PS into PDF with fax group 4 encoding: unexpected result

[akapuma]

Hello,

I'm trying to convert some PS files into PDF with fax group 4 encoding (monochrome with very good compression). I'm using the following command line:

gm convert -colors 2 -monochrome -compress group4 -density 300x300 -page A4 +dither in.ps out.pdf

(Remark: without "-colors 2", "green" becomes "white". Tested with IM).

Formerly, I used ImageMagick. The result is good, but it's very slow. Then, I tried GraphicsMagick. It's much faster, but the result is not OK:

the "+dither" - command is ignored
the pictue is not centered

For testing, I used the file "testpage.ps" from this site: http://anders.geekhouse.no/Testpage/

Can somebody help me?

Best regards

akapuma

[Bob Friesenhahn]

On Wed, 14 May 2014, akapuma wrote:

Hello,

I'm trying to covert some PS files into PDF with fax group 4 encoding (monochrome with very good compression).
I'm using the following command line:

gm convert -colors 2 -monochrome -compress group4 -density 300x300 -page A4 +dither in.ps out.pdf

(Remark: without "-colors 2", "green" becomes "white". Tested with IM).

Formerly, I used ImageMagick. The result is good, but it's very slow. Then, I tried GraphicsMagick. It's much
faster, but the result is not OK:

• the "+dither" - command is ignored
• the pictue is not centered

For Testing, I used the file "testpage.ps" from this site: http://anders.geekhouse.no/Testpage/

I am not sure about the centering but I achieve much better results by
arranging the arguments in a more suitable order so they are actually
used:

gm convert -density 300x300 in.ps +dither -colors 2 -monochrome
   -compress group4 -page A4 out.pdf

The reasining behind the reordering is

-density 300x300 is needed by Ghostscript to render in.ps at correct
resolution.

+dither influences -colors 2

If you don't mind doing the critital steps in Ghostscript, then this
will obtain vastly better throughput since Ghostscript will then
produce bilevel output directly:

gm convert -type bilevel -density 300x300 in.ps -compress group4
    -page A4 out.pdf

or perhaps this with Ghostscript producing Grayscale output rather
than RGB:

gm convert -type grayscale -density 300x300 in.ps +dither -colors 2
    -compress group4 -page A4 out.pdf

I see a lot of aliasing problems in the output due to using +dither.

Smoother output can be obtained by rendering at twice the density
needed and then resizing the image by 50%. Using +dither only seems
to cause harm to the output.

gm convert -type grayscale -density 600x600 in.ps -resample 300x300
    -colors 2 -compress group4 -page A4 out.pdf

gm convert -type grayscale -density 600x600 in.ps -resample 300x300
    -type bilevel -compress group4 -page A4 out .pdf

Bob

Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/