Detect Original Picture From Scanned Image

When I scan a small picture, the scanner outpu开发者_如何学Ct is a big picture. How can I detect original picture from scanned image?

I think you are confusing physical size with resolution. The photograph you are scanning obviously has a physical size — you can measure this with a ruler. When you scan however, you end up with a picture that has a resolution (pixel size) rather than a physical size and this is controlled by the DPI setting in your scanning software. DPI means dots-per-inch and controls how many dots (pixels) will be created for each inch of your photograph — 300 dpi would result in 300 pixels across for each inch width of your photograph and 300 pixels down for each inch of height. As an example, a photograph 10 inches square would produce a 10 * 300 * 10 * 300 = 9 million pixel (or approximately 9 mega-pixel) image.

Physical size will not actually have much meaning on the computer until you come to print your image again*. Then, you can specify the output DPI — how many pixels make up each inch of your printed document. So if you were to scan at 300 dpi and print at 150 dpi, you would end up with an image that is twice the size of your original (as less pixels are making up each inch so those scanned pixels go twice as far). For example, the 9 million pixels of the above example would be printed 150 for every inch across and down (150² per square inch) thus would produce an image that is 20 inches square.

Physical size and resolution are intertwined, however. The more dpi you scan at, the more information about the original document you preserve which means you can print at much larger sizes without the image appearing pixelated. As an extreme example, if you were to print at 1 dpi, you could make those scanned pixels spread over a huge number of inches but each square inch would be a block of solid colour.

_{* The web mixes things up in that images are usually handled in pixel form thus their size on screen is dependent upon the user's monitor and page zoom settings.}