Re: Fax compression in 6.0.8-3

(Cc:'d to the magick-developers list, as followups probably belong there)

On Fri, 1 Oct 2004, Rob Foehl wrote:

> I'm trying to convert a couple formats to TIFF for faxing using ImageMagick 
> 6.0.8-3, and I can't seem to specify any compression types. For example:
>
> $ convert -compress Fax -density 200x200 pdf:test.pdf tiff:test.tiff
> $ identify -verbose test.tiff
> Image: test.tiff
>  Format: TIFF (Tagged Image File Format)
>  Geometry: 1700x2200
>  Class: DirectClass
>  Colorspace: RGB
>  Type: Palette
>  Depth: 8 bits
>  Endianess: MSB
> [...]
>  Rendering-intent: Undefined
>  Resolution: 200x200
>  Units: Undefined
>  Filesize: 10.7mb
>  Interlace: None
>  Background Color: white
>  Border Color: #DFDFDF
>  Matte Color: grey74
>  Dispose: Undefined
>  Iterations: 0
>  Compression: None
>  Orientation: TopLeft
>  Software: ImageMagick 6.0.8 10/01/04 Q16 http://www.imagemagick.org
>  Document: test.tiff
>  Comment:  Image generated by GPL Ghostscript (device=pnmraw)
>
>
> The same command used to work in 5.5.x.  This is the first 6.0.x release I've 
> tested.  I found a possibly-related issue described here:
>
> http://studio.imagemagick.org/magick/viewtopic.php?t=2314
>
> No luck with either PerlMagick or the convert binary.

I tracked this one down...  It's losing the compression setting because 
the input image isn't monochrome; this is also mentioned in the URL above.

5.5.7 coders/tiff.c looks like this around line 1656:

      case FaxCompression:
      {
        SetImageType(image,BilevelType);
        compress_tag=COMPRESSION_CCITTFAX3;
        break;
      }

Whereas 6.0.8 looks like this, line 1447:

      case FaxCompression:
      {
        if (IsMonochromeImage(image,&image->exception) != MagickFalse)
          compress_tag=COMPRESSION_CCITTFAX3;
        break;
      }

So, explicitly converting the image to monochrome should "fix" this..

$ convert -compress Fax -density 200x200 -monochrome pdf:test.pdf 
tiff:test2.tiff
$ identify -verbose test2.tiff
Image: test2.tiff
  Format: TIFF (Tagged Image File Format)
  Geometry: 1700x2200
  Class: PseudoClass
  Colorspace: RGB
  Type: Bilevel
  Depth: 1 bits
  Endianess: MSB
[...]
  Histogram:
     69605: (  0,  0,  0)       black
   3670395: (255,255,255)       white
  Rendering-intent: Undefined
  Resolution: 200x200
  Units: Undefined
  Filesize: 27kb
  Interlace: None
  Background Color: white
  Border Color: #DFDFDF
  Matte Color: grey74
  Dispose: Undefined
  Iterations: 0
  Compression: Fax
  Orientation: TopLeft
  Software: ImageMagick 6.0.8 10/01/04 Q16 http://www.imagemagick.org
  Document: test2.tiff
  Comment:  Image generated by GPL Ghostscript (device=pbmraw)


My question now is, why was this behavior changed?  It seems natural that 
the image be converted to monochrome automatically (if necessary) when fax 
compression is requested, since it has to be monochrome; at the very 
least, this should probably generate an error rather than silently 
dropping the requested compression, IMHO.

> Also having a separate PerlMagick order of operations problem with density 
> settings.  The following code properly sets density in the output:
>
> my $img = Image::Magick->new();
> $img->Set(density => '200x200');
> $img->BlobToImage($pdf);
> $img->Write('tiff:testpl.tiff');
>
> Setting the density anywhere after the BlobToImage call does nothing. This 
> issue (or similar) was mentioned here:
>
> http://studio.imagemagick.org/pipermail/magick-bugs/2004-August/001827.html

Same question here still applies.

-Rob