Calculating Image Resolutions in Flex

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Sunday, September 20th, 2009

Custom book builders are great Rich Internet Applications that we’ve made at Rain. One core component of custom book builders is allowing users to upload their own photos. The most common user error is uploading and using a photo that cannot be printed at a high quality print resolution. It is our job to let the user know if the image he or she has selected for a particular ImageWell (a rectangle where a photo can be placed) can be printed.

Basics of printing

Printers print according to dpi (dots per inch). Say a 1200 px x 1800 px image is printed onto a 4″ x 6″ sheet of paper. The dpi would then be:

• width: 1200 px / 4 inches = 300 dpi
• height: 1800 px / 6 inches = 300 dpi

Print resolutions can vary anywhere from 150 dpi (low/medium quality) to 300 dpi (high quality) and up.

Our Example

First we need to define two variables:

1. Final print resolution dpi
2. Warning dpi

For this example we will use a final print resolution of 300 dpi and we will show a warning if the image resolution is lower than 200 dpi. The user has uploaded a low resolution 400 x 500 photo to our photo book application. The user has placed the photo into an ImageWell that is 1200 x 1200 (square). Then the user scales the image to 200% which crops the photo and lowers the dpi.

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