Convert to grayscale mode vs Remove Color

[Peter Duniho]

I'm trying to figure out what the most accurate way to convert a color image to a grayscale is. This topic -- davidkchan "what is the best way to convert a color photo to black and white" 7/24/03 9:59pm </cgi-bin/webx?50@@.1dea0177> -- says that converting the image mode to grayscale is the same as using the Remove Color command from the Enhance menu.

But it's not. When I tried both, the images were noticeably different. I suspect that converting to grayscale mode is the more accurate, because the image had a fair amount of green in it, and converting to grayscale mode resulted in the lighter image.

I'm basing that on my recollection of the weighting that is normally done when converting RGB to grayscale, in which the green is given the highest weight, and the blue the lowest, because physiologically, the green is perceived as a "brighter" color.

Oddly enough, when I use the eyedropper tool to pick up the green, and then edit the color in the color chooser, dropping the saturation to 0% and leaving everything else unchanged, I get yet a third shade of gray, even lighter than what converting the image to grayscale mode does.

So, can anyone shed some light on these differences? Why do the Remove Color command, converting to grayscale mode, and manually changing the color by reducing the saturation to 0% all produce difference results?

The documentation doesn't explain exactly what converting to grayscale mode does (that is, what the underlying algorithm is), but I would have thought that at the least, removing all the saturation using the Remove Color command and removing all the saturation in the color chooser dialog would do the same thing.

[Schraven Robert]

Peter,
reading your message I think you are more informed about this subject then I am, but may be I can push things forward a bit.
What I understand from remove color is that the rgb colours are reduced to zero but the system is still set for colour. If under these circumstances you print an image you will finmd that the rgb cartridges are the first ones to go and not the black one!! Where as with greyscales you tell the soft ware to change colours into grey shades and the soft ware translates as it were the original rgb colours into greys, thus the result will be different.
I hope I made sense.

Robert

[Ralph Brannon]

If you print using only black ink, you will probably get yet another look, in some cases I have found that gives me the look I am looking form. (Some of the old epsons let you chose just black instead of color)

Ralph
<http://www.darkstar.us>

[Peter Duniho]

Follow up:

I've done a little experimenting and have some additional information. I've pretty much answered my own question for myself, but I'm still unclear as to WHY the software does what it does. So, that's my remaining question: why is the software designed the way it is?

See the remainder of this message for what I found out about "the way it is"...

First, just to clarify: my question concerns strictly the on-screen, image editing behavior. While it's true that there's even more depth to the complexity when a printer is introduced, I don't necessarily want to have to print out an image to get the image I want. :)

Anyway, my experiment involved drawing three solid rectangles, each one pure red, green, and blue respectively. I then used each method to remove the color.

Changing the color mode gave me the expected results: the rectangles all came out different shades of gray, with the green being the lightest, and the blue being the darkest. This is consistent with normal conversion practices.

Using the Remove Color command resulted in all of the rectangles being the same shade of gray. As near as I can tell, the red, green, and blue values are averaged, but without any weighting. Thus, each rectangle winds up as a 33% gray (since each started out with one color at 100% and the others at 0%).

I find it bizarre that a photo editing program with a pedigree firmly based in serious, professional graphic artistry would include such an amateurish color-to-black-and-white conversion feature (it's the algorithm a freshman computer science major would use, before he learns about human visual perception).

However, since the mode conversion *does* do the right thing, I suppose it's not that big of a deal. I can always export layers I want to be black & white to a new image, change the mode, and bring them back in to my original piece. Just seems a little silly to me, that's all.

Anyone have any insight? In what situation would straight non-weighted averaging be a useful way to convert from color to black and white? I have to figure there are people out there using the feature...why do they consider it useful?

Pete

[Nancy S]

Peter,

I think you will get much better results with the following method, compliments of Scott Kelby. Weighting of colors can be addressed with this technique, it emulates the capability of full Photoshop's "Channel Mixing".

* have your image in RGB
* create a Levels Adjustment layer above your image but DON'T make any changes, just click OK
* create a Hue/Saturation Adjustment Layer above Levels and move the saturation slider all the way to the left, removing all color, click OK
* image now appears as a black/white photo, but it is not, it is RGB
* in Layers Palette, click on Levels thumbnail to bring up the histogram
* where the dialog box says RGB, use drop down to select one color channel
* watching your image, move any/all of the three sliders
* switch to the second color channel and tweak
* switch to the third color channel and tweak

Better? You have a lot more control with this technique, your eye is your guide as to how much contrast etc you want to build into the image. Technically this is still an RGB image, if you need grayscale, change the mode, it won't change the image's appearance.

[Nancy S]

Peter,

Last step,
* flatten image

[Chuck Snyder]

Nancy, that's pretty neat! Is that from Scott's new book for Elements? I
just ordered it this morning...

:-)

Chuck

[Nancy S]

Chuck,

Yes, Scott Kelby's "Photoshop Elements for Digital Photographers"...it is a must-have in my opinion.

Nancy

[Richard Coencas]

Callum,

You don't have Lab mode in Elements.

[Peter Duniho]

Nancy,

Thanks for the advice. That sounds like a very useful procedure, and I'm going to go check it out ASAP. I'm new to Photoshop Elements (and Photoshop, of course...but I don't have that program), and eager to learn about all these things it can do.

That said, I was mainly trying to understand why the difference between the various methods to remove color. The color mode change does what I want, basically. I was just surprised the Remove Color didn't. I think Chuck's answer most directly addresses my original question (though it still doesn't explain why that particular method is considered useful).

Thanks very much for everyone's input. I don't know that I've really found out what I wanted to find out, but I've learned lots of other interesting things in the process. :)

[Richard Coencas]

Peter,

The main usefulness of remove color to me is that by keeping the image in RGB it gives you the ability to maintain or add color back to an image. For example you have a photo of a red rose. You select the background around the rose and then use the remove color option. The result is a B/W image with a red rose. Can be very interesting effect. But for high quality grayscale conversion I would use the one of the methods using adjustment layers, like Russel Brown's technique. You get the most control that way.

Rich

[Peter Duniho]

You select the background around the rose and then use the remove color
option. The result is a B/W image with a red rose

Well, except that the B/W image left as the background doesn't respect the perceptual differences between red, green, and blue.

To do what you describe, I think I would actually separate the flower and the background into two different layers, and then change the color mode for the background layer. This would have the same effect, but would keep the B/W image closer to the perceptual impact of the original.

More importantly, the Remove Color feature could have used the same algorithm that the grayscale color mode uses when converting. The usefulness you're describing (which is very true) is in its ability to remove color from an area of the image, not in the algorithm it uses to convert color to grayscale.

[Byron Gale]

"Nancy S" wrote in message news:2ccd0c09.8@webx.la2eafNXanI...

> Peter,
>
> Here is a variation on the former
>
> <http://www.russellbrown.com/tips/photoshop.html>

Nancy,

Have you had success with the method Russell demonstrates?

I followed the instructions you credit to Scott Kelby, with wonderful
results.

However, Russell's method does not work for me in PE2. The adjustments made
on the center Hue/Sat layer have no effect on the top Hue/Sat layer. In his
vernacular, the "filter" does not affect the "film". If I turn off
visibility of the "film", I can see the effects on the "filter". With both
the film and filter visible, though, nothing I do on the filter layer causes
any discernable difference.

I wonder if this is a "Big Photoshop" only process, or if I'm twisted up.
(I hate resetting prefs...!#@$%)

Byron

[Byron Gale]

Doh! I forgot to change the blending mode of the "filter" layer to Color...
never mind.

"Byron Gale" wrote in message
news:0CDFE5A369FCB9A1C4A7361A6CAEE7D8@in.webx.la2e afNXanI...

> "Nancy S" wrote in message news:2ccd0c09.8@webx.la2eafNXanI...

> > Peter,
> >
> > Here is a variation on the former
> >
> > <http://www.russellbrown.com/tips/photoshop.html>

>
> Nancy,
>
> Have you had success with the method Russell demonstrates?
>
> I followed the instructions you credit to Scott Kelby, with wonderful
> results.
>
> However, Russell's method does not work for me in PE2. The adjustments

made

> on the center Hue/Sat layer have no effect on the top Hue/Sat layer. In

his

> vernacular, the "filter" does not affect the "film". If I turn off
> visibility of the "film", I can see the effects on the "filter". With

both

> the film and filter visible, though, nothing I do on the filter layer

causes

> any discernable difference.
>
> I wonder if this is a "Big Photoshop" only process, or if I'm twisted up.
> (I hate resetting prefs...!#@$%)
>
> Byron
>
>

[Chuck Snyder]

Peter, you can't change the mode to grayscale on a single layer; it affects
the whole image.

Chuck

[Chuck Snyder]

> Peter, you can't change the mode to grayscale on a single layer; it

affects

> the whole image.

BUT...having said that, here's a potential workaround:

1. Take the image you wish to make partially grayscale and do an
Image>Duplicate Image.
2) Turn the duplicate image into Grayscale (Image>Mode>Grayscale)
3) Turn the duplicate image back from Grayscale into RGB (Image>Mode>RGB
Color).
4) Move or Copy and Paste the grayscale-turned-back-to-RGB into the
original image as a new layer.
5) Erase a portion of the pseudo-grayscale layer to expose the color below.

I think that will do it...

Chuck

[Peter Duniho]

"Chuck Snyder" <csnyder@houston.rr.com> wrote in message
news:F9A4827C3B59B9177FFAA8A1D31F5F4D@in.webx.la2e afNXanI...

> Peter, you can't change the mode to grayscale on a single layer; it

affects

> the whole image.

Yes, I know. I took it as granted that would be understood, and that anyone
reading my message would understand the layer in question needs to be copied
to a new image, where it can be converted and then copied back.

Thanks,
Pete

[Peter Duniho]

Follow-up/Summary:

Well, I've had a chance to play with the techniques suggested, so I thought I'd write back one more time with my impressions.

First, I've learned a LOT about how to use the layers and what they can do. Some pretty flexible adjustments in there, that's for sure.

I especially enjoyed playing with the Color blend mode using the Hue/Saturation adjustment layer type. One can accomplish similar results using the Levels adjustment layer type, but only by sacrificing color resolution. Using the Color blend mode and the Hue/Saturation adjustment, you get the best of both worlds. Neat!

Now, all that said: while one can approximate a true perceptual color-to-gray conversion (what changing the image mode to Grayscale does) using the techniques described here, they all ultimately fail in one important respect: loss of color resolution (or, since it's a grayscale image, gray-level resolution).

For example, I can use the Level adjustment layer technique to adjust the relative red, green, and blue levels to match what the human eye perceives. But the only option there is to scale the levels downward. You can compensate by reducing the range of the input levels but that also loses color resolution.

So, only by using the mode conversion can you get a true weighted averaging of the color channels, taking full advantage of the full 8 bits of grayscale range available.

I really appreciate all of the techniques offered. Even though none match the color mode change for the particular application I initially asked about, they are incredibly useful for LOTS of other things. In fact, I suspect that the underlying ideas behind the techniques are the "bread and butter" of many Photoshop artists, seeing how much control one gets over the nature of the colors in the image with them.

Thanks so much for all the help, all of you...

Pete

[Chuck Snyder]

Pete, one other technique to try would be a Gradient Map Adjustment Layer,
possibly coupled with a Curves Adjustment Layer (an add-on you're going to
want if you don't have it already). Just playing with them for a little
while, I was able to get some way toward the Grayscale equivalent, although
I don't know how to use Gradient Map well enough to bring it to closure.
Susan S. is our resident expert on Gradient Maps; perhaps she'll offer her
advice on its use. Just another one of those arcane features of Elements...

Chuck

[Peter Duniho]

"Chuck Snyder" <csnyder@houston.rr.com> wrote in message
news:95336ACB1E7ED52A398F399553D3A76D@in.webx.la2e afNXanI...

> Pete, one other technique to try would be a Gradient Map Adjustment Layer,
> possibly coupled with a Curves Adjustment Layer (an add-on you're going to
> want if you don't have it already).

Yeah, I saw the Gradient Map option. I have no idea how to use it
though...haven't had a chance to play with it yet.

Once I found out about the color-blending modes for layers, I had fun making
my own color wheel though. It looks especially nice when you use a radial
gradient fill for the circles, instead of just using solid colors. Lots of
different effects that way.

Though, I did find myself wondering if there's a way to change the blending
mode for multiple layers at once. It got a little tiresome clicking back
and forth between layers to repeat the blend mode selection for each.

Anyway, I guess I'll have to look up the Curves adjustment layer too.

Thanks,
Pete