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[DPRG] Camera needed at RBNO

Subject: [DPRG] Camera needed at RBNO
From: Chuck McManis cmcmanis at mcmanis.com
Date: Mon Oct 20 15:54:01 CDT 2003

At 12:49 PM 10/20/03 -0700, William James wrote:
>I need to take a 300dpi picture of Beerbot. My phone
>camera has a resolution of 640 x 480. I don't believe
>that is enought. [sic] First, how do you figure out
>the DPI of a specific resolution.

This is a very common area of confusion ...

Cameras don't have a "dots per inch" resolution, they have a resolving 
resolution. Printers on the other hand have a "printing" resolution.

Imagine a camera, the focal plane of the imaging element has some number of 
pixels (in your case 640 x 480). If the width and height ratios of those 
pixels were exact (they aren't but assume for the moment they are), and the 
camera was focused on a robot 18" away, such that the robot, whose 
dimensions were 12" on a side, filled the imaging area. (slightly less than 
a 90 degree field of view) Then with your camera in that configuration you 
can "resolve" things that are as small as .02" on a side (that's a little 
less than 1/32nd of an inch).

As it is unlikely that your robot has many features this small, imaging it 
at 640 x 480 would provide acceptable resolving resolution for most 

Then there is the issue of reproducing this image in print.

Printers, and especially color printers, have a "printing resolution" 
(usually specified in dots per inch) that identify how accurately those 
printers can place their ink. For example a 300 dpi printer, can print 
black and white ink dots that are slightly more than 3 mils apart. 
Technically that makes it a 150 lines/inch printer.

Now when you get to color, these printers often use a technique where they 
overprint dots in a several base colors to create a new color. The 
interesting (and glossed over) fact is that the more "room" they have to 
print their inks, the more colors they can reproduce. The resolution at 
which they can print their full gamut is called the "screen resolution." 
Gamut is a term for the set of all possible colors that can be reproduced, 
and screen resolution comes from the days of silkscreen printing where the 
holes in the screen determined the size of the printed dots. Interestingly, 
an HP 970 which is called a 300DPI printer has a screen resolution of about 
125 DPI. You can demonstrate this to your self by using a program like 
Corel Photopaint (or Adobe Photoshop) and create two versions of an image, 
one at 300 DPI and one at 125 DPI. Print them both to your printer, they 
should be exactly the same size on the page, with one having better color 
fidelity than the other. If you've got a standard color wheel type image 
its best, but anything with a lot of color variation will do.

Now getting down to brass tacks...

If you have an image that is nominally a 72 DPI image (CRT screen 
resolution for a 640 x 480 screen, 13" diagonal display area) You can 
re-sample it in most imaging tools to change its DPI to something you would 
find more appropriate for printing (100 - 200 DPI). Different programs do 
this in different ways, however remember that for printing you want both a 
"size" (which is under your control) and a "dpi" which is tuned for the 
printer you are printing to. The number of pixels will be "dpi * size in 
inches" and if the number of pixels in the resampled image is larger than 
the original, artifacts in the form of "blockyness" can be introduced. When 
the resampled image has fewer pixels than the original you can get 
artifacts in the form of "aliasing."

So if the consumer of your image wanted to print it as a 2" x 1.5" image 
next to a paragraph describing your robot, then resampling a 640 x 480 
pixel image to an image that is 2" x 1.5" and prints at 300DPI (600 x 450 
pixels) will be able to use your image with no change. If they want to 
print it at 4 x 3" (1200 x 900 pixels) then they will have to resample it 
to the larger size. Depending on their resampling algorithm this will 
either make it look pixellated or fuzzy.

Another consideration is that typically you will crop the image before you 
print it, and cropping takes away pixel information. Typically a raw image 
size of 2048 x 1600 is usable in nearly any media short of film projected 
in a theater environment.

I know, a lot more than you asked for and probably not very helpful, I'll 
stop typing now ...


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