Hue / Saturation / Luminosity / Selective color

[snibgo]

Your command converts RGB (3 channel) input to CMYK (4 channel), then converts that to gray (3 channel). I don't know why you go via CMYK.

On v6.9.2-5:

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convert im3.png -colorspace Gray im4.png -clut x1.png

convert im3.png -colorspace CMYK -colorspace Gray im4.png -clut x2.png

x1.png and x2.png are identical.

EDIT: My copy-paste removed the "-colorspace Gray" in the first command.

[varape]

Re Snibgo.

Thanks for the information.
At start we tested it without CMYK before the greyscale and the result was not as good.
So we decided to use cmyk before and the result was clearly better.
So i dont know why we have a difference on this and not you but we do have a difference on our side.

Can you please post the image generated on your side ?
Thanks

[snibgo]

Sure:

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f:\web\im>%IM%convert im3.png -colorspace CMYK -colorspace Gray im4.png -clut clt_x1.png

f:\web\im>%IM%convert im3.png -colorspace Gray im4.png -clut clt_x2.png

f:\web\im>%IM%compare -metric RMSE clt_x1.png clt_x2.png NULL:
0 (0)

The files are:

[varape]

Thanks so much to have post the results here !

So in fact you get exactly the same result than us on the online server.
But not the same that the one we generated on local and that we are trying to get.
We are also going to test again the CMYK conversion, if the code is simpler, clearly better for server ressource.

But i really dont understand why on local we have a different result. That maybe means also that finally the result on local we get is not the good "normal" one we should get. So strange...

[fmw42]

His results seem to match your "perfect" results as far as I can see.

I get the same results as your "perfect" result from both of these without CMYK. Note I could not recompile IM 6.8.8.3 with PNG. So I converted your images to TIF and processed the TIF

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convert infile.tif -colorspace gray gradient.tif -clut -resize 602x602 -background white -gravity center outfile1.tif

im6883 convert infile.tif -colorspace gray gradient.tif -clut -resize 602x602 -background white -gravity center outfile2.tif

http://www.fmwconcepts.com/misc_tests/c ... tfile1.tif

http://www.fmwconcepts.com/misc_tests/c ... tfile2.tif

What API are you using? Is it Imagick? If so, try checking your phpinfo.php file and see if it tells you what version of IM it thinks you are using. Also check your version of Imagick and also be sure you have the same version of libpng on both systems. Perhaps you should post the API code you are using -- there might be an error in your commands. Might you have more than one version of Imagemagick on your system.

try

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type -a convert

or

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which -a convert

in a PHP exec(command).

To see if it is your API commands or an issue with PNG, try using TIF as I did. Does that work better or the same.

[varape]

Hello Fred

Ok, we check all this and come to you tomorrow.
But for the results of Snibgo, no it's not at all like the perfect result we posted.

Look the image below, you will see the green cross that show the difference of colors.
The top image is the perfect result we would like
The bottom image is the Snibgo one.



Thanks

[fmw42]

OK. Thanks. I see now. My (tif) results (using -colorspace CMYK or not) are the same as snibgo's and thus not your "perfect" result. I cannot say why that is happening except for the things you should check about versions of PNG and your API. Which API is it -- Imagick? If so, check to see if you have more than one version of IM on your system, if your libpng versions are the same and if your API code is correct. For the latter, try using PHP exec() or command line on both systems. Is that any better?

[snibgo]

My results, like Fred's, are not identical to the "perfect" results. This can be seen in the cyan, yellow and magenta "pipes" at the left and bottom. Our yellow pipe has become white, where the "perfect" result has a light gray. I don't know why this is.

To get a result that seems "perfect", use "-grayscale Rec709Luminance" instead of "-colorspace Gray". (The documentation http://www.imagemagick.org/script/comma ... #grayscale say these are the same, but it seems they are not the same.)

[fmw42]

To get a result that seems "perfect", use "-grayscale Rec709Luminance" instead of "-colorspace Gray". (The documentation http://www.imagemagick.org/script/comma ... #grayscale say these are the same, but it seems they are not the same.)

Good catch snibgo! I was just in the process of checking out the linear vs nonlinear issue when I saw your post. (Though I was (wrongly) trying to go the other way to make the old linear equivalent to the new non-linear gray using -set colorspace RGB and that did not seem to work). See viewtopic.php?f=4&t=21269 for a discussion of linear vs non-linear and the IM versions where things changed.

Aside: -colorspace gray currently should be the equivalent of Rec709Luma (not luminance) and used to be equivalent to Rec601luma and Rec601luminance at various times.

In the old 6.8.8.3 system, grayscale was linear (and probably equivalent to Rec601Luminance). So as you found out, to replicate that you need to use a linear grayscale in the current system to match the older system. I am pretty sure that -colorspace CMYK has nothing to do with this. IM will/should convert to the same grayscale image from RGB or from CMYK.

So try

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convert infile.png -grayscale rec709luminance gradient.png -clut -resize 602x602 -background white -gravity center outfile4.png

convert infile.png -grayscale rec601luminance gradient.png -clut -resize 602x602 -background white -gravity center outfile5.png

There will be slight difference, so try both to see which matches better.

The odd thing is that my im6883 result should not have matched my current IM (6926) result above and that was puzzling me.

[snibgo]

A quick test with hald:10 shows that, yes, "-colorspace Gray" is equivalent to "grayscale Rec709Luma", and no other.

Fred, can you correct http://www.imagemagick.org/script/comma ... #grayscale ?

[fmw42]

Fred, can you correct http://www.imagemagick.org/script/comma ... #grayscale ?

Yes, I will try to do that tomorrow.

[varape]

Hey Guys you simply rocks !

We did the test on local and online and it's just perfect
So much tanks to have took time to help people that needs it.
This forum is great, Fred, Snigbo, you are the best !

Just a thing to contribute, we tried the -grayscale but it seems to doesn't exists in 6.8.3-3 (on our local it didn't works).

Thanks again for you help; you rock !

[fmw42]

The -grayscale was introduce after 6.8.3-3 (about 6.8.3-10), so that when the grayscale linear was change to grayscale non-linear, there would be a way to go back to the linear format in more current versions of IM. In 6883, it is always linear and you must use -colorspace RGB to make grayscale images non-linear as directed in the Colorspace link I mentioned. But I had tried that briefly with no success. Then I saw user snigbo's post and realized that we needed to go in the other direction, so I did not follow up to try to make it work the other way as you seem to be trying.