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Image Resolution

Resolution The text resource I have does not indicate that horizontal and vertical resolutions are encoded in GIF's. As for BMP and other formats, it depends on the application that saved the file whether this information is encoded or not.

I must reiterate, an image file cannot intrinsically have "resolution". Resolution refers to the number of image elements or data points that represent a physical area and so can only be relative to the input source (e.g. a scanner or in some cases a digital camera) or the output device (printer or screen). The stored image file itself contains a fixed number of data elements, along with instructions on how to arrange them (how many on the X-axis, "width", and the Y-axis, "height") and how to color the pixels or dots. Any "resolution" information encoded in the file can only be for archival purposes to indicate the resolution at which the original image source was acquired. Let me give an example:

If I focus my digital camera on a 4" x 3" subject entirely filling the camera view and take a photo at the 1600 x 1200 pixel setting, I can reliably say I have an image with a resolution of 400 pixels per inch. But if I aim my camera out across my back yard and press the shutter, who is to say what resolution the image is? I have 1600 x 1200 pixels worth of image information, but I couldn't begin to guess what area -- square inches (!) -- this represents.

You could describe the resolution of a digital camera in terms of the number of pixels relative to the physical size of the CCD device that actually captures the image, just as you define the number of granules per inch (I don't recall the correct term) contained in a celluloid photographic film. My Epson PhotoPC encodes a "resolution" of 144 pixels per inch in the JPEG files it creates, but I'm not sure what that represents, since at its 640x480 image size, that computes to 4+" x 3+", clearly not the size of the CCD. In fact, it may only be inserted as an instruction to graphics applications as to what size to print the image. My Nikon CoolPix does not encode resolution at all.

Here is another example:

Say I have a file that contains a 300x300 pixel image. If I display it on my home computer screen measuring approximately 14" x 11" with display area set at 800w x 600h pixels, the effective resolution is (roughly) 57w x 54h pixels per inch and the image will physically occupy an area of 5.25" x 5.5" on my screen. However if I view the same image on my laptop screen measuring 9.75" x 7.25" also at 800w x 600h pixels, the effective resolution is 82w x 82h pixels per inch and the image occupies 3.625" x 3.625" of physical space. The image file has not changed and cannot possibly predict what my physical output display area is going to be.

So Jon McGrew's response to your question really was not quite accurate. If you look at the the font information in the Windows "Display" properties, it probably will tell you that your "small font" size is 96 dpi; "large font" is 120 dpi. And if you use Windows 'GetDeviceCaps' API function to read the X & Y resolutions of the screen device, it will tell you that the pixels per "logical" inch are 96. If a graphics application is telling you that a GIF image has a resolution of 72 dpi, it literally must be fabricating this information, because it is not in the file (unless the application put it there as a custom field or my knowledge about file formats is incomplete -- either is possible) nor does it fit with Windows' view of its own world. If a scanner includes the horizontal and vertical resolutions of the image in a file it saves, that information should be reliable (and possibly useful for initializing a printed output).

Technically, pixels per inch are not really the same as dots per inch, however both are so nebulous that we use them interchangeably. Pixels are exclusively in the province of electronic display screens (I forget the exact derivation of the term); "dots" are used to describe input sources like scanners and output sources like printers. If you scan a one-inch square image at 300 dpi, the scanner will gather 300x300 pieces of data, and if you display them on the computer monitor, each piece of data will occupy one pixel. On my monitor, that will cover a 5-inch square area. If you have a 300x300 pixel image and print it at 300 dpi, you will have a one-inch by one-inch print. And if you have a 300x300 image-file and print it at 600 dpi, it will occupy on-half by one-half inch, unless you set the output size at one inch (or two or three), in which case the printer driver or graphics application will have to interpolate the additional data needed to reach the specified resolution. The "resolution" of the stored image has not changed (in fact it doesn't exist), but the resolution of the output has.

After all this ranting, the crux of the matter is, to prepare image files for printed publication, you need to know what physical area the printed images are to take up on the page. If you want a 4" x 3" printed image of good quality, then you need to provide, for use in Quote Quad in this instance, at least 1200 x 900 pieces of data -- you can call them pixels of you like, since you will be preparing the image using a computer screen. When they are printed they'll be dots.

As always, my disclaimer is that you know full well I am neither a computer professional nor graphics specialist, and I stand ready to be corrected by anyone who knows more than I do.

(from W.R.Parke e-mail of 6/18/00)

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