing, though Costco seems to have their printers set to 320 DPI.
Along the way the Cathode Ray Tube (CRT) was invented and eventually turned into the television picture tube and computer monitor. Early CRTs had electrons hitting chemicals spread evenly across the glass screen. These chemicals glowed when the electrons hit them but were fuzzy. To improve the picture sharpness for television a metal sheet (mask) with holes was placed between the screen and the electron gun so that the screen lit up only at the holes. These lit spots were called pixels, as in small pictures. The standard (still today) was 640 pixels wide and 480 pixels high (640x480). Since then monitors have steadily increased in the number of pixels and progressed through VGA, SVGA, XVGA and etc. Television only recently added High Definition (1080x720).
For about 20 years both the monitors and printers were making steady improvements and computers were becoming common. Talk about PPI and DPI became so popular and casual that most people forgot that there was a real difference. In the '70s we would say "Hi, what's your zodiac sign?" In the '80s we would say "Hi, what's your PPI?" Few people cared whether they were saying it right or not because that wasn't the point. To date some people still use the two interchangeably.
Today monitors come in many different sizes with many different PPI resolutions making it difficult to create anything that looks the same on all (or even most) monitors. Fortunately the software on most computers has the ability to accept most files no matter what DPI or PPI they were created in and display them in an acceptable manner. Because of this most people have never had to learn about PPI. Still there are rare applications that use the standard 72 PPI or can only handle images no larger than 640 x 480 pixels, and that creates a lot of confusion. People like me who, occasionally, mix up PPI and DPI don't help.
Printers, on the other hand, have not been held to the rigid standard of the CRTs. After the teletypewriters most of the early printers were of the impact type. The heads on these drove metal pins onto printer ribbons causing small dots on the paper that looked like characters or images. Later versions shot ink directly onto the paper (bubble jet and such) but were still limited to a fixed number of dots-per-inch and each dot had the same amount of ink. Many innovations later, the current generation of good printers have complex heads that print different sized dots and are no longer limited to fixed spacing. And for photographic prints the ink flows enough to blend colors to the point that you couldn't find a dot even if you had a microscope. Therefore the concept of dots-per-inch is nearly, but not quite, obsolete.
Most LPA photographers, not being computer geeks, only run into PPI and DPI when they go to print at a store or send files to be shown in our competitions. Without a strong background in computers or photoshop this new concept causes a lot of anxiety, even though technically it's fairly easy to do.
To reiterate, PPI and DPI are often misunderstood, used interchangeably, and cause a lot of anxiety. At the same time they are becoming less and less meaningful or useful to the average person. Still, there are the occasional applications where either pixels (i.e. LPA projector) or DPI (i.e. Costco printers) should be specified in order to get exactly the results you want. Within a few years software development will make the PPI and DPI issues a thing of the past, except for a few diehards.
In fact, places like Costco already have software that will convert your photos from whatever DPI your files are in to what their machines use. If you're curious you can try sending in a photo at 320 DPI and the same photo at the original DPI and see if you can tell the difference. If you can't then don't bother converting your photos to 320 DPI. Most users of Costco, WalMart, and those places don't convert their photos and are happy with the results. If you're a highly competitive type then you will probably want to mother over each and every dot. And that leaves only the LPA projector to come into the 21st century.
And you thought history was boring and useless. :-)
Pixels Per Inch (PPI) and Dots Per Inch (DPI) are often misunderstood, used interchangeably, and cause a lot of anxiety. To understand how all this came about requires a little bit of history.
Back a hundred and fifty years ago, before the invention of the telephone, electronic digital communication was the telegraph. The basic telegraph consisted of two wires, a battery pack, a telegraph key to send messages with, and a clicker at the other end to hear the signal. All letters and symbols were made up of a combination of short (dots) and long (dashes) pulses, usually in Morse Code. You've no doubt seen these in western movies.
Jumping past the invention of the telephone, telegraphy was automated by connecting (simply stated) a wired-up typewriter to a another wired-up typewriter so that what someone typed at one end would come out at the other end. This was called teletype. To make them faster these operated on power-off (zeros) and power-on (ones) and went through a number of different codes. One popular code was named after its inventor, Baudot, for which "baud rate" is named. The code that has endured is the ASCII code, which is still widely used today by computers.
Many innovations later, these teletypewriters were better and faster but could still print only the typewriter characters and were limited to fixed spacing. Each character had the same amount of space on the paper and this was referred to as characters-per-inch (CPI) or pitch. Ten and 12 pitch are still commonly used today. At some point someone figured out that they could make pictures (crude as they were) by printing periods (or other characters) in patterns. These periods were called dots and the clarity of the picture was measured in dots-per-inch (DPI).
A few innovations later and printers were built specifically to create pictures with dots. Because they were not printing characters they could be made to print at a higher pitch and 300 DPI was settled on as a standard because it was hard for people to see the dots. We still use 300 DPI for most printing, though Costco seems to have their printers set to 320 DPI.
Along the way the Cathode Ray Tube (CRT) was invented and eventually turned into the television picture tube and computer monitor. Early CRTs had electrons hitting chemicals spread evenly across the glass screen. These chemicals glowed when the electrons hit them but were fuzzy. To improve the picture sharpness for television a metal sheet (mask) with holes was placed between the screen and the electron gun so that the screen lit up only at the holes. These lit spots were called pixels, as in small pictures. The standard (still today) was 640 pixels wide and 480 pixels high (640x480). Since then monitors have steadily increased in the number of pixels and progressed through VGA, SVGA, XVGA and etc. Television only recently added High Definition (1080x720).
For about 20 years both the monitors and printers were making steady improvements and computers were becoming common. Talk about PPI and DPI became so popular and casual that most people forgot that there was a real difference. In the '70s we would say "Hi, what's your zodiac sign?" In the '80s we would say "Hi, what's your PPI?" Few people cared whether they were saying it right or not because that wasn't the point. To date some people still use the two interchangeably.
Today monitors come in many different sizes with many different PPI resolutions making it difficult to create anything that looks the same on all (or even most) monitors. Fortunately the software on most computers has the ability to accept most files no matter what DPI or PPI they were created in and display them in an acceptable manner. Because of this most people have never had to learn about PPI. Still there are rare applications that use the standard 72 PPI or can only handle images no larger than 640 x 480 pixels, and that creates a lot of confusion. People like me who, occasionally, mix up PPI and DPI don't help.
Printers, on the other hand, have not been held to the rigid standard of the CRTs. After the teletypewriters most of the early printers were of the impact type. The heads on these drove metal pins onto printer ribbons causing small dots on the paper that looked like characters or images. Later versions shot ink directly onto the paper (bubble jet and such) but were still limited to a fixed number of dots-per-inch and each dot had the same amount of ink. Many innovations later, the current generation of good printers have complex heads that print different sized dots and are no longer limited to fixed spacing. And for photographic prints the ink flows enough to blend colors to the point that you couldn't find a dot even if you had a microscope. Therefore the concept of dots-per-inch is nearly, but not quite, obsolete.
Most LPA photographers, not being computer geeks, only run into PPI and DPI when they go to print at a store or send files to be shown in our competitions. Without a strong background in computers or photoshop this new concept causes a lot of anxiety, even though technically it's fairly easy to do.
To reiterate, PPI and DPI are often misunderstood, used interchangeably, and cause a lot of anxiety. At the same time they are becoming less and less meaningful or useful to the average person. Still, there are the occasional applications where either pixels (i.e. LPA projector) or DPI (i.e. Costco printers) should be specified in order to get exactly the results you want. Within a few years software development will make the PPI and DPI issues a thing of the past, except for a few diehards.
In fact, places like Costco already have software that will convert your photos from whatever DPI your files are in to what their machines use. If you're curious you can try sending in a photo at 320 DPI and the same photo at the original DPI and see if you can tell the difference. If you can't then don't bother converting your photos to 320 DPI. Most users of Costco, WalMart, and those places don't convert their photos and are happy with the results. If you're a highly competitive type then you will probably want to mother over each and every dot. And that leaves only the LPA projector to come into the 21st century.
And you thought history was boring and useless. :-)