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------- Additional comments from [EMAIL PROTECTED] Sun Mar 16 17:01:16 +0000 2008 ------- > ... "DPI" [...] is the ratio between the image dimension in pixels [...] > and the image dimension in inches This is mostly nonsense. DPI was and is *dots per inch* defining the characteristics of an output device (more correctly DPI is used for printers, PPI for monitors and SPI for scanners). DPI is the measure of how many dots of ink or toner a printer can place within an inch (or centimeter), and not pixels per inch. [A printer dot is not a pixel, see the explanatory section at the end.] It never defines a picture characteristic. [If you wish, you can view it somewhat the other way round: the image dimension in inches when printed is the ratio between the image resolution in pixels and the LPI for that printer, where LPI is usually much smaller than DPI, and only for pure black-and-white lineart, LPI = DPI. Unfortunately, LPI does not accurately reflect inkjets, because these machines use a different error diffusion dithering technology.] DPI is a *hardware* characteristic of the output device (more precisely of the printers halftone grid for laserjet printers). It is a physical characteristic of the printer. What you are referring to is probably more closely related to PPI: pixels per inch. IF people are more comfortable with PPI, than lets use PPI instea of DPI. [The printing-industries term for resolution is LPI, lines per inch. I opt for PPI however, because it is likely that LPI will increase the misunderstanding. Even Adobe chooses PPI in its products. Though, please remember that PPD printer descriptor files contain the LPI values.] HOWEVER: - PPI is very different from DPI - PPI requirements are much smaller than the hardware DPI of the output device [they are very distinct measures, so it makes little sense to compare them, BUT if you want to print in colour on a 600 DPI printer, it will suffice to have a 60-100 PPI image] Even IF you consider editing the image in an external application prior to printing, it is enough to add only a 50% to 100% overhead over the capabilities of the output device. Therefore, IF you plan to print on an 100 LPI printer, then 150-200 PPI will suffice most of the time, and 300 PPI will almost certain be overkill. [Please ignore anything that sounds bigger than 400 because any search for such a printer or output device will fail.] Just as a real example: high quality art books and glossy magazines are usually printed at 200 LPI. IF for reasons of prepress editing you add some buffer pixels (50%), you will end with 200 + 100 = 300 PPI images. Therefore, eveything that is above 300 PPI (LPI) is just a colossal waste of time and resources for over 99.99% of users. [Higher SPIs are needed for the scanner, especially if the original image gets resized, but this is a different story.] ===================== The next section tries to explain the vast difference between DPI and LPI. RATIONALE: ========== Now, unfortunately, printers can print only a dot of a single color, be it black (BW printers), or either one of black, cyan, magenta or yellow dots. They can not print any other colour (including gray colors). So, you can not print directly any of the 16.8 million colours in the RGB colourspace, but just some 4 colours (on some printers 6 colours). To actually achieve any of the many other colours, the printers will print a grid of dots consisting of these fundamental colors. Therefore, for every pixel in the original image, the printer will need to print up to 100 dots (and even more) to get a similar colour to the original colour. > For example, some grid sizes for a 600 dpi printer are: > 1x1 shows 2 shades (black or white, 600 dpi Line art) ** > ** 1x1 is not halftones, it is simply called "line art" mode. > 6x6 shows 37 shades of gray, reducing image resolution to 600/6 = 100 lpi > 7x7 shows 50 shades of gray, reducing image resolution to 600/7 = 85 lpi > 8x8 shows 65 shades of gray, reducing image resolution to 600/8 = 75 lpi > 10x10 shows 101 shades of gray, reducing image resolution to 600/10 = 60 lpi [This applies also after editing a low-colour image, because most editing effects will introduce new colours - so you can't print in lineart anymore.] Also, newer printers advertising resolutions of 1,200 DPI and higher, while partially correct, are highly misleading. They are still bound to 300 (360) LPI, therefore diverting the usefulness of DPI values. For further details, see also: http://www.scantips.com/basics03.html http://desktoppub.about.com/cs/intermediate/a/measure_lpi.htm http://desktoppub.about.com/cs/intermediate/a/measure_dpi.htm http://www.scantips.com/basics3b.html [Note: imagesetters mentioned in the articles are very different machines!] This is why I strongly advocate the use of pragmatic values: - Let user choose between 3 predefined OPTIONS: -- screen: 72 PPI (or LPI; for screens, PPI is the more accurate term) -- medium: 150 LPI (pixel equivalent, or PPI) -- high: 300 LPI (pixel equivalent, or PPI) --------------------------------------------------------------------- Please do not reply to this automatically generated notification from Issue Tracker. 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