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ImageMagick resizer color/profile consistency


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What are your experiences with profile/color consistency when using Image Magick for resizing?
I know @horst is probably the person with the most experience on this, (hope you can chime in here Horst)

Problem:
I noticed that In some cases Image Magic resized jpgs appear darker than the original, and after some digging it appears to involve various factors concering both the image preparation (Photoshop's save for web and even general profile handling before that) and the way the Image Magic resizing process is setup.

- Images with an embedded (srgb) profile that were exported via Photoshop's "save for web" function with "convert to srgb" and "embed profile" ON, somehow result in muted colors and a darker image, (actually it looks very much like when you would assign an srgb profile to an image that was already converted to srgb before, not dramatic, but quite noticeable with e.g. reds and cyans.)

- I tried multiple variations, with embedded and excluded srgb profile, "convert to srgb" on and off, but the result appears to be the same darker, muted image. Need to find time to do more structured testing though..

Possible causes:
- The way the srgb profile is embedded in the jpg
- The way the Imagick module detects/ignores profiles
- Colorspace handling changes between imagick versions
- One of the above plus these rather involved technical reccomendations (tldr: convert to linear RGB, resize, convert back)
http://www.4p8.com/eric.brasseur/gamma.html#ImageMagick
http://www.imagemagick.org/Usage/resize/#resize_colorspace

Would be nice to get a discussion going here. I am out of my league with this, technical knowledge wise but I'll try to keep up :)

Cheers guys,

Phil

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2 hours ago, phil_s said:

Images with an embedded (srgb) profile that were exported via Photoshop's "save for web" function with "convert to srgb" and "embed profile" ON, somehow result in muted colors and a darker image

When exporting web images (assets from design mockups) I follow this guide and it works great: https://www.viget.com/articles/the-mysterious-save-for-web-color-shift

  • Working Space: sRGB
  • Proof Setup: Monitor RGB
  • Proof Colors: On (doesn't affect export - just ensures you see the same colour in Photoshop as you will in the browser)
  • Save For Web
    • Embed Color Profile: unchecked
    • Convert to sRGB: unchecked
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hey @phil_s,

we already follow the rules you linked too, with a bit more individuality:

But it looks like your local workflow isn't correct. I suggest to read your first link to brasseur, but in regard of Photoshop and how it should be used.

Using photoshop in short:

  • don't use "save for web"!
  • only work in 16 bit depth mode! Everytime!! (No, - no exceptions!)
  • At the last step, procede reduce the color depth from 16 bit to 8 bit, but only for variations that you want to share / upload, etc! KEEP your 16 bit ORIGINAL locally!
  • If you don't have original images in 16bit, first step after opening it in photoshop is, to increase the colordepth from 8 to 16. Then do all your stuff, and at last step, reduce the colordepth back to 8 bit.

For using images as a master source in PW, I highly suggest(ed) to save images in Photoshop as jpegs in 12 (=100%) quality. (NOT with saving for web, just the regular "save as jpeg" method). Don't deliver your original image in PW, never. If you need it in its original dimensions, call it with $image->width($image->width). This way, you get a variation with original dimensions but lower quality, your default image quality. Or pass an array with different quality setting with the image to the pageimage / imagesizer.

EDIT:
If you embedd an ICC profile or not, is not relevant. But you need to use / have the image in sRGB!! This is assumed by all web image render engines. GD, for example, isn't able to read ICC profiles. Some IMagick installations also don't / cannot.

Edited by horst
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13 hours ago, horst said:

we already follow the rules you linked too, with a bit more individuality:

  • look for a gamma setting shiped with the image:
  • if there isn't one available, assume default value:
  • go into 16bit mode (if available in this imagick engine. there are also only 8bit engines!)
  • now do all imagemanipulations, ...
  • and recalculate and set gamma back
  • and set back the colordepth to 8 or lower, if the original has lower depth

Makes sense, thanks for the detailed explanation!
Can you elaborate on what the profile check / assign @line 214/215/216 does? Is this only for checking if the image profile is Grayscale?

What caught my eye initially in the brasseur guide was the following part, at the end of the gamma section:

Quote

Now about the enhanced and the newer features. Starting from ImageMagick 6.7.5-1 you can use better than the -gamma parameter. The -colorspace parameter allows to truly switch from sRGB colorspace to linear RGB colorspace and back to sRGB. Like this:
 

convert in.jpg -colorspace RGB -resize 50% -colorspace sRGB out.jpg

This would imply that there is a more accurate and I would guess more convenient way to do the whole conversion dance?

Quote

 

  • don't use "save for web"!
  • only work in 16 bit depth mode! Everytime!! (No, - no exceptions!)
  • At the last step, procede reduce the color depth from 16 bit to 8 bit, but only for variations that you want to share / upload, etc! KEEP your 16 bit ORIGINAL locally!
  • If you don't have original images in 16bit, first step after opening it in photoshop is, to increase the colordepth from 8 to 16. Then do all your stuff, and at last step, reduce the colordepth back to 8 bit.

 

This sounds perfectly reasonable, except for the ubiquitous "don't use save for web" caveat. "Save for web" removes the EXIF data, removes attached image profiles, reduces the image to 8bit if >8bit, and converts it to sRGB. You could create an action that does this for you, except save for web is much easier to explain to clients.

FWIW since we're comparing notes: I'm using Lightroom/CaptureOne for basic corrections, further edits / archival files are 16 Bit .tiffs rendered in prophoto RGB/Adobe RGB depending on the source, which I manually convert so srgb (usually since I want to control the rendering intent of the conversion), before using "save for web" with neither "convert to srgb" nor "embed profile" checked, which leaves me with a clean jpg with no profiles or meta data embedded/attached. (I use itpc for tagging and cataloging and I don't want these to show up in the web version). 

Quote

EDIT:
If you embedd an ICC profile or not, is not relevant. But you need to use / have the image in sRGB!! This is assumed by all web image render engines. GD, for example, isn't able to read ICC profiles. Some IMagick installations also don't / cannot.

Very true. This also means that with GD, any image NOT converted to sRGB will still be interpreted as such. If you forget to do this with wide gamut profiles (e.g. Prophoto) it will lead to quite funky effects :)

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6 minutes ago, phil_s said:

Can you elaborate on what the profile check / assign @line 214/215/216 does? Is this only for checking if the image profile is Grayscale?

yes, it first checks if we have a grayscale image. If yes, we use grayscale as workspaceColorspace, otherwise \IMagick::COLORSPACE_SRGB.

If you want to test it out, you also can change this in line 184 and use \IMagick::COLORSPACE_RGB (without the s! / sRGB and RGB)

Also, if you feel adventures (==abenteuerlich?), you may test with disabled and enabled gamma conversion. Maybe you can experience some differences?

To test without gamma correction, simply comment out the line 246. I'm interested in seeing / discussing anything you find. :)

-----

The truth with Imagick is, at least with the PHP extension, that you cannot use any version check for detecting features. There are so many possibilities to compile it. If there is one thing not supported, the php extension simply seems to omitt that and silently return a 0. :(

And with the CLI extension, (using convert), I tried to include all needed manipulations into one call. If we would call it multiple times, to determine some image attributes, it would to much slow down the process.

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