Re: anybody use sips to change image dpi?

FROM : David Masters
DATE : Sat Jul 08 00:05:51 2006

Michael,

On 7 Jul 2006, at 21:54, Michael B. Johnson wrote:

> I'm trying to use sips to turn some 200dpi full rez images into 72 
> dpi thumbnails (because of a bug in the NSImageCell), and this
>
> sips -s format jpeg -s formatOptions normal src.jpg --resampleWidth 
> 256 -s dpiHeight 72.0 -s dpiWidth 72.0 --out dst.jpg
>
> yields a 256 wide 200 dpi (if that was the source image's dpi).
>
> Anybody see what I'm missing?


Not entirely sure, but I think the problem lies with some of the 
additional metadata in the JPEG file.

I ran into something similar before: I opened up .jpg files (saved 
from Photoshop) in Preview, and then used Tools -> Get Info -> 
Details to see what was in the metadata. The relevant information 
looks something like this:

File Name    src.jpg
   DPI Height    200
   DPI Width    200
   Pixel Height    600
   Pixel Width    800

  Exif Properties
   Pixel X Dimension    800
   Pixel Y Dimension    600

  JFIF Properties
   X Density    200
   Y Density    200

  TIFF Properties
   X Resolution    200
   Y Resolution    200

When you run...

sips -s format jpeg -s formatOptions normal src.jpg --resampleWidth 
256 -s dpiHeight 72.0 -s dpiWidth 72.0 --out dst.jpg

...this only appears to change the JFIF Properties X and Y Density - 
as you would expect for a JPEG file. However, other applications read 
the other Properties so Photoshop and Preview think it is 200dpi, but 
GraphicConverter, for example, thinks it is 72 dpi.

Probably the best solution is to somehow remove the additional 
metadata, but one workaround is to convert to an intermediate format...

sips -s format png -s formatOptions normal src.jpg --resampleWidth 
256 -s dpiHeight 72.0 -s dpiWidth 72.0 --out intermediate.png

...and then convert to jpg...

sips -s format jpeg intermediate.png --out dst.jpg

One slight annoyance (apart from having to do two steps) is that the 
conversion from the png format's of 2835 pixels per metre(!) results 
in 72.00901 dpi being reported in some applications for the JPEG 
resolution. Possibly a different intermediate format would work better.

Hope this helps!

David


David Masters, Software Imagineer
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