FAQ-Resolution, Is 200 PPI clear enough?

I received many emails from Easy Mosaic users.
They ask me to provide a high resolution version of easy mosaic.

I get confused at the begining. I am very sure my program already provided enough resolution for most situations. Hummm. I stop nag here, and show you a sample:

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Show Time:

This is a photomosaic I created, at 200PPI resolution.  The size is about 40×32 inch.

I take it to a photofinishing shop and printed the photomosaic, then I placed it over the floor, ready to take some photos for it, there are two harddriver on it, as a size reference.
I use a Fuji Finepix S6500 to take a photo at 2 meter distance.

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Now I will get closer and take a new photo.  1.5 meter distance

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Now, more closer, and a new photo. at 1 meter distance

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More.  at 40cm distance.

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More closer, 20 cm distance,  I placed a quater and a dime on the photo.

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 The last one. 5cm distance.
 Look, our photomosaic keeps great details at such close distance.
Average person cannot see smaller pixels with out a magnifier. So we are not going more closer.

click for larger view

Showtime is over, Now let me nag.
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I talked with some user, then I found the source of this question.
The fact is,  the user has been told that his printer can printing a high resolution image at 1200DPI.
So he think he may need a 1200DPI photomosaic for the printer.
It is a big confusion.  You dont need a 1200DPI image for a 1200DPI printer.

The photomosaic rendered by computer is a digital image, The image size is calculated by PIXELS. for example, a 1000×1000 pixels image. It stored as ‘010111000101101′ binarys, so  people can not see a digital image, until we use an output device to display it.
For example, your computer monitor is a output device, Your printer is a output device, the lcd viewfinder of your digital camera is a output divice.

We use PPI to define the resolution of output device. PPI = Pixels per Inch. For example, If your output resolution is 200 PPI, the 1000×1000 pixels image will be outputed as a 5×5 inch result.  The common computer monitor’s resolution is 96PPI, so, a 1000×1000 pixels will take about 10×10 inchs on your screen.  The commercial photofinishing devices work at 200 to 300PPIs,  FujiFilm’s Frontier LP7900, work at 300PPI, then we can get a 3.3×3.3 inch printed photo.

DPI only refers to the printer, Every pixel output is made up of different coloured inks (usually 4 or 6 colours, depending on your printer). Because of the small number of colours, the printer needs to be able to mix these inks to make up all the colours of the image. So each pixel of the image is created by a series of tiny dots.

A 1200 dpi printer uses 1200 dots of ink in every inch to make up the colours. If you were printing a 150 PPI image, then every pixel would be made up of 64 smaller ink dots (1200 DPI x 1200 DPI) / (150PPI x 150PPI).

The more ink dots we used to printing a pixel, the better color depths we can reach.
While printing ture colored images, a 1200DPI printer can be thinked as a 150PPI devices.

Summary: You rarely need your image resolution to be higher than 200 PPI for most printers.