View Full Version : RGB to black and white...

10 October 2004, 12:45 AM
Awhile back on either the Lightwave forums or here, someone came up to the connections and asked for suggestions on how to use or render black-and-white digital imagery. At one point a member replied pointing out that Adobe Photoshop has different methods of rendering out an RGB coded image into grayscale (or technically RGB grayscale). It interested me, not to a great extent but it did make me realize I should be careful with Photoshop's (or any other program for that matter) way of converting modes.

So the other day I was playing around with Photoshop and after mixing a bunch of different gradients, I thought about one specific way of rendering the spectrum.

I took an angle gradient with the spectrum preset and spread it across a new transparent 512x512 image making an equal circle. Then I created a new layer and took the radial gradient and made a hotspot gradually increasing in intensity linearly toward the center. I changed the radial's blending mode to luminosity and got this (along with preceding images for further explanation)...

If you already know all of this then my apologies, I just like making stuff look technical.

The first image is the composite (obviously), the second is the same but white points have marked full intensities for each channel, the third is just a quick curves adjustment for effects as well as to see the bulb of the three colors, and the fourth and fifth are different methods of rendering black-and-white. I went and figured out that Photoshop's saturation adjustments do something similar to a simple average by the equation (R+G+B)/3. And the grayscale conversion is; to what I have read about color conversions on the web, is the correct version taking into account the importance of the three primary colors in the addictive system (76*R+150*G+29*B)/256.

My question is that why do the three primary colors go in reverse in apparent scale in these images even though that green should be the greater importance? And if using the technically correct conversion is really necessary? I have changed various photographs to black-and-white using the desaturate method and have liked it over the accurate grayscale conversion. Or maybe it is just me. I don't know. I just wanted to know other artist's opinions on it.

Thank you.

10 October 2004, 06:22 AM
Good info to know. I don't think everyone know this stuff so some folks should appreciate your taking the time to work out and post this for them.

Did you also try using the Channel Mixer filter with the 'Monochrome' option selected? That produces different results as well.


10 October 2004, 08:28 AM
I'm not sure why converting to greyscale has that effect but I've always hated it ( Very flat looking ), I usually use a saturation adjustment layer and then darken or lighten the colours selectively to generate the seperation I want.

10 October 2004, 11:30 AM
Due to the different luminance values of each colour, greyscale conversion is not a simple operation. Try to set a Hue-Sat adjusment layer with no effect, and another Hue-Sat adjusment layer over the first one, with Sat=0, and blended as Color or Saturation. Then manipulate each channel value on the first Hue-Sat layer, untill the final greyscale image looks right to you.
you have a simple tutorial on this on the Tutorials of Adobe Home Page.
Shake converts to grayscale on this ratio: R: 0.3, G: 0.59, B: 0.11
Here is a color wheel grayscaled with Shake:

10 October 2004, 02:05 PM
To convert to grey using perception, try this:
1. new layer
2. fill with 50% grey
3. set blending mode to sat

Greens and yellows will be brighter, while blues will be darker.
Has do to with Y in XYZ for L in Lab.

Now, manipulating S in HSB is very different and usually 'flat'.
Has to do with Max(RGB) and Min(RGB) for finding S.
All pure hues will be Sat = 100% and will result in L = 50%.
So, there is no L variation across all hues - not based on perception.

10 October 2004, 03:52 PM
Do you have a method for doing this in CMYK?


10 October 2004, 04:14 PM
Dozens of way to do this

10 October 2004, 10:04 PM
...gonna keep it to yourself?


10 October 2004, 11:57 PM
Sorry, wasn't talking to you, just talking about gray scale conversions in general.

10 October 2004, 06:48 PM


11 November 2004, 03:57 PM
fantastic and very informative thread guys...
i have a question for you.

I love the style of the old hollywood portrait photos...they seem very lush to me...i was told this was because in those old days there was more silver in the photographic process and also they are using large format film...was wondering how one would go about converting a 3d render into this old fashioned style...or some tips/methods that i could try. lighting etc will help extra...but was looking for help on the conversion process from colour or should i bother...

someone like hurrell did a lot of negative manipulation also which helps...smoothing and bloom type stuff.

i'm not a photographer so its trying to convey some of that sort of appearance in a 3d scene would be all appreciated.

11 November 2004, 10:16 PM
another way of making grey scaled images is by using adjustments layers. By doing it through the following method you have more control over the transition from color to grey then with the standard greyscale mode or desaturate.

first step, first adjustments layer: Hue/Saturation

do not change any settings yet, just add the H/S adjustment layer and click 'OK'. The image ofcourse has not changed

second step, second adjustments layer: Hue/Saturation

now take the saturation down to -100

So far, this effect is exactly the same as the standard greyscale conversion (Mode > greyscale)

Now for the real trick :)

double click on the first H/S adj. layer and now you can edit the lightness, saturation and hue from the original Reds, Yellows, Cyans,...

This technique I learned from a Total Training DVD for Photoshop and it has proven to be quite a nice technique for getting just the greyscale image you want, at least for me personally that is.

Hope this is a bit relevant/helpfull.


11 November 2004, 10:25 AM
sounds good will have to do some experimenting with the various ideas/methods presented...sounds like more control with all of them which is essentially the important point.

11 November 2004, 12:41 PM
b_d (, you forget to mention to set the blending mode of the first H&S adj. layer to color.
Without it the technique is not going to work.

11 November 2004, 01:25 PM
Set colour? Works dandy for me in normal mode.
I think the difference is in which sliders are used.
When using HSB for this, I stick to the Lightness slider because that's what is after in the end result. Use the drop-down and the fall-off sliders, then tweak L for the selected hues.
Know what I mean?

11 November 2004, 02:48 PM
Set colour? Works dandy for me in normal mode.Sorry, but it doesn't. I did some research for you too to show you that you're wrong;

11 November 2004, 03:25 PM
No, I am not wrong.
Notice on the second page, and subsequent pages, that Russell moves the H and S sliders.
In my post I talk about moving the L slider.
Like I said, the sliders are the difference.

When converting to B&W, it makes more sense to directly convert H to L using the L slider.
Use the drop-down and the fall-off sliders.
Adjust the L slider.

11 November 2004, 04:39 PM
R_D cleary said: "double click on the first H/S adj. layer and now you can edit the lightness, saturation and hue from the original Reds, Yellows, Cyans,..."

I replied that you need color blending mode to do that.

You said: "Set colour? Works dandy for me in normal mode"

I say it doesn't. He was clearly talking about using saturation and hue sliders and without setting the blending mode to color any of the adjustments to these sliders have Z E R O effect.

It's important that you read the content of the post I was referring to.

11 November 2004, 04:51 PM
When converting to B&W, it makes more sense to directly convert H to L using the L slider.I think it's normal that we now all want to hear why your technique is better than Rusell Brown's, not what I call an average guru... :rolleyes:

11 November 2004, 04:56 PM
I have read and I understand perfectly.
In order for the H and S sliders to have an affect, the blending mode has to be Colour.

I'm saying you don't have to use H and S and set to Colour.
Rather, I'm suggesting leaving the mode Normal and adjusting L.
Be direct with converting hues to greyscale.

Want blues darker to bring out clouds?
Use the drop-down and fall-offs to select the range, then bring L down.
Want the red brick building to be brighter against the now darker sky?
Use the drop-down and fall-offs to select the range, then pump L up.
Green shrubbery in front of the building? Same thing.

Simple and direct.
Take it or leave it.
And with that, I'm done.

11 November 2004, 05:14 PM
I have read and I understand perfectly.
In order for the H and S sliders to have an affect, the blending mode has to be Colour.Geez man, that's what I'm trying to tell the whole time!

11 November 2004, 05:34 PM
you guys are funny...heh heh.
runs off to hide from flames...and practice using my sliders in whatever mode i want..
keep smiling guys its all long as we have some control...its ideas we want at the end of the day.
:thumbsup: :love: :thumbsup:

11 November 2004, 05:49 PM
Sorry guys, I wasn't 100% clear/correct. Indeed when doing it my way, so 'normal mode', the H & S sliders don't do anything, only the L slider has effect. Didn't mean for you guys to start a fuss about it, still getting used to the high quality level of posting here... :shrug:

Didn't know myself that by choosing color for the bottom H&S layer youn could also change the other parameters. Thanx for the tip Elysian.

I did notice that when doing it like that you have to make very suddle changes to the sliders, otherwise you get some mean artefacts.


11 November 2004, 09:56 PM
Elysian is quite the connoisseur of marshmellow roasting - gotta watch what you say around him...


11 November 2004, 12:55 AM
...says the guy who started with the name calling by saying, I quote: "For the imagining impaired..." :rolleyes:

11 November 2004, 05:14 AM
I guess I shouldn't take that 'too' personally... after all, I'm not the first person you've taken it upon yourself to belittle...


11 November 2004, 09:43 AM
i know you guys love each other really...can't you just kiss and make up...heh heh.
:love: :love:

11 November 2004, 01:10 PM
lol... allright allright...
peace elysian...


11 November 2004, 07:12 PM
Peace man, but you have to admit; it's fun to have some spice on the board once in a while ha! :D

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