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file size get bigger while sampling down

Posted: 2011-06-24T20:01:12-07:00
by yorua007
'The out put of 'identify -verbose' of the original file is:

Code: Select all

Image: bandpass_EOF-6.png
  Format: PNG (Portable Network Graphics)
  Class: DirectClass
  Geometry: 2082x1927+0+0
  Resolution: 300x300
  Print size: 6.94x6.42333
  Units: Undefined
  Type: TrueColor
  Endianess: Undefined
  Colorspace: RGB
  Depth: 8-bit
  Channel depth:
    red: 8-bit
    green: 8-bit
    blue: 8-bit
  Channel statistics:
    red:
      min: 0 (0)
      max: 255 (1)
      mean: 232.503 (0.911775)
      standard deviation: 68.5806 (0.268944)
      kurtosis: 6.59494
      skewness: -2.88456
    green:
      min: 0 (0)
      max: 255 (1)
      mean: 232.503 (0.911775)
      standard deviation: 68.5806 (0.268944)
      kurtosis: 6.59494
      skewness: -2.88456
    blue:
      min: 0 (0)
      max: 255 (1)
      mean: 234.317 (0.91889)
      standard deviation: 66.1858 (0.259552)
      kurtosis: 7.58625
      skewness: -3.05019
  Image statistics:
    Overall:
      min: 0 (0)
      max: 255 (1)
      mean: 174.831 (0.68561)
      standard deviation: 116.773 (0.457933)
      kurtosis: -1.33604
      skewness: -0.798471
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    ps:Level: Adobe-3.0 EPSF-3.0

    ps:SpotColor-0: Helvetica-Bold
    signature: 4576e3c0a629022836d213b5398237e733c42101b78bdcf5a6612753976ab350
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 366KiB
  Number pixels: 3.826MiB
  Pixels per second: 42.51MiB
  User time: 0.090u
  Elapsed time: 0:01.089
  Version: ImageMagick 6.5.7-8 2010-12-02 Q16 http://www.imagemagick.org
After using the command:

Code: Select all

convert bandpass_EOF_6.png -resample 150 -depth 8 bandpass_EOF-5_1.png
the file size changed from 366kB to 438kB which I didn't expect, and I can not figure it out why.

The output of 'identify -verbose' of the new file is :

Code: Select all

Image: bandpass_EOF-5_1.png
  Format: PNG (Portable Network Graphics)
  Class: DirectClass
  Geometry: 1041x964+0+0
  Resolution: 150x150
  Print size: 6.94x6.42667
  Units: Undefined
  Type: TrueColor
  Endianess: Undefined
  Colorspace: RGB
  Depth: 8-bit
  Channel depth:
    red: 8-bit
    green: 8-bit
    blue: 8-bit
  Channel statistics:
    red:
      min: 0 (0)
      max: 255 (1)
      mean: 231.746 (0.908808)
      standard deviation: 63.9992 (0.250977)
      kurtosis: 6.27581
      skewness: -2.77255
    green:
      min: 0 (0)
      max: 255 (1)
      mean: 231.746 (0.908808)
      standard deviation: 63.9992 (0.250977)
      kurtosis: 6.27581
      skewness: -2.77255
    blue:
      min: 0 (0)
      max: 255 (1)
      mean: 233.636 (0.916219)
      standard deviation: 61.8409 (0.242513)
      kurtosis: 7.27206
      skewness: -2.94106
  Image statistics:
    Overall:
      min: 0 (0)
      max: 255 (1)
      mean: 174.282 (0.683459)
      standard deviation: 114.583 (0.449347)
      kurtosis: -1.31453
      skewness: -0.79118
  Rendering intent: Undefined
  Interlace: None
  Background color: white
  Border color: rgb(223,223,223)
  Matte color: grey74
  Transparent color: black
  Compose: Over
  Page geometry: 1236x1061+73+69
  Origin geometry: +73+69
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2011-06-25T09:17:42+08:00
    date:modify: 2011-06-25T09:17:25+08:00
    ps:HiResBoundingBox: 593x509+0+166
    ps:Level: Adobe-3.0 EPSF-3.0

    ps:SpotColor-0: Helvetica-Bold
    signature: a9dff7db30d5529478d04e51625e2bcf8049b5c0784e1c748f9c47c8f5662730
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 438KiB
  Number pixels: 980KiB
  Pixels per second: 23.93MiB
  User time: 0.040u
  Elapsed time: 0:01.039
  Version: ImageMagick 6.5.7-8 2010-12-02 Q16 http://www.imagemagick.org

Re: file size get bigger while sampling down

Posted: 2011-06-24T20:15:32-07:00
by anthony
Simply put the image becomes more complex, and does not compress as well.

For example a binary image that is downsampled (made smaller) will get grayscale values that will nor compress as well as the original binary picture.

This does not happen for real-life images, which are complex to start with, but happens very often with artificially generated images.

Without more information (such as a sample) we can not help you more.

You may also like to look at the recent discussion about GIMP images being smaller than IM images.
which looks at PNG compression results.
viewtopic.php?f=1&t=18928

Re: file size get bigger while sampling down

Posted: 2011-06-24T23:16:42-07:00
by yorua007
Thanks for your quick reply. In fact the "bandpass_EOF-6.png" was convert from a eps file:
http://dl.dropbox.com/u/16633290/bandpass_EOF-1.eps
with the command:

Code: Select all

convert -density 300 bandpass_EOF-1.eps -trim -depth 8 bandpass_EOF-6.png
What I want is a png file with 150x150 resolution. I use the option '-density 300', because I think the result may could be better using the "super sampling" technique mentioned in http://www.imagemagick.org/Usage/text/#postscript. However I noticed that the file size got much bigger using this technique.

Re: file size get bigger while sampling down

Posted: 2011-06-24T23:53:02-07:00
by anthony
The output from ghostscript conversion to a image format that is highly aliased. Supersampling is well recommended.

However if the EPS is only a wrapper around a raster image, somehow extracting that raster at the 'ideal' resolution, would be a better option.