When It's Important to Know the DPI and When It Doesn't Matter
  
PSS ID Number: Q139421
Article last modified on 03-28-1996
 
3.50
 
IRIX
 

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The information in this article applies to:
 
 - Softimage Toonz for IRIX, version 3.5
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SUMMARY
=======
 
This article describes the difference between DPI (dots per inch) and size
(in pixels) and explains why you sometimes need to know the DPI.
 
MORE INFORMATION
================
 
DPI gives the number of dots packed into a one-inch space on a single line.
This doesn't means that an image with a higher DPI will be smaller; it just
means that the image will have more definition because it uses more dots
(or pixels) to define the same aera.
 
A dot is not the same thing as a pixel. Dots are always discussed in
relation to a unit of measurement (an inch). Dots per inch are relative
units of measurement (dots relative to inches), whereas pixels are absolute
units of measurement. DPI tells you how good the resolution of the image
was while it was scanned, and pixels tell you how big the actual image is
or how much space it will take up on your computer screen.
 
Pixels define the size of the image. For example 1024 by 768 pixels means
1024 pixels wide by 768 pixels high.
 
DPI tells you how many pixels of that same image you would need to exactly
match the size of the original image if you were to print that same image.
 
For example, if you have two images, both 1024 by 768 pixels, but one image
is at 85.33 DPI and the other is at 128 DPI, you can think of the one with
the higher DPI value as having its pixels packed tighter together than the
one with a lower DPI. In fact, the first image (85.33 DPI) was scanned at
field 12 and the second one at field 8. This means that 1024 divided by
85.33 equals approximately 12 (or field 12). So, if you scan a field 12 at
85.33 DPI, you will get an image that is 1024 pixels wide. In the same
manner, 1024 divided by 128 equals 8 (or field 8).
 
Better resolution equals higher resolution values and higher DPI and the
more dots you pack in a one-inch space, the higher the DPI will be.
However, this doesn't mean that the bigger the size of the image (say 1024
by 768 pixels instead of 640 by 480), the higher the resolution and the
higher the DPI.
 
DPI is directly related to the scanned field. It doesn't affect the size of
the image. The relationship between field size in traditional animation and
DPI can be defined as "For the same output size, the smaller the field, the
higher the DPI." Why? Because to keep the same file size, each pixel in the
image needs to represent a smaller area, thus packing them tighter.
 
For example, think of a file as a sardine can. Each sardine represents one
pixel. Some cans have a few big sardines, while others may have many
more sardines packed in the same can size. Of course, to pack more sardines
into the same can size, you have to take smaller sardines because the can
itself is the same size. Dots are like the sardines and files are like the
cans that contain sardines. The more dots you pack in, the higher the
resolution will be.
 
The confusion comes from the fact that you need to know the DPI (or field
size) when you scan in an image but not when you output to video or film.
Most of the time the DPI value is irrelevent when you output. When you
output to video or film what is important is the file size itself: 640 by
480 pixels or 720 by 486 pixels for video or 2K (2048 by 1536 pixels) or 4K
(4096 by 3072 pixels) for 35mm film.
 
Differences Between Video and Film Output
-----------------------------------------
 
In video, what's important is the number of lines. PAL, SeCAM, and NTSC
video use different numbers of lines but the width is the same. That's why
in Toonz you hear discussions about the vertical fit for TV. The DPI
question is irrelevent.
 
In film, though, what's important is the number of pixels that you want to
have on a single line. As with TV, the DPI question is irrelevent because
whatever you scanned in will never give you good results if you try to
match the DPI. That's why in Toonz you hear discussions about the
horizontal fit for film.
 
NOTE: Softimage Inc. is a wholly owned subsidiary of Microsoft Corporation.
 
Additional reference words: 3.50
KBCategory: kbgraphic
KBSubcategory:
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Copyright Microsoft Corporation 1996.