density based on size

[Lars-Daniel]

Hi there,

is there a way to choose the resolution (ppi) based on the final size in millimeters?

Right now, I'm using a script like this to get 550 x 550 mm output (the image is 4300 x 4300 px) :

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echo (4300*2.54/55) | tr -d $'\r' | bc -l

= 198.58181818181818181818

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convert -density 198.58181818181818181818 -units PixelsPerInch input.tif output.jpg
exiftool -XResolution=198.58181818181818181818 -YResolution=198.58181818181818181818 output.jpg

Would be more easy to use -extent in millimeters. Is this possible?

Best,
Lars-Daniel

[fmw42]

I do not see why you would want to use -extent. It does not set print density. It only pads or crops the image to the specified size. It does not allow mm units only pixels or percent.

This is how I would do it using IM's fx calculator and saving as a variable. Proper IM syntax has the input raster image right after convert, though in this case it really does not matter, since all your arguments are settings and not operators.

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size=4500
mm=55
dens=$(convert xc: -format "%[fx:2.54*$size/$mm]" info:)
convert input.tif -density $dens -units PixelsPerInch output.jpg

[Lars-Daniel]

It would be nice to let IM do the change of the density automatically based on printing size the user gives it it. This would help Windows users.

[fmw42]

See -resample at http://www.imagemagick.org/script/comma ... p#resample.

[Lars-Daniel]

See -resample at http://www.imagemagick.org/script/comma ... p#resample.

That's the point: I do NOT want to resample it. Only density should change, image dimensions (pixels) shouldn't change.

Let's say the image is 1000 x 1000 px and has 300 ppi by design; this would mean 84,67 x 84,67 mm when printed 1:1.
I'd love to have a function like this: convert -resample 50x50mm -density only <input> <output>

Then the image's density should be changed to 508 ppi without any resizing.

[fmw42]

Post your request to the Developers forum for a new function. I do not think you can supply mm on -resample and -density "only" would need to be a -define rather than a -density option. Proper IM 6 syntax has the input image right after convert for raster images.

So it could be something like:

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convert image -units pixelsperinch \
-define:resample:density=only -define:resample:units=mm \
-resample 50x50 output

Perhaps the second define is enough and the first would be implied by the second?

Or better, just a new function such as -setdensity 50mm, e.g.

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convert image -setdensity=WxH+[mm,cm,in] output

In any case, this would convert your 50 mm print size to a density for the appropriate units and pixel dimensions.

But let the IM developers decide what is best. Perhaps they have a better way.

[snibgo]

"-resample" curently does two things: it resizes, and it changes the metadata density setting. As there is already an operator devoted solely to changing the density setting, it would be perverse to change "-resample" so that it, too, could merely change the density.

The units for the required operator are units of length (inches, mm or whatever) so creating this as a new operator makes more sense than shoe-horning an existing operator.

IM is open source. Perhaps the OP could do this himself.