dpi=ppi?

Discussion in 'Digital Photography' started by didi, Dec 10, 2003.

  1. didi

    didi Guest

    Hello,

    is the resolution in dpi equal to the resolution in ppi?

    Thanks

    Titi
     
    didi, Dec 10, 2003
    #1
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  2. didi

    Trev Guest

    "didi" <> wrote in message
    news:...
    > Hello,
    >
    > is the resolution in dpi equal to the resolution in ppi?
    >
    >

    No. Resolution is measured in pixels per inch. the printer converts them
    into drops of ink at a variable rate up to the set, drops per inch ink
    setting.
     
    Trev, Dec 10, 2003
    #2
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  3. didi

    Don Coon Guest

    "didi" <> wrote in message
    news:...
    > Hello,
    >
    > is the resolution in dpi equal to the resolution in ppi?
    >
    > Thanks
    >
    > Titi


    In proper use, no although even major manufaturers tend to misuse them.

    ppi refers to the a printing density -- ie. "print at 300 pixels per inch".
    dpi refers to the number of dots of ink an injet printer can lay down in an
    inch of travel --- ie. 720dpi or 1440 dpi.

    My inkjet on normal setting expects 300ppi, my laser printer expects 600ppi.
    My Epson inkjet lays down 360, 720 or 1440dpi.
     
    Don Coon, Dec 10, 2003
    #3
  4. didi

    mark herring Guest

    Answer below is generally accepted. However, DPI and PPI do tend to get
    used interchangeably---eg scanners are speced in DPI.

    Regardless of the initials used---keep straight the difference between FILE
    resolution and whatever metric applied to the printing process

    --
    ******************
    Mark Herring
    Pasadena, CA, USA
    private e-mail: just say no to "No"

    *
    "Trev" <> wrote in message
    news:...
    >
    > "didi" <> wrote in message
    > news:...
    > > Hello,
    > >
    > > is the resolution in dpi equal to the resolution in ppi?
    > >
    > >

    > No. Resolution is measured in pixels per inch. the printer converts them
    > into drops of ink at a variable rate up to the set, drops per inch ink
    > setting.
    >
    >
     
    mark herring, Dec 10, 2003
    #4
  5. didi

    Jim Townsend Guest

    didi wrote:

    > Hello,
    >
    > is the resolution in dpi equal to the resolution in ppi?
    >
    > Thanks
    >
    > Titi



    A pixel is a dot unless you're talking about printers in which case it's
    referring to ink drops which are completely different. Image scanners are
    measured in dots too and those dots are the same as pixels.

    So pixels are dots unless you're talking about printers.. Just to add to the
    confusion, cameras are measured in megapixels rather than megadots.

    It's impossible to set the DPI of an image in Adobe Photoshop because P.S. only
    allows you to set the PPI.. But when discussing this, it's OK to for tell
    people that you set the DPI since they are the same.. That is unless you're
    talking about the resolution of your printer....

    No wonder so many people get confused :)
     
    Jim Townsend, Dec 10, 2003
    #5
  6. didi

    imbsysop Guest

    "Jim Townsend" <> wrote in message
    news:...
    > didi wrote:
    >
    > > Hello,
    > >
    > > is the resolution in dpi equal to the resolution in ppi?
    > >
    > > Thanks
    > >
    > > Titi

    >
    >
    > A pixel is a dot ....


    just for nit-picking .. if I correctly remember the definition a "pixel" has
    no dimension .. :)
     
    imbsysop, Dec 10, 2003
    #6
  7. Jim Townsend <> wrote in
    news::

    > A pixel is a dot unless you're talking about printers in which case it's
    > referring to ink drops which are completely different. Image scanners
    > are measured in dots too and those dots are the same as pixels.
    >
    > So pixels are dots unless you're talking about printers.. Just to add
    > to the confusion, cameras are measured in megapixels rather than
    > megadots.
    >
    > It's impossible to set the DPI of an image in Adobe Photoshop because
    > P.S. only allows you to set the PPI.. But when discussing this, it's OK
    > to for tell people that you set the DPI since they are the same.. That
    > is unless you're talking about the resolution of your printer....
    >
    > No wonder so many people get confused :)


    Correct! And you are confused to :)

    A dot is not a pixel. A dot has dimension, a pixel not.
    You never talks about Megadot pictures for that reason.

    The reason why you cannot set DPI in Photoshop is that DPI
    has no meaning for digital pictures. DPI is the printer's
    ink drop resolution, which is much higher than the printer's
    actual resolution.

    Then - sometimes DPI is mixed with PPI, e.g. in scanners.
    The resolution of a scanner (meassured in DPI) also is the
    maximum PPI for that scanner.


    Roland
     
    Roland Karlsson, Dec 10, 2003
    #7
  8. didi

    Jim Townsend Guest

    Roland Karlsson wrote:

    > Jim Townsend <> wrote in
    > news::
    >
    >> A pixel is a dot unless you're talking about printers in which case it's
    >> referring to ink drops which are completely different. Image scanners
    >> are measured in dots too and those dots are the same as pixels.
    >>
    >> So pixels are dots unless you're talking about printers.. Just to add
    >> to the confusion, cameras are measured in megapixels rather than
    >> megadots.
    >>
    >> It's impossible to set the DPI of an image in Adobe Photoshop because
    >> P.S. only allows you to set the PPI.. But when discussing this, it's OK
    >> to for tell people that you set the DPI since they are the same.. That
    >> is unless you're talking about the resolution of your printer....
    >>
    >> No wonder so many people get confused :)

    >
    > Correct! And you are confused to :)


    No I'm not.. :)

    I was *trying* to make it sound as complicated as I could to show the folly of
    accepting dots and pixels as the same thing.

    My tongue was placed firmly in the side of my mouth as I typed :)

    Here's a better answer: Dots vs pixels.


    For an image scanner, a dot is the smallest area of a document that it can
    resolve. The information from this 'dot' is converted to an image pixel.
    (This is where the dot/pixel connection gets made).

    Scanning a 1 inch x 1 inch document at 1200 dots per inch yields an image
    that's 1200 x 1200 pixels.

    Printer dots are of course made of ink (or toner or whatever). They lay down
    so many dots per inch, but these dots have nothing to do with pixels. The
    area the ink covers is fixed. Pixels on the other hand can vary in the area
    they occupy on the paper.

    If you print an image at 300 Pixels per inch, then 300 pixels get spread over
    an inch of paper. That would mean the information from each pixel will take
    up 1/300 of an inch of paper.

    If you pint an image at 72 Pixels per inch, then 72 pixels spread over an inch
    of paper would make each pixel fill up 1/72 inch of paper.

    Note that whether the pixel takes up 1/300 of an inch or 1/72 of an inch, the
    printer still prints the *same amount of dots*. It will take less ink dots to
    fill up a 1/300 inch area. It will take more dots to fill a 1/72 inch area..

    Finally.. Dots and pixels are often used interchangeably. It's important to
    know the context.. This will avoid mixing printer dots with image pixels.
     
    Jim Townsend, Dec 11, 2003
    #8
  9. didi

    imbsysop Guest

    On Wed, 10 Dec 2003 19:00:33 -0600, Jim Townsend <>
    wrote:

    >Roland Karlsson wrote:
    >
    >> Jim Townsend <> wrote in
    >> news::
    >>
    >>> A pixel is a dot unless you're talking about printers in which case it's
    >>> referring to ink drops which are completely different. Image scanners
    >>> are measured in dots too and those dots are the same as pixels.
    >>>
    >>> So pixels are dots unless you're talking about printers.. Just to add
    >>> to the confusion, cameras are measured in megapixels rather than
    >>> megadots.
    >>>
    >>> It's impossible to set the DPI of an image in Adobe Photoshop because
    >>> P.S. only allows you to set the PPI.. But when discussing this, it's OK
    >>> to for tell people that you set the DPI since they are the same.. That
    >>> is unless you're talking about the resolution of your printer....
    >>>
    >>> No wonder so many people get confused :)

    >>
    >> Correct! And you are confused to :)

    >
    >No I'm not.. :)
    >
    >I was *trying* to make it sound as complicated as I could to show the folly of
    >accepting dots and pixels as the same thing.
    >
    >My tongue was placed firmly in the side of my mouth as I typed :)
    >
    >Here's a better answer: Dots vs pixels.
    >
    >
    >For an image scanner, a dot is the smallest area of a document that it can
    >resolve. The information from this 'dot' is converted to an image pixel.
    >(This is where the dot/pixel connection gets made).
    >
    >Scanning a 1 inch x 1 inch document at 1200 dots per inch yields an image
    >that's 1200 x 1200 pixels.


    ... :) it's the wrong concept and the wrong example .. :)

    by using the scanner example you are boiling down the pixel definition
    to a narrow concept of a number light/color detections per length unit
    which in turn define a "size" forthe "dot/pixel" .. which is just the
    mechanical way scanners work .. by the same concept pixel "size" would
    is related to the "scanning resolution" which is only true for a
    scanner ..
    FWIW
     
    imbsysop, Dec 11, 2003
    #9
  10. didi

    Jim Townsend Guest

    imbsysop wrote:

    > On Wed, 10 Dec 2003 19:00:33 -0600, Jim Townsend <>
    > wrote:


    >>For an image scanner, a dot is the smallest area of a document that it can
    >>resolve. The information from this 'dot' is converted to an image pixel.
    >>(This is where the dot/pixel connection gets made).
    >>
    >>Scanning a 1 inch x 1 inch document at 1200 dots per inch yields an image
    >>that's 1200 x 1200 pixels.

    >
    > .. :) it's the wrong concept and the wrong example .. :)
    >
    > by using the scanner example you are boiling down the pixel definition
    > to a narrow concept of a number light/color detections per length unit
    > which in turn define a "size" forthe "dot/pixel" .. which is just the
    > mechanical way scanners work .. by the same concept pixel "size" would
    > is related to the "scanning resolution" which is only true for a
    > scanner ..


    I see what you're saying, but I didn't mean it that way :)

    I used the scanner dot/pixel example to show where I believe this inextricable
    link between dots and pixels came from. (After all, that was the original
    poster's question).

    Scanners have been around for a lot longer than digital cameras. Using DPI to
    describe scanned images has always been commonplace. With the advent of
    digital cameras, the DPI term stuck.

    When obtaining a pixel, the information *must* be sampled from a finite area.
    Of course once the actual pixel is created, it has no size.

    I didn't mean to impart the idea that pixels have size.
     
    Jim Townsend, Dec 11, 2003
    #10
  11. didi

    Don Stauffer Guest

    Depends on context. For printers, NO. Normal inkjet printers cannot
    print dots of various density, so cannot print a fulltone image with a
    single dot per pixel. They must make a pseudo halftone pattern, which
    takes many dots to print each pixel.

    For a scanner, yes. However, some of us prefer the term Samples per
    Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    ppi for image pixels per inch.

    BTW, not that the image in the camera itself has no physical dimensions,
    so ppi is irrevalent when speaking of the camera.

    didi wrote:
    >
    > Hello,
    >
    > is the resolution in dpi equal to the resolution in ppi?
    >
    > Thanks
    >
    > Titi


    --
    Don Stauffer in Minnesota

    webpage- http://www.usfamily.net/web/stauffer
     
    Don Stauffer, Dec 11, 2003
    #11
  12. didi

    imbsysop Guest

    On Thu, 11 Dec 2003 08:34:32 -0600, Jim Townsend <>
    wrote:

    >imbsysop wrote:
    >
    >> On Wed, 10 Dec 2003 19:00:33 -0600, Jim Townsend <>
    >> wrote:

    >
    >>>For an image scanner, a dot is the smallest area of a document that it can
    >>>resolve. The information from this 'dot' is converted to an image pixel.
    >>>(This is where the dot/pixel connection gets made).
    >>>
    >>>Scanning a 1 inch x 1 inch document at 1200 dots per inch yields an image
    >>>that's 1200 x 1200 pixels.

    >>
    >> .. :) it's the wrong concept and the wrong example .. :)
    >>
    >> by using the scanner example you are boiling down the pixel definition
    >> to a narrow concept of a number light/color detections per length unit
    >> which in turn define a "size" forthe "dot/pixel" .. which is just the
    >> mechanical way scanners work .. by the same concept pixel "size" would
    >> is related to the "scanning resolution" which is only true for a
    >> scanner ..

    >
    >I see what you're saying, but I didn't mean it that way :)


    I know :) but is kind of a tricky concept to explain .. while the
    only examples we can think of imply pixels must have a "size" :) ..
    and especially as people just mixup the PPI/DPI concept and then get
    caught in the idea "how many dpi do I need to ..." :)
     
    imbsysop, Dec 11, 2003
    #12
  13. didi

    Matt Guest

    On Thu, 11 Dec 2003 08:35:08 -0600, Don Stauffer
    <> wrote:

    >Depends on context. For printers, NO. Normal inkjet printers cannot
    >print dots of various density, so cannot print a fulltone image with a
    >single dot per pixel. They must make a pseudo halftone pattern, which
    >takes many dots to print each pixel.
    >

    But it gets even more interesting when you consider that (for
    dye-based printers) a single ink dot can be overlayed multiple times
    to create various densities and hues, taking advantage of the
    transparency of dyes (as opposed to pigments). In fact, this is one
    reason why unsatisfactory results can occur when attempting to change
    between dye and pigment-based inks for the same printer, since
    printheads & drivers are specifically designed for each in many cases.


    >For a scanner, yes. However, some of us prefer the term Samples per
    >Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    >ppi for image pixels per inch.
    >
    >BTW, not that the image in the camera itself has no physical dimensions,
    >so ppi is irrevalent when speaking of the camera.
    >
    >didi wrote:
    >>
    >> Hello,
    >>
    >> is the resolution in dpi equal to the resolution in ppi?
    >>
    >> Thanks
    >>
    >> Titi
     
    Matt, Dec 11, 2003
    #13
  14. didi

    Jim Townsend Guest

    Don Stauffer wrote:


    > For a scanner, yes. However, some of us prefer the term Samples per
    > Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    > ppi for image pixels per inch.


    This would be ideal.. It certainly would eliminate a lot of confusion.
     
    Jim Townsend, Dec 11, 2003
    #14
  15. didi

    Ray Murphy Guest

    ----------
    In article <>, Jim Townsend
    <> wrote:


    >Don Stauffer wrote:
    >
    >
    >> For a scanner, yes. However, some of us prefer the term Samples per
    >> Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    >> ppi for image pixels per inch.

    >
    >This would be ideal.. It certainly would eliminate a lot of confusion.


    RM: It's interesting that while we in Australia have been using metric
    measures for a few decades, we prefer to use ppi and dpi rather than
    p/mm or d/mm. It's just as well too :))

    Ray
     
    Ray Murphy, Dec 11, 2003
    #15
  16. didi

    Canopus Guest

    "Ray Murphy" <> wrote in message
    news:...
    >
    > ----------
    > In article <>, Jim Townsend
    > <> wrote:
    >
    >
    > >Don Stauffer wrote:
    > >
    > >
    > >> For a scanner, yes. However, some of us prefer the term Samples per
    > >> Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    > >> ppi for image pixels per inch.

    > >
    > >This would be ideal.. It certainly would eliminate a lot of confusion.

    >
    > RM: It's interesting that while we in Australia have been using metric
    > measures for a few decades, we prefer to use ppi and dpi rather than
    > p/mm or d/mm. It's just as well too :))
    >
    > Ray


    LOL...they are international standard speak. Interestingly, ppi are not
    really pixels per inch, it was developed in the early days of computing when
    computer screens of (I think it was Macs) were a standard size and the size
    you saw an image on the screen was the size it printed out in dpi. From
    what I remember it was 75ppi which was also how print on the screen was
    measured. When computer screens started coming out in different sizes then
    ppi became somewhat meaningless, but, still handy. Really though it should
    be regarded as pixels per virtual inch.

    Rob
     
    Canopus, Dec 11, 2003
    #16
  17. didi

    didi Guest

    Thank you all for the answers. In my case I was talking about just a
    picture resolution for a scanner. I scanned a pic with 300 dpi setting
    and when I look at it in photoshop it says 96 ppi. I was wondering if
    it was screwed up.

    How can I know exactly a picture resolution and how can I increase it
    from 96 to 150??

    Thank you


    Don Stauffer <> wrote in message news:<>...
    > Depends on context. For printers, NO. Normal inkjet printers cannot
    > print dots of various density, so cannot print a fulltone image with a
    > single dot per pixel. They must make a pseudo halftone pattern, which
    > takes many dots to print each pixel.
    >
    > For a scanner, yes. However, some of us prefer the term Samples per
    > Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    > ppi for image pixels per inch.
    >
    > BTW, not that the image in the camera itself has no physical dimensions,
    > so ppi is irrevalent when speaking of the camera.
    >
    > didi wrote:
    > >
    > > Hello,
    > >
    > > is the resolution in dpi equal to the resolution in ppi?
    > >
    > > Thanks
    > >
    > > Titi
     
    didi, Dec 11, 2003
    #17
  18. didi

    Ray Murphy Guest

    ----------
    In article <brajjd$19jku$-berlin.de>, "Canopus"
    <> wrote:


    >
    >"Ray Murphy" <> wrote in message
    >news:...
    >>
    >> ----------
    >> In article <>, Jim Townsend
    >> <> wrote:
    >>
    >>
    >> >Don Stauffer wrote:
    >> >
    >> >
    >> >> For a scanner, yes. However, some of us prefer the term Samples per
    >> >> Inch, spi, to avoid any confusion, using dpi strictly for printers, and
    >> >> ppi for image pixels per inch.
    >> >
    >> >This would be ideal.. It certainly would eliminate a lot of confusion.

    >>
    >> RM: It's interesting that while we in Australia have been using metric
    >> measures for a few decades, we prefer to use ppi and dpi rather than
    >> p/mm or d/mm. It's just as well too :))
    >>
    >> Ray

    >
    >LOL...they are international standard speak.


    RM: They (PPI, DPI + SPI) are not international speak in Europe,
    although I'm not sure if they constitute the majority of countries-
    although it's pixels/cm (not pixels/mm as I mentioned above).
    Printing screeens in much of Europe have been Lines/cm for many years
    as well - probably ever since they or the metric or system was
    invented.

    >Interestingly, ppi are not
    >really pixels per inch, it was developed in the early days of computing when
    >computer screens of (I think it was Macs) were a standard size and the size
    >you saw an image on the screen was the size it printed out in dpi. From
    >what I remember it was 75ppi which was also how print on the screen was
    >measured. When computer screens started coming out in different sizes then
    >ppi became somewhat meaningless, but, still handy. Really though it should
    >be regarded as pixels per virtual inch.


    RM: I think just about everyone refers to the squares in a
    "Photoshop-style" image as "pixels" (for picture cells), so we're
    pretty much stuck with the term no matter where it came from, so if we
    want to make say a checkerboard pattern 1 inch square (not a virtual
    inch square) with 10 pixels on each line we would say 10 ppi wouldn't
    we, rather than 10dpi?

    >
    >Rob


    Ray
     
    Ray Murphy, Dec 11, 2003
    #18
  19. didi

    Ray Murphy Guest

    ----------
    In article <>,
    (didi) wrote:


    >Thank you all for the answers. In my case I was talking about just a
    >picture resolution for a scanner. I scanned a pic with 300 dpi setting
    >and when I look at it in photoshop it says 96 ppi. I was wondering if
    >it was screwed up.
    >
    >How can I know exactly a picture resolution and how can I increase it
    >from 96 to 150??
    >
    >Thank you
    >


    RM: Is it possible that your picture is being *displayed* at 96ppi but
    is really 3 times bigger in size?
    If this is the case you would need to change the resolution in your
    graphics program to 300 ppi and then the size would be reduced back to
    normal.

    If it's not this, it is highly unlikely that you have scanned it at
    300.

    Ray
     
    Ray Murphy, Dec 11, 2003
    #19
  20. didi

    Ray Murphy Guest

    ----------
    In article <>,
    (didi) wrote:


    >Thank you all for the answers. In my case I was talking about just a
    >picture resolution for a scanner. I scanned a pic with 300 dpi setting
    >and when I look at it in photoshop it says 96 ppi. I was wondering if
    >it was screwed up.
    >
    >How can I know exactly a picture resolution and how can I increase it
    >from 96 to 150??


    RM: In Photoshop you go to the menu at the top "Image" and then down
    to "Image size". You will then see the specifications of your job. If
    you want to change the number of pixels per inch to something else,
    just place your cursor in the appropriate box and delete the existing
    number and type in the new number BUT be careful because if you want
    to retain all your pixels you must click on the appropriate box to
    retain "file size".
    If you have done it correctly the file size will remain he same, but
    otherwise it will change.
    Once you have done this - play around with the various sizes and
    number of pixels to see what's going on.
    It would be best to do this with a COPY of your original image so you
    cannot get into trouble.

    Ray
     
    Ray Murphy, Dec 11, 2003
    #20
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