Photo dpi confusion

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Gramps

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Jan 10, 2003
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I have a dpi question. I am confused about the relationship between printer dpi and the dpi of the image being printed. If I set my printer to print at 600 dpi, and the image in my photo program is set at 150 dpi, how does that interact? Will the printer still print it at 600 dpi or what? I understand pixels pretty well but
am very confused about this. For any help, I would be forever grateful.
Gramps
 

DaveBurnett

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Nov 11, 2002
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Its as simple as:
The printer will print a 4x4 (600/150 by 600/150) dot for each pixel in the image.
 

Gramps

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Thanks very much Dave. I figured it was too simple for my even simpler mind to latch onto. Appreciate it.
Gramps
 
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It isn’t that simple at all.

150 photo program PPI is more than 600 printer DPI. The printer makes a lot of dots to producer a single color pixel. That is why most graphics people use DPI for dots per inch and PPI for pixels per inch.

The printing spooling algorithm is complex. The printer can seldom get a pixel right with the available colors – especially if it is a 4 color printer. So it adjusts the dots in the adjacent pixel to give the impression of the first being correct, but then it has to adjust adjacent pixels to account for that. So even if you said your program sent the image at 50 PPI the printer would still not print pure pixels of color. In the first place the adjacent boundaries wouldn’t be perfect and in the second it would be adjusting within the pixel to offset color errors in the adjacent ones.
 
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