Pixels are dimensionless units.
DPI and PPI are only meaningful when you are printing or displaying.

300dpi is the basic resolution used in the print industry (magazines)
72ppi is the basic resolution used for screen displays (monitors)

When you display or print a digital image you have to map the pixels of your image to the display elements on your monitor or the dots of ink from your printer. You could simply match them up one to one. The resolution of your image would determine how big the displayed image will be. So let’s say we want to do this the easy way and print a single dot of color for each pixel in our image (no interpolation).

At 300 printed dots per inch –

2mp image (1600 x 1200) = 5″ x 4″
4mp image (2272 x 1704) = 7.5″ x 5.5″
8mp image (3456 x 2304) = 11.5″ x 7.5″

The quality should be great at these sizes. What if I wanted to print them at larger sizes? Let’s imagine I wanted to print my 2mp image at 8×10″. This would be double the size with 4x the number of pixels needed. Each pixel in our original image would need to be mapped to 4 dots on the print. Simplistically we would be enlarging the pixels 4 times and the quality would be poor (lots of jaggies). Somehow we need to create an additional 3x the original data and have it look good. You don’t have to only print at 1:1 to get good looking pictures. Images can be interpolated or resized. Some of the interpolation methods are better than others. The default bi-cubic image resize in Photoshop is pretty feeble. Resizing down isn’t necessarily easy, even though you are ‘just’ throwing data from your image away. You have to choose carefully what to throw away. Resizing up is somewhat harder – you are adding data to your image that wasn’t in the original. Some of the other, more advanced methods include:

  • Lanczos
  • Hermite
  • Pyramid
  • Mitchell
  • Bell
  • Fractals

When you are working with your digital images you can safely ignore PPI and DPI. These only become important when you display or print them. Most of today’s inkjet printers can produce output in the neighborhood of around 300 dots per inch when they print in high quality mode. They love to claim 720 or even 1440 dpi, but this is misleading (multiple dots for a single color).

I always disliked the requirement to do math and the poor quality that I was experiencing with Photoshop. I read some recommendations about a program called QIMAGE. It has a lot of the advanced interpolation methods built-in and is really easy to use. It knows about your printer and how to get the best out of it. I tried it and fell in love. It’s very easy to use. You specify how big you want the image(s) and what size paper it’s being printed on. The program will automatically fit as many images onto your paper as possible. It’s a labor of love from a one man shop and he’s very responsive and provides great service. He has slowed down recently, but people used to complain that he updated the software too often. Take a challenge and use the trial to compare your current at home printing program with Qimage. Even if you never print larger than your ‘no interpolation’ size, I think you’ll like the results and how the program works. It runs on Vista and has multi-core support.

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