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Save (BMP) As (JPG) reduces dpi


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I scanned a 3.5"x5" pic on a Canon 4200F at 600dpi and outputting as BMP using CanoScan Toolbox (4.6.1.2). I scan to BMP so I can open the image with Paint.net (4.0.9) and Save As JPEG and control the JPEG quality settings (which I can't figure out how to do with the Canon tool).

 

Windows Explorer shows the Horizontal & Vertical Resolution of the original BMP as 600dpi and the JPEG as 599dpi. Any ideas why the JPEG image has reduced resolution?

 

If I scan the pic and output it as JPEG, then use Paint.net to Save As (a different) JPEG, the dpi doesn't change from 600dpi. Even if I modify the image before Save As the dpi doesn't change.

 

Is it normal for Paint.net to change the dpi?

Edited by wumpus
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DPI is actually mostly a shell game as far as computers are concerned, unless you're planning on printing anything.  My guess is that Windows is doing the calculations differently on the JPG than on the BMP, but if your file still has the proper dimensions, it should be unchanged.

 

One thing to note is that JPG is a lossy file format; I'd recommend PNG if possible.  If you get a good PNG compression plugin (like OptiPNG), it'll be smaller than a JPG anyway while maintaining source quality.

 

Hope that helps!

 

The Doctor: There was a goblin, or a trickster, or a warrior... A nameless, terrible thing, soaked in the blood of a billion galaxies. The most feared being in all the cosmos. And nothing could stop it, or hold it, or reason with it. One day it would just drop out of the sky and tear down your world.
Amy: But how did it end up in there?
The Doctor: You know fairy tales. A good wizard tricked it.
River Song: I hate good wizards in fairy tales; they always turn out to be him.

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JPG being a lossy format is why I was scanning to BMP then using Paint.net to save to JPG so I could edit the image without saving a lossy from a lossy. CanoScan Toolbox 4.6.1.2 only has BMP or JPG output support. I'll look into the PNG plugin.

 

 

According to Exif Pilot the XResolution and YResolution exif tags are set by Paint.net as they're not present in the original JPG image saved by the scanner.

 

The scanner -> JPG, re-saved by Paint.net -> JPG has "600/1" in both XResolution and YResolution exif tags

The scanner -> BMP, Save As by Paint.net -> JPG has "299999/500" in both XResolution and YResolution exif tags.

 

very odd.

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According to Exif Pilot the XResolution and YResolution exif tags are set by Paint.net as they're not present in the original JPG image saved by the scanner.

 

Ah, I see.  Well, my conclusion was incorrect, but everything else still holds.  DPI is a farce (most computers display an image at 96DPI regardless of the image's stated resolution), and OptiPNG is a better idea than JPG.

 

The Doctor: There was a goblin, or a trickster, or a warrior... A nameless, terrible thing, soaked in the blood of a billion galaxies. The most feared being in all the cosmos. And nothing could stop it, or hold it, or reason with it. One day it would just drop out of the sky and tear down your world.
Amy: But how did it end up in there?
The Doctor: You know fairy tales. A good wizard tricked it.
River Song: I hate good wizards in fairy tales; they always turn out to be him.

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Thank you for your patience. OptiPNG plugin is working Just Fine. I'm trying to figure out how to set the "Date Taken" and "GPS Location" metadata for PNG. Looks like there isn't a standard, like EXIF for JPEG. I may open another forum question about that if I can't figure it out. Thanks again.

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No problem!  Glad you're getting things sorted out.

 

Incidentally, I don't know of a way to edit EXIF within Paint.NET, but there are tools available on the internet that can.

 

The Doctor: There was a goblin, or a trickster, or a warrior... A nameless, terrible thing, soaked in the blood of a billion galaxies. The most feared being in all the cosmos. And nothing could stop it, or hold it, or reason with it. One day it would just drop out of the sky and tear down your world.
Amy: But how did it end up in there?
The Doctor: You know fairy tales. A good wizard tricked it.
River Song: I hate good wizards in fairy tales; they always turn out to be him.

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