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Changing the dpi value stored in an image DOES NOT improve it's quality...
Authored by: dilvish1984 on Apr 22, '11 03:20:53PM

The procedure you describe has absolutely no effect on the data representing the pixels in the image (so would give no improvement to the printed output). The number you're modifying simply tells the software displaying the image how big a pixel is intended to be (1/72 of an inch or 1/300th of an inch in your example). Most software actually ignores this value, anyway, especially on raster-based images (as opposed to vector based, such as SVG files). Any software that actually does try to honor the new resolution you've given the image will simply display (or print) it at a much smaller apparent size (e.g. a 4 inch by 5 inch image originating at 72dpi would be displayed as 0.96 inches by 1.2 inches after having its resolution changed to 300dpi, regardless of the output device, screen or printer, in software that uses this value correctly).

So, all you've done by changing this value is to point out a flaw in any software that ignores it (which is most software, in my experience) and shrunk the output size of the image for any software that doesn't. You haven't actually improved the quality at all.

Now, if you were to actually scale the image data by, for example, changing the width and height values in that same Preview dialog (while the "Resample image" checkbox is on), you WILL actually "improve" it's quality (in other words add data), but this data is interpolated by the software and can often times create artifacts (jaggies), though the algorithms have greatly improved since the early days. What the OP is trying to accomplish is to create an image that actually contains more accurate data in the first place rather than relying on later upscaling.



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