[magick-users] Re: Trouble converting 300dpi jpeg to 72dpi jpeg
with same size
Michał Skalski
mskalski at enigma.com.pl
Tue Nov 13 06:07:11 PST 2007
I tried (on Win2k, Version: ImageMagick 6.2.2 05/04/05 Q8
http://www.imagemagick.org)
convert 32.jpg -density 72x72 32-2.jpg
and this does what you want (I checked in properties of file in Windows
Explorer)
Note, that dpi is only a hint for printing software how many pixels it
should 'stretch' on one inch on a sheet of paper and is written to
header of JPEG file - it doesn't have any special meaning (or it should
not have) for normal raster image processing. If you only want to write
data do header data, you can try other software for processing EXIF data
in JPEG files, but for me ImageMagick sets this density accordingly.
Check excellent site of Anthony: http://www.imagemagick.org/Usage - I
think I saw something about density on this site.
Below is summary of files I converted (note Resolution)
File 32.jpg is original one, without saved any density information.
Files 32-1.jpg and 32-2.jpg are almost binary identical except for four
bytes where resolution is written.
c:\>convert 32.jpg -density 300x300 32-1.jpg
c:\>convert 32.jpg -density 72x72 32-2.jpg
c:\>identify -verbose 32.jpg
32.jpg JPEG 348x498 DirectClass 24kb 0.060u 0:01
Image: 32.jpg
Format: JPEG (Joint Photographic Experts Group JFIF format)
Geometry: 348x498
Class: DirectClass
Type: TrueColor
Endianess: Undefined
Colorspace: RGB
Channel depth:
Red: 8-bits
Green: 8-bits
Blue: 8-bits
Channel statistics:
Red:
Min: 0 (0)
Max: 255 (1)
Mean: 146.259 (0.573566)
Standard deviation: 84.3146 (0.330646)
Green:
Min: 0 (0)
Max: 255 (1)
Mean: 156.33 (0.61306)
Standard deviation: 86.979 (0.341094)
Blue:
Min: 0 (0)
Max: 255 (1)
Mean: 155.334 (0.609152)
Standard deviation: 93.7502 (0.367648)
Colors: 67613
Rendering-intent: Undefined
Resolution: 72x72
Units: Undefined
Filesize: 24kb
Interlace: None
Background Color: white
Border Color: #DFDFDF
Matte Color: grey74
Dispose: Undefined
Iterations: 0
Compression: JPEG
Quality: 50
Orientation: Undefined
JPEG-Colorspace: 2
JPEG-Sampling-factors: 2x2,1x1,1x1
Signature:
d44cdde450ead13fd0844ac92ee82c9c02539a891e85a3fcb0ddc06fb80eebe1
Tainted: False
Version: ImageMagick 6.2.2 05/04/05 Q8 http://www.imagemagick.org
c:\>identify -verbose 32-1.jpg
32-1.jpg JPEG 348x498 DirectClass 24kb 0.040u 0:01
Image: 32-1.jpg
Format: JPEG (Joint Photographic Experts Group JFIF format)
Geometry: 348x498
Class: DirectClass
Type: TrueColor
Endianess: Undefined
Colorspace: RGB
Channel depth:
Red: 8-bits
Green: 8-bits
Blue: 8-bits
Channel statistics:
Red:
Min: 0 (0)
Max: 255 (1)
Mean: 146.178 (0.573249)
Standard deviation: 84.2115 (0.330241)
Green:
Min: 0 (0)
Max: 255 (1)
Mean: 156.308 (0.612974)
Standard deviation: 86.8731 (0.340679)
Blue:
Min: 0 (0)
Max: 255 (1)
Mean: 155.215 (0.608686)
Standard deviation: 93.5585 (0.366896)
Colors: 68074
Rendering-intent: Undefined
Resolution: 300x300
Units: PixelsPerInch
Filesize: 24kb
Interlace: None
Background Color: white
Border Color: #DFDFDF
Matte Color: grey74
Dispose: Undefined
Iterations: 0
Compression: JPEG
Quality: 50
Orientation: Undefined
JPEG-Colorspace: 2
JPEG-Sampling-factors: 2x2,1x1,1x1
Signature:
d1641c9ce7e06436e7d8e686a12c3565d1754194b7e2f73adb8febc730dfe2c7
Tainted: False
Version: ImageMagick 6.2.2 05/04/05 Q8 http://www.imagemagick.org
c:\>identify -verbose 32-2.jpg
32-2.jpg JPEG 348x498 DirectClass 24kb 0.050u 0:01
Image: 32-2.jpg
Format: JPEG (Joint Photographic Experts Group JFIF format)
Geometry: 348x498
Class: DirectClass
Type: TrueColor
Endianess: Undefined
Colorspace: RGB
Channel depth:
Red: 8-bits
Green: 8-bits
Blue: 8-bits
Channel statistics:
Red:
Min: 0 (0)
Max: 255 (1)
Mean: 146.178 (0.573249)
Standard deviation: 84.2115 (0.330241)
Green:
Min: 0 (0)
Max: 255 (1)
Mean: 156.308 (0.612974)
Standard deviation: 86.8731 (0.340679)
Blue:
Min: 0 (0)
Max: 255 (1)
Mean: 155.215 (0.608686)
Standard deviation: 93.5585 (0.366896)
Colors: 68074
Rendering-intent: Undefined
Resolution: 72x72
Units: PixelsPerInch
Filesize: 24kb
Interlace: None
Background Color: white
Border Color: #DFDFDF
Matte Color: grey74
Dispose: Undefined
Iterations: 0
Compression: JPEG
Quality: 50
Orientation: Undefined
JPEG-Colorspace: 2
JPEG-Sampling-factors: 2x2,1x1,1x1
Signature:
d1641c9ce7e06436e7d8e686a12c3565d1754194b7e2f73adb8febc730dfe2c7
Tainted: False
Version: ImageMagick 6.2.2 05/04/05 Q8 http://www.imagemagick.org
Nicolas Dhomont pisze:
> Thanks for your answer Michal.
>
> I tried :
>
> convert -units PixelsPerInch fromFile -density 72x72 toFile
>
> But 'toFile' is still in 300dpi. 'toFile' file size is even more
> bigger than 'fromFile' file size. lol
>
> Nicolas
>
> Michał Skalski a écrit :
>> Try -density option instead of -resample
>>
>> Michał
>>
>> Nicolas Dhomont pisze:
>>> Hello
>>>
>>>
>>> I'm trying to convert 300dpi CYMK jpeg to 72dpi RGB jpeg
>>>
>>> I already manage to convert from CYMK to RGB by doing the following :
>>>
>>> convert fromFile -antialias -colorspace RGB toFile
>>>
>>> I'm now trying to convert from 300 dpi to 72dpi with the following :
>>>
>>> convert -units PixelsPerInch fromFile -colorspace RGB -antialias
>>> -resample 72x72 toFile
>>>
>>> Size of toFile is reduced by a 4,16 ratio (= 300 / 72) but still in
>>> 300 dpi. (It means that if fromFile is 2.66inch wide, toFile is
>>> 2.08inch wide for exemple). But I want exactly the opposite : a file
>>> in 72 dpi with same size.
>>> I've also played with the -density option unsuccessfully.
>>>
>>> As converting is quite simple with photoshop, I guess convert can do
>>> same but I can't find out how.
>>>
>>>
>>>
>>> Any help would be highly appreciated
>>>
>>> Nicolas
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