Legacy ImageMagick Discussions Archive

from http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=445574

> with the debian logo from
> http://www.debian.org/logos/openlogo-nd-50.png
> and can easily reproduce the problem, by comparing the output of
> the following two commands:
>
> convert openlogo-nd-50.png test.pdf
> convert openlogo-nd-50.png -density 72 test.pdf
>
> According to the documentation, the default density value is 72 dpi,
> and the default unit for the density option is dpi, so the output
> pdf files should look the same.
> However, they don't: the second command results in a pdf file with
> a much smaller picture.
>
> Strangely, when trying with another debian picture, namely from
> http://www.debian.org/Pics/debian.png
> the bug doesn't occur: both pdf's look identical.

I can confirm this bug with a more recent version

>As far as I can see, the default conversion is done at 72 dpi, as it
>should; but the _other_ testcase (i.e., with the -density 72 option)
>does _not_ use 72 dpi (but perhaps 72 dots per cm).

Thank you

Bastien

This particular image has units of pixels per centimeter:

• -> identify -verbose openlogo-nd-50.png
  Image: openlogo-nd-50.png
  Format: PNG (Portable Network Graphics)
  Class: PseudoClass
  Geometry: 50x61+0+0
  Resolution: 28.34x28.34
  Print size: 1.76429x2.15243
  Units: PixelsPerCentimeter
  Type: Palette
  Endianess: Undefined
  Colorspace: RGB
  Depth: 8-bit

To get the results the user is looking for, use this command:

• convert openlogo-nd-50.png -units pixels-per-inch -density 72 2.pdf

Ok will forward to the user. But according to the documentation:

http://www.imagemagick.org/script/comma ... hp#density

>this option specifies the image resolution to store while encoding a raster image or the canvas resolution while rendering (reading) vector formats such as >Postscript, PDF, WMF, and SVG into a raster image. Image resolution provides the unit of measure to apply when rendering to an output device or raster image. >The default unit of measure is in dots per inch (DPI). The -units option may be used to select dots per centimeter instead.

>The default resolution is 72 dots per inch, which is equivalent to one point per pixel (Macintosh and Postscript standard). Computer screens are normally 72 or 96 >dots per inch while printers typically support 150, 300, 600, or 1200 dots per inch. To determine the resolution of your display, use a ruler to measure the width of >your screen in inches, and divide by the number of horizontal pixels (1024 on a 1024x768 display).

Therefore convert does not behave as expected, the two image should be the same 72 should means 72 dpi and by default you should use 72 dpi
May be a documentation problem?

Regards

The default resolution is 72DPI. That's a density of 72 and units of pixels-per-inch. Both are needed to specify the resolution. The density option is just a number. Its meaning requires the units option as well. We will work towards improving the documentation for density.

Any news of this documentation bug?