First, a correction or clarification to what I was saying. PPI and DPI are not the same. PPI is used for on-screen display, DPI is used for printing. The quick and dirty with DPI is that printers can print dots, but each dot represents only (typically) the standard CMYK (cyan, magenta, yellow, black) colors.. so if you have a 600 PPI image and print it with a 600 DPI printer, you're not going to get an exact representation. One source recommended a 1200 dpi printer for 300ppi images (1200dpi / 4 possible colors per dot = 300 ppi effective printing? dunno.. it seems not exactly correct somehow but think that's what their rationale was). > What you are doing in Photoshop is simply changing the number of... Wow.. what a great jumble of words you got here. Let's bullet it: * ..simply changing number of samples * You are not changing the DPI of the image.. * .. still show the image in any dpi you want, you just change the ability of the image to retain it's quality over a larger range of dpi.. What is a 'sample' to you/ Yes I am changing the dpi/ppi of the image as indicated by the "pixels per inch" next to the setting I'm changing. And finally.. huh?? Yeah, of course a higher ppi image is going to retain it's quality when you blow it up. I'm really not sure what this paragraph is saying but it doesn't seem to address any argument or make any point other than saying "No.. you're wrong". > Surprise, surprise -- why is that? GIF's were developed for web pages... Why the contradictory tone? All you did was re-state what I said.. but in a tone that seems antagonistic... kinda funny actually.. just don't know where you're going with that. > 72 ppi on-screen vs 72 dpi on printer Yeah.. as I stated (and it makes some kind of sense even if the 300ppi -> 1200dpi thing might not be totally true), a 72 ppi image on screen printed on a 72dpi printer is NOT going to look exactly like it will on-screen.. even if 72 ppi matched 72 dpi (assuming each printer dot could do a full 256, 32k, or more colors..which it cant) it's still a 72 dpi image. So it's going to be bigger and pretty clunky looking. If you moved the piece of paper away from you til the image was about the same size (to you) as the one the screen, it'd probably look pretty much the same.. assuming you had a printer that could match the color variation per dot as your computer was displaying per pixel... which they don't. This next part is my favorite... > jpeg faq links > You will not find any mention of dpi. Why? Because DPI is not > relevant to the image...it only pertains to the display of the image. How is dpi or ppi not relevant to the image? So are you saying that programs like Photoshop and devices like printers just 'invent' or conjure up some magic numbers and use those for dpi and ppi when they're displaying or printing? GIFs and older image file formats may not bother to store that information because it's just known that GIFs are 72ppi. That's just a hard fact. Hell, maybe GIF89a or something may have provisions for storing ppi info... but not that I've seen. PNG came around as a freeware replacement for GIF and I'm sure it has ppi information in them... as do jpeg files. Here's a faq link of my own for ya: http://www.fileformat.info/format/jpeg/ Notice the part right up top about the file header data: typedef struct _JFIFHeader { BYTE SOI[2]; /* 00h Start of Image Marker */ BYTE APP0[2]; /* 02h Application Use Marker */ BYTE Length[2]; /* 04h Length of APP0 Field */ BYTE Identifier[5]; /* 06h "JFIF" (zero terminated) Id String */ BYTE Version[2]; /* 07h JFIF Format Revision */ BYTE Units; /* 09h Units used for Resolution */ BYTE Xdensity[2]; /* 0Ah Horizontal Resolution */ BYTE Ydensity[2]; /* 0Ch Vertical Resolution */ BYTE XThumbnail; /* 0Eh Horizontal Pixel Count */ BYTE YThumbnail; /* 0Fh Vertical Pixel Count */ } JFIFHEAD; horizontal resolution, vertical resolution, units used for resolution. "Units, Xdensity, and Ydensity identify the unit of measurement used to describe the image resolution. Units may be 01h for dots per inch, 02h for dots per centimeter, or 00h for none (use measurement as pixel aspect ratio)." What do ya know? DPI... or.. for the rest of the non U.S. world.. DPCM. Guess DPI information is stored within the jpeg format after all. > You will not find any mention of dpi. Why? Because the faqs you pointed to decided not to mention it... not because it doesn't exist. > Go ask a chicken what "eggs per package" it's egg will be put in, a > dozen, a dozen and a half, two dozen, or what? Eggs, like pixels, > haven't friggen a clue as to what packing they're going to be put in. Beautiful. *applaud* I have no adequate response to this that I feel will do any good at untangling the logic presented. So... back to PHP and more rational discussions? -TG = = = Original message = = = -TG: At 10:51 AM -0400 5/8/06, <tg-php@xxxxxxxxxxxxxxxxxxxxxx> wrote: >Actually I don't believe this is exactly right.... from playing >around with Photoshop you can see that you change an image's >resolution under Image -> Image Size and if you turn off "Resample >Image", it will retain the same pixel dimensions. What you are doing in Photoshop is simply changing the number of samples in the image. You are not changing the dpi of the image. In fact, you can still show the image in any dpi you want, you just changed the ability of the image to retain it's quality over a larger range of dpi, nothing more. >GIF's are limited to 72dpi (or ppi if you prefer.. pixel per inch) >which matches what is typically displayable or desirable for web >content. But when you go to print that webpage, you really see the >lack of quality in that GIF image. Surprise, surprise -- why is that? GIF's were developed for web pages and the typical monitor was 72 dpi (now, they vary greatly). When you take a GIF and print it, you now have a different medium at which to view the image. If you can set your printer to 72 dpi, and your monitor to 72 dpi, then you'll see very little difference between the monitor image and the printer image. However there are some differences in the way a monitor and paper shows an image (brightness and so on). >If you use a JPG and set it for around 300 dpi/ppi, it prints out a LOT nicer. Yes, and if you have images that are capable of 2880 dpi, and you print them at 2880 dpi, they will look even better, but so what? You have missed the point. Outside of some newer cameras that embed capture information into the headers of the image file, jpg's do not have dpi information in them. I suggest you read: http://www.faqs.org/faqs/jpeg-faq/part1/ http://www.faqs.org/faqs/jpeg-faq/part2/ You will not find any mention of dpi. Why? Because DPI is not relevant to the image -- it only pertains to the display of the image. Go ask a chicken what "eggs per package" it's egg will be put in, a dozen, a dozen and a half, two dozen, or what? Eggs, like pixels, haven't friggen a clue as to what packing they're going to be put in. Again, DPI is a way to display an image and is not something intrinsic within the image itself. tedd ___________________________________________________________ Sent by ePrompter, the premier email notification software. Free download at http://www.ePrompter.com. -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
