Scanner resolution details - conversion method

Four, pixel, inch and resolution conversion method

When dealing with digital images, it is often necessary to switch between inches, pixels or pixels per inch. Here are some basic methods.

1. Convert pixels to inches

Digital images are usually represented by pixels in their size, but printouts are generally expressed in inches or centimeters. When a digital image is printed out, it is necessary to perform conversion so as to know how large an image file and the size of the printout are. For example, a 1500~1200 pixel image is output on a device with an output accuracy of 300 dpi. The calculation method is:

Width: 1500/300 = 5 (inches)

Height: 1200/300=4 (inches)

However, if the output device's accuracy is increased to 600 dpi then the same image can only be printed with a 2.5 inch x 2 inch thumbnail.

2. Convert inches to pixels

Scanning is the reverse of the printout. The image that is usually scanned is represented by a size, and the scanned document is represented by a pixel. This requires converting inches to pixels. The method is to multiply the inch by the scanning resolution. For example, a 3 inch x 4 inch image is scanned with 300 dpi and the following pixels are calculated:

Width: 3X300=900 pixels

Height: 4X300=1200 pixels

Application examples;

If you want to use a 300dpi color inkjet printer to output a 10-inch wide image, how much documents do you need?

If only the width is considered, the number of pixels in the horizontal direction is: 10300 = 3000, that is, 3000 pixels are needed.

If the original is a 35mm film, then its width is about 1.375 inches, so you can calculate the required scanning resolution: 3000 ÷ 1.375 = 2182 (dpi), which is the case when the original is 35mm film , need to determine the input resolution when scanning.

If the original is a 5-inch by 7-inch color photo, how high a scanning resolution is required? Similarly, 3000 ÷ 7 = 429 (dpi) can be calculated. 429 dpi is the input resolution that the original needs to be determined.

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