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Old Oct 18, 2003, 2:11 PM   #1
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Default Printer dpi mystery


I'd like to think that I understand the printer/scan/screen dpi/ppi saga well. However, I'm still puzzled about how printer manufacturers determine their printer's dpi. Canon i950, for instance, advertises a printer resolution of 4800x1200 dpi. As far as I understand, 1200 is the vertical resolution while 4800 is the horizontal one. The horizonal resolution is limited by the construction of the print head itself. Is vertical resolution limited by the precision of paper feeding mechanism? Why is there such a vertical/horizontal discrepancy? Does the printer simply tone down it's horizontal capability from 4800 to 1200 in order to produce an even image? After all, what good is a huge horizontal resolution if you can't match that vertically? How can you judge the impact of these numbers on print quality? Do these numbers even matter, or is the size of the dot all there is to it?

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Old Oct 18, 2003, 6:30 PM   #2
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DPI for printers are dots per inches. That is individual dots sprayed on the paper. A 4800*1200dpi resolution means that the printer can lay down (in theory) that many dots with precision, in other words, it can assign 4800*1200 individual places for the ink dots. This is a number of precision, a theoretic number. When it comes to how many dots layed down, the ink dot sizes matters.

So does resolution matter for printers? yes it does, better resolution should (in theory at least) produce less grainy result. It should also give more sharpness. So it does matter, but a super high resolution is pretty worthless if the ink dots are too big. So it's a combination of both.

To answer your question about the huge difference in horizontal/vertical resolution, i have no idea actually. But we can all be certain that resolution is kind of a hype, just as megapixels for digicams often are used in advertising.
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Old Oct 20, 2003, 1:59 PM   #3
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Printer dpi is not terribly important anymore. For example, Epson printers have the highest dpi (5760x1440) yet their photos are often still noticably grainy (SP 925/825) even with very high res digital images. HP and Canon, even with the lower 4800x1200 dpi are not, in my experience, grainy with the same image.

Something you also need to take into account are ink droplet sizes. Epsons are usually 3 or 4 picoliters (1 picoliter = 1 billionth of a liter), Canons are 2 to 5, depending on detail needed and HP are 4-27 depending on detail needed.

At 1 picoliter, the human eye cannot percieve a difference between dots. 6 color printers not only have a larger color gamut but those 2 photo colors trick the eye into seeing .6 picoliter ink drops.

Keep this in mind when looking at photo printers.

Here's more specifics:

i950/i960/i850/i860. . . = 2 Picoliter
i9100 = 4 picoliter

SP 925/825/1280/2200 = 4 Picoliter
C84 = 3 Picoliter
SP 960 = 2 Picoliter

All Photosmart machines are 4 picoliters
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Old Oct 21, 2003, 11:32 AM   #4
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Ink jet printers use dithering to render color pixels. That is, many drops of ink (dots) are used for each pixel. With more dots per inch, you can either print more pixels per inch or produce more percise colors by combining more dots of color per pixel. More dots per pixel allows the printer to produce more accurate ratios of ink colors. Note that mutiple shades of ink and variable drop sizes also helps the printer control the ratios of the primary colors. I don't know a lot about the redering algorithms used by printers, but I would expect that more dots per pixel also allows a smoother transition of color between adjacent pixels.

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