Legacy ImageMagick Discussions Archive

I use Image Magick 7.0.8-Q16 on Windows 7 to convert big TIFF files.
The files are resized and the CMYK colorspace is assigned.
The new TIFF is a little bit brighter than same file converted in Photoshop.
What can I do? Thank you in advance!

I suggest you use either "-colorspace" or "-profile", but not both.

Different software will have different resizing code, so I would expect some difference, but probably not noticeable for ordinary photographs. Please quantify your finding of "a little bit brighter".

What is your Original.tif? CMYK, sRGB, etc? Does it have an embedded profile? If it is some form of RGB with no embedded profile, then that is the problem. The cure is to assign a correct RGB profile, and convert it to a CMYK profile.

I would suggest you resize the RGB image first, then convert to cmyk with a CMYK profile. You may need two profiles -- the first is sRGB.icc and the second is CMYK (usually USWebCoatedSWOP.icc), if your input RGB tiff has no color profile.

OK I use only the -profile command. And resize the RGB image first, then convert to cmyk with the CMYK profile.
So I have done many tests but without success. The IM Image is a little bit brighter than the PS image.
Please can you have a look to my images. Thank you for your help!

Original TIF: https://www.dropbox.com/s/nulz2bu74krca ... g.tif?dl=0
Photoshop result: https://www.dropbox.com/s/9sr1em2xfms8l ... S.tif?dl=0
ImageMagick result: https://www.dropbox.com/s/4x9j0o1ygxcln ... M.tif?dl=0

Your input has an embedded profile, so your IM command isn't assigning a new profile but converting to the new profile. If you assign in Photoshop, that's the problem.

Your PS and IM versions use different ICC profiles, so that will create a numeric difference. It will also create a visual difference when resizing.

Numerically, the IM result is a bit lighter. The mean CMYK (on a scale of 0.0 to 1.0) for IM is 0.268561, and for PS is 0.276897. The difference is 0.008336, and is just visible in this example.

Do you get this difference only if you resize?

I get the difference also when I am not resize.
So the problem is the ICC profile.
But what can I do, to get same result as in Photoshop?

Harald wrote:But what can I do, to get same result as in Photoshop?

For a start, do the same operations with the same data. Your IM command converts the profile to a new one. Ensure your Photoshop process also converts to a new profile, and use the same one as IM.

I use also Convert to profile in Photoshop. And I get the difference.

In Photoshop Convert to profile dialog;
Source colorspace: eciRGB v2
Target colorspace: ISO coated v2 300% (ECI)
Engine: Adobe (ACE)
Intent: Relative Colormetric
Checked Use Black Point compensation
Checcked Use dither

Maybe IM use different conversion options?

Harald wrote:Intent: Relative Colormetric
Checked Use Black Point compensation

But you didn't use those settings in IM.

Harald wrote:Checcked Use dither

IM can't dither when converting to a new profile.

OK, I think this is the solution. I convert in PS without dither and now I get the same result as in IM.
I will do more tests tomorrow.

Why did IM not support dither when converting to a new profile?
Maybe a stupid question but I am not an expert.

IM delegates profile work to LCMS. I don't know if LCMS can dither when converting to a new profile.

I'm not sure that dithering when converting to a new profile is a good idea. It reduces accuracy, so it seems more sensible to defer this until the final step of image processing, if you need it at all.