Talk:Dots per inch: Difference between revisions

Since inches are now only an American unit* the spellings on this page should be in American English. Similarly, a page on the British monarch or Australian government would use British spellings, and a neutral article (i.e., one on molecular biology) could use either or both spellings. SteveSims 04:39, 5 January 2007 (UTC)[reply]

*Excluding a few non-English speaking countries.

I asure you, the inch is still widely used in the UK. A page on the BRITISH monarch or AUSTRALIAN government is a different matter, because its actualy about a particular country, Dots per inch is universal. -OOPSIE- 14:07, 26 August 2007 (UTC)[reply]

I hadn't noticed before that a separate article existed for DPI until Bobblewik helpfully pointed it out. I have arrogantly decided to simply redirect DPI to this article, wiping out the previous contents of that article. Here is why:

• In a general sense, there was no information in that article that is not already in this one.
• Several factoids from that article are either vague or misleading, and are more adequately explained in this article or in related articles Pixels per inch and samples per inch. Specifically:
  • DPI always refers to a physical representation of pixels per length unit. - Not true. DPI most correctly refers to printing resolution - dabs of ink on paper. The term is often used to specify what would more accurately be called pixels per inch; this distinction is explained in the current article.
  • Only when outputting this image to a physical medium with a certain size (say printing on to paper 20cm by 15cm) does the DPI get defined. - This is confusing, subtly wrong, and partially contradictory to the previously-mentioned sentence that DPI always refers to pixels.
  • The resulting DPI depend both on the resolution of the image... -- Not true. Even if DPI is used broadly, encompassing printer and monitor output, it still refers to a physical characteristic of an output device. The resolution of a particular image displayed on that output device has no bearing on that device's DPI capability. And DPI doesn't really make any sense in describing an image with such-and-such number of pixels; the "DPI" of an image only makes sense when given some number of inches. You can print a 5x5-pixel image on a 1200dpi printer; the output is 1200dpi. Printing a 1000x1000-pixel image does not change that.

Of course, the history is preserved, if anyone wants to extract any useful nuggets of information from it. -- Wapcaplet 20:57, 12 Aug 2004 (UTC)

From an editor: Draw a 1-inch black line on a sheet of paper and scan it. If the resulting image shows a black line with a width of, say, 300 pixels, then does the scanner not capture at 300 PPI? 128.83.144.239

I moved the above anon comment from the article to here. It was posted by way of explaining the setence "A digital image captured by a scanner or digital camera has no inherent "DPI" resolution until it comes time to print the image..." I'm not sure how it helps to explain this point, however; indeed, it seems to stem from the very misunderstanding of DPI that is being explained (that DPI and PPI are not the same thing, and SPI is yet another thing entirely). -- Wapcaplet 23:42, 23 Sep 2004 (UTC)

DPI is mostly used to to tell how big a resolution an image should be printed at. Of course, this should be really be "pixels per inch"! But since it has become a standard, I think it should be explained here.--Kasper Hviid 18:33, 9 Nov 2004 (UTC)

• I am not sure what you mean; the details of what it means to print an image at a certain DPI are, I think, fairly well-explained in the article at present. DPI is all about printing; there is a separate article on the related but different term pixels per inch. In printing an image, three things influence the output quality: DPI (the physical capability of the printer), the number of pixels in the image, and the space in which it is to be printed. As far as I know, there is no term to describe "the number of pixels printed in a one-inch space on the paper," though pixels per inch is probably the most appropriate. The resolution of an image sent to the printer (that is, the number of pixels) is unrelated to DPI of the printer. Maybe this should be explained better in the article? -- Wapcaplet 00:12, 10 Nov 2004 (UTC)

• I has always throught of DPI as "How many pixels should be printed per inch"? This is a wrong, but common use of the word. As you said, there is no term to refer to "the number of pixels printed in a one-inch space on the paper", but this is probably why the word dpi has been used instead. For instance, at www.lexmark.com they tell that "resolution is measured in dpi (dots per inch) which is the number of pixels a device can fit into an inch of space." And at www.olympus-europa.com, they tell that a 640 x 480 pixels image at 150 dpi will end up as 10.84 x 8.13 cm in the print. Since this has become a commonly used standard, it deserves to be accepted in the article as a official use of the word dpi, along with a note that this really is wrong use of the word. The "pixels per inch" article dont tell anything about pixels per inch in print, but only about the screens resolution, something I have never understood the point of. Kasper Hviid 09:54, 10 Nov 2004 (UTC)

I wasn't able to find the references you gave on lexmark.com or olympus-europa.com, but it doesn't surprise me that DPI would be used in this broad way in documentation intended for the general consumer. I've seen scanner software that uses DPI to mean samples per inch. "Dots" is a fairly general term for most people; pixels, color samples, and ink spots could all be "dots" to most people. I don't think there's any call for distinguishing an "official" use of the word. It's official if people use it that way, and (I suppose) if it's defined that way in the dictionary, which it is. It's really only the more technical among us who care to differentiate DPI from PPI and SPI.

As for the purpose of describing screen resolution, I suppose it's probably useful in calibrating a computer display to a printing device. If a print shop needs to have things displayed on their computer monitor at the same size they will be printed, monitor PPI is useful. I thought about adding to the pixels per inch article to include the idea of pixel density on paper, but while it makes sense to me, I know of no other instances of PPI being used in that way. If you find such a usage, let me know! -- Wapcaplet 22:59, 10 Nov 2004 (UTC)

Since pixels are actually dots (PDF), it comes out that PPI and DPI are actually the same thing. --88.153.32.35 12:50, 24 June 2006 (UTC)[reply]

---

Yes, since dpi and ppi are obviously used interchangeably, this interchangeability should be explained here... so that novices looking here for definitions can understand what they find to read in the real world. It seems extremely arrogant to say "Wrong" and "misuse" when obviously pixels per inch has always been called dpi. And still is.

Do some few say ppi? Yes. Do the vast majority say dpi. Absolutely yes.

1. All scanner ratings are specified as dpi, obviously meaning pixels per inch. They dont say "samples per inch", they all say dpi, which we all know means pixels per inch. Scanners create pixels, not ink dots. Who are you to call every scanner manufacturer wrong?

2. All continuous tone printers (dye subs, Fuji Frontier class, etc) print pixels, and call their ratings dpi too (colored dots also called pixels). Who are you to call all these manufacturers wrong?

3. The most current JPG image file format specification claims to store image resolution in "dots per inch". The most current TIF file format specification claims to store image resolution in "dots per inch" They are referring to pixels... there are no ink dots in image files. Who are you to call these authors of the most common file format specifications wrong?

http://www.w3.org/Graphics/JPEG/jfif3.pdf (page 5)

http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf (page 38)

4. Google searches on 7/14/06 for

"72 dpi" 17,200,000 links

"72 ppi" 124,000 links

(138 times greater use of dpi... a couple of magnitudes more useage)

You may be aware that 72 dpi topics are never about printer ink dots.

When calling everyone else wrong, a wise man would reevaluate his own position. The Wikipedia author who claims misuse of dpi is obviously dead wrong. It is probably only his wishful thinking that the world OUGHT to be as he wishes it to be, but it is just his imagination, and this Wiki definition is definitely WRONG.

The two terms are obviously interchangeable. Wake up, look around, where have you been? Pixels per inch has ALWAYS been dpi. Yes, dpi does also have another use. So what? Almost every English word has multiple meanings and uses. However which term is best is not important here - this is certainly not the place to decree it (as attempted). Both terms are obviously used with the same meaning (pixels per inch) and that matter is long settled. Say it yourself whichever way you prefer to say it, but we obviously must understand it both ways. Because we see it everywhere both ways. So this both-ways phenomena needs to be explained in the definitions here. Without bias. About how the real world really is, not about how some author might dream it ought to be.

WHAT IS IMPORTANT is that beginners need to know the two terms are used interchangeably everywhere, with both terms meaning pixels per inch, simply so they can understand most of what they will find to read about the subject of imaging. There is no reason to confuse them even more by telling them everything they read is wrong. Wiki is wrong. The Wiki definition can only totally confuse them.

Beginners do need to know the two concept differences (your two definitions), but once the concepts are known, then the terms are almost arbitrary.. We could call them "thingies per inch". The context determines what it means (like all English words), and if the context is about images, dpi can only mean pixels per inch (ppi can mean that too). If the context is about printer ratings, then dpi can only mean ink dots per inch. 71.240.166.27 03:20, 14 July 2006 (UTC)[reply]

DPI is the CORRECT term for the target resolution at which an image is to be printed or displayed. It is a value stored in a digital file which indicates the current target printing resolution of that file. To use it otherwise is to sow confusion out of some misguided ideology. —Preceding unsigned comment added by 24.128.156.64 (talk) 18:19, 10 October 2007 (UTC)[reply]

I see several printers advertised with "4800 x 1200 color dpi" and such. Is this some kind of industry conspiracy to redefine the term "dpi"? Or am I misunderstanding something? Example: [1] -- Anon

• Nope, sounds like the most appropriate and correct possible usage of DPI to me, assuming those figures are what the printer is actually capable of. Now, if a scanner is advertised with some DPI, then Samples per inch is what is actually meant. Many times a scanner is advertised with its interpolated sampling resolution, since that number is often much higher than the actual optical resolution; a good consumer scanner may only be able to capture 1600 samples per inch, but the samples are often scaled (either in the hardware or in the scanning software) to much higher resolution, such as 19,200, and of course "19,200 DPI" looks better in an advertisement than "1600 DPI." Whether printer manufacturers use a similar strategy, I don't know; I do know that the reason the two DPI figures are often different is that one of them is a horizontal resolution, determined by how finely the printing heads can be controlled, while the other is a vertical resolution, determined by how finely the paper feed roller can be controlled. Finally, if you see a digital camera advertised with some DPI, buy a different brand, since DPI has no meaning in that context unless they are referring to the quality that might be achieved in printing a digital photo at a certain size (and then, pixels per inch is probably more appropriate). -- Wapcaplet 18:11, 28 Nov 2004 (UTC)

Yes, it's a mixture of morons being stupid and marketing people trying to pull the wool over peoples' eyes. They're conflating dots per inch with dots themselves, image size (48x12 kpixels across) with spatial image resolution (note that the "DPI" in the prior figure does NOT contain actual inches or any other real-world spatial measurement, dead givaway of moronity.) I'll put this back in the article. 76.126.134.152 (talk) 11:47, 2 June 2008 (UTC)[reply]

I am concerned about the following statement:

This is due to the limited range of colors typically available on a printer: most color printers use only four colors of ink, while a video monitor can often produce several million colors. Each dot on a printer can be one of only four colors, while each pixel on a video monitor can be one of several million colors; printers must produce additional colors through a halftone or dithering process.

Computer displays work in a similar fashion to printers: they use a combination of different amounts of the primary colors (in this case, the additive primaries: red, green, and blue) to produce a wide range of visible colors. Most printers use the (subtractive) primaries and black in different combinations and patterns.—Kbolino 02:18, 10 February 2006 (UTC)[reply]

No they don't. Video displays can actually produce darker or lighter versions of the same color in each of their subpixels by altering the amount of light produced. Printers, on the other hand, can not blend their ink with the color of the paper (I.E.: white) to produce darker or lighter shades/tones due to the ink's nontransparency. Instead, they must place smaller blobs of a given color of ink in order to (on white paper) make a less saturated tone or bigger blobs to make a more saturated shade. Admittedly, some printers actually CAN change the color of their ink by mixing all 4/5/6/8 colors together into one big blob, like the solid ink printers I've drooled over for years. 76.126.134.152 (talk) 11:47, 2 June 2008 (UTC)[reply]

"The DPI measurement of a printer is dependent upon several factors, including the method by which ink is applied, the quality of the printer components, and the quality of the ink and paper used."

This is not true, or at least is confusing.

The DPI in the printing direction is dependent on the head firing frequency and the linear print speed. The DPI in the advance direction (perpendicular to the printing direction) is dependent on the spacing of actuators (e.g. nozzles for inkjet) on a head, and the angle of the heads. Each of these can be multiplied by use of interleaving/"weaving" using multiple passes and/or multiple heads.

What the sentence above may have been trying to get at is that different print modes can use different firing frequencies, linear speeds, interleaving factors, etc., and the effect of ink and media settings in print drivers is often to change the print mode (possibly in addition to other software settings that don't affect DPI). Also, different print head technologies may improve at different rates in terms of firing frequency, actuator spacing, etc.

On the subject of advertisements, I strongly suspect that some of the dpi figures quoted in printer adverts are inflated. You can inflate a dpi figure by:

• counting different colors as more than one dot (this may be what "4800 x 1200 color dpi" means -- I expect it is really 1200 x 1200 in 4 colors)
• counting each dot printed by a variable-dot head as more than one dot
• saying "equivalent dpi" and making up a random number.

This kind of creative arithmetic is all the result of trying to munge various resolution and quality factors into a single number for marketing purposes. It's similar to how clock speed used to be used to indicate how "fast" a processor was. At its worst, it can lead to distorted technical decisions that maximize DPI with no improvement in, or even at the expense of quality (just as the Pentium 4 design was distorted to maximize clock speed).

It's unlikely that someone could see a visible improvement in resolution above about 1000 dpi with the unaided eye at a normal reading distance. The extra quality that you can get from a higher dpi than that is not due to an increase in resolution; it's due to a reduction in "graininess" (and possibly better hiding of head defects) from using smaller drop volumes, which requires you to use more dots in a given space to achieve the same ink density.

To meaningfully compare printers, you need at the very least to know the volume of ink in a drop (for inkjet heads as of 2006, this can vary from about 1 to 80 picolitres), including what "subdrop" volumes are possible in the case of variable-dot heads, as well as the real dpi figure in each direction. The overall quality will also depend on halftoning algorithms, the gamut of the inks used, color management, positioning accuracy of the printer mechanism and any encoders, head defects and how well the print mode hides them, etc. The intended application is also significant: to give an extreme example, there's no point in achieving "photographic" resolution in a printer that will be used to print billboards -- although color gamut would still be very important for the latter.

DavidHopwood 00:16, 5 June 2006 (UTC) (working for, but not speaking for, a printer manufacturer)[reply]

"There are some ongoing efforts to abandon the dpi in favor of the dot size given in micrometres (µm). This is however hindered by leading companies located in the USA, one of the few remaining countries to not use the metric system exclusively."

I wouldn't blame US companies for this, even though I'm an enthusiastic S.I. advocate. Software interfaces to RIP packages and driver APIs require dpi, and there's no compelling reason to change them. Despite this, it is possible for a printer controller implementation to be internally almost S.I.-only. DavidHopwood 01:20, 5 June 2006 (UTC)[reply]

This is pretty trivial, but should the case (DPI or dpi) be standardized in this article? The last section uses dpi while the others use DPI. --MatthewBChambers 09:13, 2 October 2007 (UTC)[reply]

It should use DPI. It's an abbreviation. I've fixed it. --jacobolus (t) 10:55, 2 October 2007 (UTC)[reply]

Both of the external links are low quality links to pages by writers with an ideological axe to grind and a limitied understanding of the topic. —Preceding unsigned comment added by 24.128.156.64 (talk • contribs)

So be WP:BOLD, add some better sources! --jacobolus (t) 04:29, 13 October 2007 (UTC)[reply]

I disagree, the "Myth" link was the first place where I have understood why and how to reset dpi without losing quality. RomaC (talk) 14:42, 31 March 2008 (UTC)[reply]

This sentence "Therefore it is meaningless to say that a digitally stored image has a resolution of 72 DPI." is just simply, clearly unequivocally false. It is also misleading in a way that exacerbates existing confusion among users. —Preceding unsigned comment added by 24.128.156.64 (talk) 16:44, 12 October 2007 (UTC)[reply]

Hmm, there seems to be some disagreement here about the origin of the term DPI. The position of the article is that it has its origins in printers, while 24.128.156.64 says that it had its origins in digital file formats. Does anyone have a reference to support either position? Personally, I think it's the printers. Rocketmagnet 17:10, 12 October 2007 (UTC)[reply]

I don't think it is only an issue of origin. It is, most importantly, an issue an issue of use. Users of graphics software get confused on this issue as it is. To deny the fact that all professional graphics software and most amateur graphics software allows the editing of a value called DPI which gets stored with the file adds to that confusion. Here is an example of a page using DPI correctly: http://msdn2.microsoft.com/en-us/library/ms838191.aspx—Preceding unsigned comment added by 24.128.156.64 (talk) 17:23, 12 October 2007 (UTC)[reply]

I don't think anyone was denying that DPI can be stored in a digital image. And you're right, people do get confused about it all the time. People often ring me up wanting a picture, and they say "I want a picture, and I need it at 300 dpi" And I say, "Well, it depends how big you're going to print it." And they say "I don't know, just give it to me at 300 dpi". Rocketmagnet 17:59, 12 October 2007 (UTC)[reply]

Which is precisely the problem. People don't seem to realise that a digital image doesn't fundamentally have DPI, in the same way that it fundamentally has a resolution. The DPI is a tag that's added on by some software, that can be used or ignored as the user sees fit. Rocketmagnet 17:59, 12 October 2007 (UTC)[reply]

I'm having trouble reconciling "Therefore it is meaningless to say that a digitally stored image has a resolution of 72 DPI." with "I don't think anyone was denying that DPI can be stored in a digital image."

On the other hand it is now clear that we're both motivated by trying to correct the same confusion among our clients (and other people in similar positions) and disagreeing about how to do that. —Preceding unsigned comment added by 24.128.156.64 (talk) 18:43, 12 October 2007 (UTC)[reply]

I'm sure we agree that a digital image fundamentally has a resolution, since it is literally made out of pixels. It is impossible to change the resolution without fundamentally changing the content of the image. Now, a digital image might also have a filename. But the filename is not fundamental to the image. I can rename the file to be whatever I want, and it does nothing whatsoever to the content of the image. It could even have no filename if I haven't saved it yet. So I would say that digital images don't fundamentally have a filename. I would say the same thing of the DPI value in the image. The image might have a DPI value stored in it, but digital images in general do not fundamentally have DPI. I could change the DPI value to be whatever I want, and it will do nothing to the content of the image.

Another example: I could e-mail a photo of a cat to a local print shop. Then call them up and say "I know the image says it's 300 dpi, but I want it printed at a width of 2 feet". That makes sense, because the DPI is not fundamental to the image. The guy at the print shop probably wouldn't even bother editing the dpi value in the image, he would set the printer to print it to the size I want. Compare it with this example: I e-mail a photo of a cat to a local print shop, then call them up and say "I know it's a photo of a cat, but please print me a photo of a house". That would be insane.

Yet another example: I add a tag to my digital image which says "50 gsm" (gsm = grams per square meter). The idea is that, when the image is printed I want it printed on that weight of paper. But is it meaningful to say that this digital image has a weight of 50 grams per square meter? No. That's nonsense. Digital images do not have weight (even if there is a GSM tag in the image). In the same way, they do not really have DPI (even if there is a DPI tag in the image). Rocketmagnet 20:23, 12 October 2007 (UTC)[reply]

Er... any Wikipedia entry which said "Therefore it is meaningless to say that computer files have names." would get corrected.

Please do not misquote me. I did not say that computer files don't have names. I said that digital images do not fundamentally have filenames. A filename is not a fundamental part of what it means to be a digital image. For example, when I play Quake, I am seeing tens of digital images per second. Not a single one of those has a filename. Nor do any of them have a DPI.

The DPI in the file DOES effect the printed output unless it is changed or overridden. It is like the icc color profile in that regard. Would you say that it is meaningless to say that graphic image files in many formats can have color profiles? —Preceding unsigned comment added by 24.128.156.64 (talk) 21:58, 12 October 2007 (UTC)[reply]

Oh god. I think we're talking at cross purposes. Do you understand the difference between "some humans wear glasses" with "glasses are not a fundamental part of what it means to be human"? Glasses can be added to humans, and it may benefit them, but there are many humans without them, and they are still human. Likewise, a DPI value can be added to an image, and it may help people when printing the image, but many images have no DPI value added to them, and it makes them no less an image.

I think you are confusing "a digital image" with "a file on a disk which contains a digital image", which are two different things. A digital image is a much broader concept. Rocketmagnet 14:36, 13 October 2007 (UTC)[reply]

The DPI value is a part of a digital image in the same way that the color profile is; in the same way that vector data or text can be added to a bitmap in a photoshop image and become part of the digital image. This is the same way in which a file name is part of a computer file. I may have been a bit confusing in that a file name is not a part of a digital image in quite that way. The example of a file name being part of a file was a metaphor and may not have made my point clearer.

A digital image is composed of more than just a bitmap, it can include color profiles, DPI, vector data, text, positional offsets, filters, information specifying display of a variety of devices, compression specifications, and other elements.

To use your metaphor, I would object to an article that said "It is meaningless to say that a person has glasses." A person can have glasses. It would be very confusing to people to say that a person cannot have glasses if it wasn't a subject everyone is so familiar with. —Preceding unsigned comment added by 24.128.156.64 (talk) 16:20, 13 October 2007 (UTC)[reply]

Perhaps we are having a problem over the meaning of "has". You could say that a human has glasses. But this would be a different meaning of "has" to its use in: "a human has DNA". In the latter example, having DNA is fundamental to what it means to be a human (all humans have DNA). In the former example, we are talking about one example of a human, not all humans. The same applies to DPI. An image may or may not have a DPI value tagged onto it.

Look, Imagine if I could tag an image with "5 ounces", then would you say that the image has a weight? Really, does it weigh 5 ounces? No, digital images don't have weight. It would be still correct to say that "it is meaningless to say that a digital image has a weight".

Tagging an image with "100 dpi" doesn't mean that it really has one hundred dots per inch. A digital image cannot actually have one hundred dots per inch, because it doesn't have a size in inches. It only has inches if you actually print it out. Surely you can see that there is a difference here? Surely? Rocketmagnet 17:30, 13 October 2007 (UTC)[reply]

It's apparent that the word "meaningless" is insufficiently clear or direct; the result we are currently witnessing is an unproductive semantic squabble. You're both essentially correct, so instead of arguing, how about we try to come up with an alternative phrasing which all can be satisfied with? --jacobolus (t) 18:00, 13 October 2007 (UTC)[reply]

As an unrelated aside, 24.128.156.64, you might try signing your comments, like this: ~~~~. :) --jacobolus (t) 18:07, 13 October 2007 (UTC)[reply]

Thanks jacobolus, wise words. I'd considered re-writing the text in the article, but I thought it would be worth coming to an understanding, in case it caused an edit war. But, yes, this discussion doesn't seem to be getting anywhere. However, it does point strongly to the misunderstanding people seem to have with the concept of DPI relating to digital images. Rocketmagnet 18:26, 13 October 2007 (UTC)[reply]

If you think re-writing will cause an edit war, then put the proposed rewrite on the talk page first. :) --jacobolus (t) 20:14, 13 October 2007 (UTC)[reply]

Here's some proposed text. Perhaps a bit clunky, but I think its correct:

DPI refers to the physical size of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor. A digitally stored image has no inherent physical dimensions, measured in inches or centimeters. Some digital file formats record a DPI value, which is to be used when printing the image. This number lets the printer know the intended size of the image, or in the case of scanned images, the size of the original scanned object. For example, a bitmap image may measure 1000×1000 pixels, a resolution of one megapixel. If it is labeled as 250 DPI, that is an instruction to the printer to print it at a size of 4×4 inches. Changing the DPI to 100 in an image editing program would tell the printer print it at a size of 10×10 inches. However, changing the DPI value would not change the size of the image in pixels which would still be 1000×1000. An image may also be resampled to change the number of pixels and therefore the size or resolution of the image, but this is quite different from simply setting a new DPI for the file.

24.128.156.64 22:45, 13 October 2007 (UTC)[reply]

I think that's pretty good. I've made a couple of small changes though. Rocketmagnet 22:48, 13 October 2007 (UTC)[reply]

Great. I took your version, changed "would tell the printer print it" to "would tell the printer to print it", put back in the page references that where in the original on the page, and put it into the article. 24.128.156.64 23:12, 13 October 2007 (UTC)[reply]