What happens when a channel gets separated?

[baxter stockman]

Hi,

I am trying to understand what happens when I use this command

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convert rose: -channel R -separate separate_red.gif

found here http://www.imagemagick.org/Usage/color_basics/#channels ?

As far as I understand it separates one channel which means the other two are set to

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0

and then grayscales that.

However, if I separate a channel by setting the two others to 0, a white pixel (255,255,255) doesn't become white, it becomes a perfect Red (255,0,0). When I grayscale it then after using

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R*0.2126 + G*0.7152 + B*0.0722

, it becomes (55,0,0) which isn't white.

The white background in the pictures I use however, always turns out white.

Where is my mistake?

Thanks!

[snibgo]

If an image is RGB, then "-separate" makes one image from each of the selected "-channel" names. Each image will be grayscale, with the same gray value that was in that channel.

So, "-channel R -separate" will make a grayscale image, by setting R=G=B= whatever was in the R channel. White will stay white because the R value was 255, and the new pixel becomes (255,255,255).

[fmw42]

In IM 6, it separates the channel and then duplicates it for the other 3 channels so that all 3 channels are the same and so it looks grayscale. In IM 7, there will be only one single grayscale channel.

EDIT: Sorry snibgo, we apparently were posting at the same time

[baxter stockman]

Ok, so IM6 doesn't use this forumula R*0.2126 + G*0.7152 + B*0.0722 ?


But how does that work with only one single grayscale channel? Will white still be white then?

[snibgo]

"-separate" doesn't use the formula R*0.2126 + G*0.7152 + B*0.0722, or any similar formula. It just copies the value found in the red channel, and uses that as the gray value. As IM6 typically processes three colour channels, it copies this same value into each of the new RGB channels.

In IM7, when there need by only one channel for gray images, the result should be exactly the same.

[baxter stockman]

Is there a difference in the file size then? It should be, the file in IM7 should be smaller, no?

[fmw42]

Is there a difference in the file size then? It should be, the file in IM7 should be smaller, no?

That is correct. It will be smaller in IM 7.

[snibgo]

Well, maybe, but the big difference will be in memory requirements, and perhaps in processing speed. IM 6 currently recognises when a image is grayscale, and makes the file smaller. But when it reads grayscale images, it creates three colour channels in memory, and processes them all.

[baxter stockman]

Ok, but I still don't understand how IM7 does that. I am trying to build a JS code that grayscales it and then takes one channel of the image just like here:

https://en.wikipedia.org/wiki/Grayscale ... lor_images

the problem that I have is everytime when I do it also turns the white background into gray. So I am wondering what's the formula for that?

[fmw42]

Yes, snibgo is correct. My mistake. When saving to a file, the channels may not be duplicated depending upon whether the file format supports grayscale or not.

[fmw42]

the problem that I have is everytime when I do it also turns the white background into gray. So I am wondering what's the formula for that?

Perhaps you should post an example image to some place such as dropbox.com and put the URL here, so we can see what you start with and what you end with. Then describe what you think you want it to look like. Perhaps you are not starting with an RGB image. Maybe it is CMYK, then the channel R would really be cyan. However, that would still have white remaining white.

This should work. Note I put +channels at the end to be safe.

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convert rose: -channel R -separate +channel separate_red.gif

[fmw42]

What version of IM and what platform are you using. If you are using an older version of IM, it might be at a time when IM was making grayscale linear gray and not sRGB gray. That would darken the image, though, still white should remain white, I think.

[baxter stockman]

Thank you so much for taking the time to explain. It's not so much the problem of IM, I am trying to rebuild it with Javascript but everytime when I do I end up with something that looks like this https://www.dropbox.com/sh/yoqcjwlp6soe ... LpaQa?dl=0

I want it to be like in the wikipedia article. So I use the forumla R*0.2126 + G*0.7152 + B*0.0722 to grayscale it and then I set channel G&B to 0.

but the outcome is not like in IM

[fmw42]

I want it to be like in the wikipedia article. So I use the forumla R*0.2126 + G*0.7152 + B*0.0722 to grayscale it and then I set channel G&B to 0.

The mistake is setting G and B to 0. Just keep the grayscale result as a single channel in some standard image format such as PNG.

If you need 3 channels, make them all the same as your grayscale result.

[baxter stockman]

Oh ok, so my function is this

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for (var i = 0; i < data.length; i += 4) {
    var grayscale = data[i]*0.2126 + data[i +1]*0.7152 + data[i +2]*0.0722;
    data[i]     = grayscale; // red
    data[i + 1] = grayscale; // green
    data[i + 2] = grayscale; // blue
}

so when I leave the green and blue out

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for (var i = 0; i < data.length; i += 4) {
    var grayscale = data[i]*0.2126 + data[i +1]*0.7152 + data[i +2]*0.0722;
    data[i]     = grayscale; // red

}

it becomes kind of blueish