问题
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
回答1:
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 devicepngalpha
.ii.
gm convert
employs as its Ghostscript output devicepnmraw
.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.
回答2:
Just wanted to add to Kurt Pfeifle answer since it pointed me to this solution. The configuration that he is referring to is found in the delegates.mgk file (graphicsmagick/1.3.19_1/lib/GraphicsMagick/config).
For me the issue was this line:
<!-- Read color Postscript, EPS, and PDF -->
<delegate decode="gs-color" stealth="True" command='"gs" -q -dBATCH -dMaxBitmap=50000000 -dNOPAUSE -sDEVICE=ppmraw -dTextAlphaBits=%u -dGraphicsAlphaBits=%u -r%s %s "-sOutputFile=%s" -- "%s" -c quit' />
I changed it to:
<delegate decode="gs-color+alpha" stealth="True" command='"gs" -q -dBATCH -dMaxBitmap=50000000 -dNOPAUSE -sDEVICE=pngalpha -dTextAlphaBits=%u -dGraphicsAlphaBits=%u -r%s %s "-sOutputFile=%s" -- "%s" -c quit' />
and my pngs come out with transparent backgrounds!
来源:https://stackoverflow.com/questions/24686623/graphicsmagic-is-unable-to-identify-background-transparency-of-pdf