#261 PNG Pixel Density Not Preserved Converting to PDF

[Dave]

Description
The pixel density stored in PNG files is not preserved when converting to PDF format. This may be related to [#253]. Not sure if it applies to other image formats as well.

Steps

1. Make a 300 dpi 8.5" x 11" image (this is a resolution of 2550x3300), and save it as input.png.
2. Run convert command:
   gm convert input.png output.pdf
3. Open output.pdf in Adobe Reader (or other PDF viewer) to check size.

Expected Result
Page Size reported by PDF viewer as 8.50 x 11.00 in.

Actual Result
Page Size reported by PDF viewer as 35.42 x 45.83 in.
This means that the output density is being set to 72 dpi even though the source is 300 dpi.

Notes
Using the -density option to force 300 dpi produces the expected 300 dpi 8.5" x 11" output PDF.
 e.g.
  gm convert -density 300 input.png output.pdf

[Bob Friesenhahn]

I am able to reproduce the problem here. It must be a problem in the PDF writer rather than with PNG. Thanks for the bug report and the hint about working with -density.

[Glenn Randers-Pehrson]

If you want the PNG to supply the density, you have to supply both
density and units to the PNG encoder when creating the PNG.

Using a recent gm snapshot, GraphicsMagick 1.4 snapshot-20140102 Q16

gm convert -size 2550x3300 -density 300x300 \
           -units PixelsPerInch xc:blue blue.png

That creates a good PNG with resolution 11811x11811 pixels/meter (300 dpi)

gm convert blue.png blue.pdf
gm identify blue.pdf

The resulting PDF 2550x3300 and has no resolution or density information.

[Bob Friesenhahn]

On Thu, 6 Feb 2014, Glenn Randers-Pehrson wrote:

If you want the PNG to supply the density, you have to supply both
density and units to the PNG encoder when creating the PNG.

Regardless, the PDF coder has an obvious problem in that it is not
using the image resolution at all. Instead it is processing ImageInfo
density (from -density option). Clearly this is due to a
misunderstanding by whomever wrote this bit of code. The fix seems
easy enough but requires care to make sure it is completely correct.

[Bob Friesenhahn]

Fixed by Mercurial changeset 14025:403d2c2b081e