[SOLVED] PNG to JPG conversion: Resolution oddities?

[mhulse]

(IM version in my signature)

Before conversion of a PNG to JPG, in Photoshop, I see:

5"x7.5"
300ppi

When I use this command:

Code: Select all

convert image.png -flatten -density 72 -depth 8  -quality 100 -units PixelsPerInch image.jpg

Photoshop shows me:

8.242"x12.363"
182ppi

Just curious why the images is 182ppi?

What I was expecting to see:

20.833"x31.25"
72ppi

Thanks!

[fmw42]

I think PS may keep its own fields for density and units and ignores those of IM.

[mhulse]

I think PS may keep its own fields for density and units and ignores those of IM.

Ahhhh, interesting!

Code: Select all

identify -verbose image.jpg
Image: image.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 1500x2250+0+0
  Resolution: 182x182
  Print size: 8.24176x12.3626
  Units: PixelsPerInch
  Type: TrueColor
  Endianess: Undefined
  Colorspace: sRGB
  Depth: 8-bit
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
  Channel statistics:
    Pixels: 3375000
    Red:
      min: 0 (0)
      max: 255 (1)
      mean: 65.8326 (0.258167)
      standard deviation: 50.1159 (0.196533)
      kurtosis: 2.71615
      skewness: 1.80886
      entropy: 0.853981
    Green:
      min: 0 (0)
      max: 255 (1)
      mean: 54.7403 (0.214668)
      standard deviation: 47.045 (0.18449)
      kurtosis: 2.34124
      skewness: 1.68187
      entropy: 0.830866
    Blue:
      min: 0 (0)
      max: 255 (1)
      mean: 43.6316 (0.171104)
      standard deviation: 49.5595 (0.194351)
      kurtosis: 2.39981
      skewness: 1.76849
      entropy: 0.812539
  Image statistics:
    Overall:
      min: 0 (0)
      max: 255 (1)
      mean: 54.7348 (0.214646)
      standard deviation: 48.925 (0.191863)
      kurtosis: 2.76248
      skewness: 1.76448
      entropy: 0.832462
  Rendering intent: Perceptual
  Gamma: 0.454545
  Chromaticity:
    red primary: (0.64,0.33)
    green primary: (0.3,0.6)
    blue primary: (0.15,0.06)
    white point: (0.3127,0.329)
  Alpha color: grey74
  Background color: white
  Border color: srgb(223,223,223)
  Transparent color: none
  Interlace: None
  Intensity: Undefined
  Compose: Over
  Page geometry: 1500x2250+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: JPEG
  Quality: 100
  Orientation: Undefined
  Properties:
    date:create: 2017-02-13T20:47:08-08:00
    date:modify: 2017-02-13T20:47:08-08:00
    jpeg:colorspace: 2
    jpeg:sampling-factor: 1x1,1x1,1x1
    signature: 6dcb71004d2a4c440ef1f3f461b74a3dd548e5d9a10801c09d9e7012788d9c6a
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 2.492MB
  Number pixels: 3.375M
  Pixels per second: 30.68MB
  User time: 0.110u
  Elapsed time: 0:01.110
  Version: ImageMagick 7.0.4-7 Q16 x86_64 2017-02-04 http://www.imagemagick.org

Oddly, looks like there's 182x182 in that output?

Thanks for your help Fred!

[fmw42]

If you post your PNG input file, I can test your conversion and open it in PS to confirm. You can upload to any free hosting service such as dropbox.com and put the URL here.

The issue may be that PNG only supports dots per centimeter and IM or PS may be automatically converting to dots per inch. Look at your input PNG in IM and PS and see if they are showing units the same?

[mhulse]

Awesome! Thank you so much for your help and time, I really appreciate it!

https://dl.dropboxusercontent.com/u/127 ... hecale.png

I obtained the full resolution image from here:

https://commons.wikimedia.org/wiki/File ... hecale.png

Thank you again for you help!

Btw, do you have a paypal donation page or email? I'd love to shoot you a small cash payment. You have been so helpful every time I have posted a question. I'm not rich, but would love to give back a little to show my thanks.

[fmw42]

Imagemagick says:


Resolution: 118.11x118.11
Print size: 12.7x19.05
Units: PixelsPerCentimeter


Photoshop says:

W=5 in
H=7.5 in
Res=299.999 pixels per inch

But you can change the units to cm in PS Image Size window.

W=12.7 cm
H=19.05 cm
Res=118.11 pixels per cm

So in fact they are equivalent.

[mhulse]

Interesting!

Thank you for checking that! Great observation.

So, this (post-conversion):

8.242"x12.363"
182ppi

Is actually 71.654 per cm!

So, if I understand correctly, the original is in pixels per cm, and after conversion, it's still using pixels per cm?

I guess I assumed using:

Code: Select all

-units PixelsPerInch

Would normalize the units? Is the "pixels per cm" hard-baked into the original image?

Maybe I should not worry about this, as PPI and PPCM are technically equivalent (that is, looking at the numbers).

[fmw42]

You told the output you wanted it to be 72 dpi. So IM converted from pixelspercm to the corresponding pixelsperin and set the density to 72.

I think 71.654 is just a precision error in the display of the data.

[mhulse]

You told the output you wanted it to be 72 dpi. So IM converted from pixelspercm to the corresponding pixelsperin and set the density to 72.

I think 71.654 is just a rounding error in the display of the data.

Ahhhh, interesting! Thank you for the clarification.

I guess I assumed that I would see 72ppi instead of 182ppi. I think I need to contemplate the math here … I think I get where this is going, but I am not fully connecting the dots. Again, this is just me being dense.

Thanks again for the help Fred, I greatly appreciate it!!!!

[fmw42]

If you leave off density 72, then you get:

Your original PNG has:
Resolution: 118.11x118.11
Print size: 12.7x19.05
Units: PixelsPerCentimeter

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convert 2_Heliconius_hecale.png -flatten -depth 8  -quality 100 -units PixelsPerInch image1.jpg

Resolution: 300x300
Print size: 5x7.5
Units: PixelsPerInch

The resolution changes to 300 because 118 px/cm = 118*2.54 px/in = 300 px/in

If you add -density 72, you get:

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convert 2_Heliconius_hecale.png -flatten -depth 8  -quality 100 -density 72 -units PixelsPerInch image1.jpg

Resolution: 182x182
Print size: 8.24176x12.3626
Units: PixelsPerInch

The resolution changes to 182 because you changed the density to 72 px/cm 72*2.54 px/in = 182.88 px/in

[mhulse]

Thank you for the help Fred! I owe you one.