It appears there is some miscommunication between digital media and print shops.
I often hear the term DPI and PPI used interchangeably, which is not true.
Hopefully this small document will shed some light on these somewhat confusing terms.

Maya will render out 72 PPI, NOT 72 DPI, there is a big difference as we will look into shortly.
When preparing your render for print, the last thing on your mind is the DPI.
You should focus on is the image size in pixel.

Pixel
Pix ‘picture’ & el ‘element’ = Pixel, is the smallest portion of information in the rendered image.
You can easily view these pixels by zooming in onto your image.

PPI
“Pixel Per Inch”  is the measurement of resolution, it refers to the actual number of pixels per inch in the image.
i.e.  100 x 100 PPI  image printed as 1  inch , will have 100 pixels per inch regardless of the printer DPI capability.
The PPI affects 2 main components of your render:  quality and size.

Quality
The higher the number of pixels per inch, the better the quality and the less chance of getting jagged edges in a rendered image.  Imagine trying to fill a square with 10 dots, now fill the same square with 100 dots.
Size
This is the important part!  As the PPI will affect the image size.  This can be adjusted in Photoshop by toggling on the Resample Image Setting.   Changing the PPI setting will increase or decrease the print size.  It will increase the images size by decreasing the PPI and vice versa.

Photoshop and other 2D application have resizing options. While working with these options you should avoid “Resample Image” since it will change the actual number of PPI in your image and there is a chance you might lose some pixels in the process.   If you uncheck “Resample Image” in Photoshop,  you will only change the image size and not affect the quality of the image.

For example:   An image that is 3000 x 3000 pixels  and you would like to print it at 300 DPI, the result would be 10” x 10” image in size (3000/300 = 10).

DPI
“Dot Per Inch” a measure of printing resolution.   This is a term that is often used by print shops or the actual printer properties.   Every Pixel that is going to be printed will be a collection of small dots per inch, the higher the value, the more “crisp” the final image going to be.  These small dots consist of the volume of ink in the each color channels (usually CYMK).
When comparing 2 images, one 10 DPI and the other is 100 DPI , the only thing you would notice is the actual size of the image. Again this is due to the fact the at the DPI is just a measurement of how the image is printed on paper

LPI
“Lines Per Inch” AKA “Line Frequency” also is a measure of printing resolution.   This is a term that is often used by print shops when outputting to certain device or paper.
When printing the gray color it will use the black ink to get the “halftone” the smaller the halftone the lighter the color will be.  Note: not all paper can handle low LPI.

SPI
Samples Per Inch, is a measurement of a the resolution in of the scanner.  To recreate an image, the scanner looks at a small part of the image and samples it according to the SPI value.
The more samples per inch, the higher the quality of the scanned image, closer the scan is to the original image because there is more information available about how the image should look.

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This entry was posted on Friday, April 10th, 2009 at 7:23 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.