GraphicsMagic is unable to identify background transparency of PDF

Question

GM is unable to identify background transparency of PDF and PNG created using "gm convert" gets white background while same PDF is converted to PNG with transparent background by IM.

$convert -verbose /var/tmp/abc.pdf /var/tmp/abc.png

/var/tmp/magick-16370Tq7WYv5U54Pa1 PNG 288x720 288x720+0+0 8-bit sRGB 20.7KB 0.000u 0:00.009
/var/tmp/abc.pdf PDF 288x720 288x720+0+0 16-bit sRGB 20.7KB 0.000u 0:00.000
/var/tmp/abc.pdf=>/var/tmp/abc.png PDF 288x720 288x720+0+0 8-bit sRGB 17c 16.6KB 0.010u 0:00.009
[ghostscript library] -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pngalpha" -dTextAlphaBits=4 -dGraphicsAlphaBits=4 "-r72x72" "-sOutputFile=/var/tmp/magick-16370Tq7WYv5U54Pa%d" "-f/var/tmp/magick-16370CVWmPbzBmjpF" "-f/var/tmp/magick-16370khy6Y-G3TgtO"

$gm convert -verbose /var/tmp/abc.pdf /var/tmp/abc.png

gm convert: "gs" "-q" "-dBATCH" "-dMaxBitmap=50000000" "-dNOPAUSE" "-sDEVICE=pnmraw" "-dTextAlphaBits=4" "-dGraphicsAlphaBits=4" "-r72x72" "-sOutputFile=/var/folders/6d/n_hv45rs1jv17nxwfjwj776cspn_3c/T/gmoCp6rG" "--" "/var/folders/6d/n_hv45rs1jv17nxwfjwj776cspn_3c/T/gmBEgWnK" "-c" "quit".
/var/tmp/abc.pdf PDF 288x720+0+0 DirectClass 8-bit 607.6K 0.000u 0:01
/var/tmp/abc.pdf=>/var/tmp/abc.png PNG 288x720+0+0 DirectClass 8-bit 0.000u 0:01
Upon further investigation, it seems "identify" from IM can correctly identify background in PDF but "gm identify" from GM cannot.

$identify -verbose abc.pdf

Image: abc.pdf
Format: PDF (Portable Document Format)
Type: Bilevel
Colorspace: Gray
Depth: 16/4-bit
Channel depth:
gray: 1-bit
alpha: 4-bit
Alpha: graya(255,0) #FFFFFFFFFFFF0000
Colors: 16
Background color: graya(255,1)
Transparent color: graya(0,0)
Version: ImageMagick 6.8.9-1 Q16 x86_64 2014-07-01 http://www.imagemagick.org

$gm identify -verbose abc.pdf

gm identify: "gs" "-q" "-dBATCH" "-dMaxBitmap=50000000" "-dNOPAUSE" "-sDEVICE=pnmraw" "-dTextAlphaBits=4" "-dGraphicsAlphaBits=4" "-r72x72" "-sOutputFile=/var/folders/6d/n_hv45rs1jv17nxwfjwj776cspn_3c/T/gmzhBEIk" "--" "/var/folders/6d/n_hv45rs1jv17nxwfjwj776cspn_3c/T/gmAPm2Po" "-c" "quit".
Image: abc.pdf
Format: PDF (Portable Document Format)
Type: grayscale
Depth: 4 bits-per-pixel component
Channel Depths:
Gray: 4 bits
Background Color: white
Comment: Image generated by GPL Ghostscript (device=pnmraw)
Signature: 215f1c08ec575526ce398d193c4df22faaea100c10255e0db747641bdaaeac49
Tainted: False

link to sourceforge: sourceforge.net/p/graphicsmagick/discussion/250738/thread/… — Ankit Khanal

Kurt Pfeifle Kurt Pfeifle · Accepted Answer · 2014-07-31T12:13:20

The reason why your (ImageMagick) convert and your (GraphicsMagick) gm convert commands produce different output is this:

Both utilities are NOT able to process PDF input files directly, both can only handle rater image formats.
In order to process PDF input files, both utilities resort to a 'delegate' program: in both cases this is Ghostscript (which CAN process PDF input files).
Both utilities however do use different 'delegate command lines' (as can be directly seen in your quoted -verbose commandline outputs:

i. convert employs as its Ghostscript output device pngalpha.

ii. gm convertemploys as its Ghostscript output device pnmraw.
Both utilities then process the output of their delegate's command into the final (raster) format file.

The problem is: the raster format 'pnmraw' does not support transparency (an alpha channel), but 'pngalpha' does. Hence, the utility which first converts PDF input to pnmraw has lost the transparent page backgrounds and replaced them by (opaque) white backgrounds.

Unless you modify your GraphicsMagick setup to make it use pngalpha in its delegate command (the same as ImageMagick uses) your gm convert will not show transparent background.

GraphicsMagic is unable to identify background transparency of PDF

2 Answers