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rattsang
02-03-2007, 11:51 AM
hi everyone

i've notice something recently...... there is a difference between taking a color image in photoshop and going to image>mode>greyscale and the image>adjustments>desaturate as you can see from the examples below:

both colors have the same luminance and saturation, just different hue, and then they are blended in the middle....

http://img185.imageshack.us/img185/6968/colorpp4.jpg

this is the desaturated:

http://img245.imageshack.us/img245/9904/desaturateqz3.jpg


this is the greyscale:

http://img58.imageshack.us/img58/802/greyscaleiy8.jpg


can somebody explain why there is this difference and which one is a more accurate representation of a 'colorless' image

thanks in advance

rat

ThePhotographer
02-03-2007, 10:40 PM
What I always do when I want to transform a colour photo into black and white, is to split the channels. Then you'll get a blue, a green and a red black and white image.

I then choose the one I like the best.

If I think it is not exactly what I want, I'll copy and paste another channel on top of it and set the opacity of that layer to something I like.

You may even want to use bits of all three channels at various parts of your image.

I use progressive selection a lot. If you need some details from one of your channels, do this and copy paste on top and set the opacity to something your like.

When you're almost done, then still play a bit with levels and contrast.

You will end up with something really nice and so much easier to obtain than when we had to do this in the darkroom.

Anyway, never just use the desaturate fonction. It will give you a grey and dull image ....

Ian Jones
02-04-2007, 01:31 AM
Desaturate is the least appealing and least accurate of the two options.

3dj
02-04-2007, 01:37 AM
The way I like to make a good grey scale image, of which there are many, is to put a hue+sat adj layer with the saturation turned all the way down, then between the image layer and the hue+Sat adj layer, put either a channel mixer layer or a selective color adj layer and adjust these values to get a great looking greyscale image. Another fast way to just get the "luminosity" of the image is to take the image into lab mode, and use the lightness channel.

Hope this is useful to someone.

-Jim

P.S. The new CS3 beta has a new method based on this under image>adjustments>black/white.

nSomnius
02-04-2007, 05:25 AM
When converting an RGB image to a Grayscale image using the mode menu you are accepting photoshops default recipe, a fixed percentage of each of the three channels blended together, I read a long while ago what these numbers were but it slips me now. B+W conversion plugins, ps's apply, channel mixer and calculations commands put the decision making process back in your hands, but in the end, desaturating an RGB image is the way to zero out the color channels without invoking a voluntary or involuntary conversion recipe.

Matt
02-04-2007, 07:08 AM
A good trick is to do a gradient map layer with the standard black and white gradient selected.

This will typically produce rich results, with nice contrast levels, and evenly distributed values, concordant with the color image.

I never save or convert to grayscale. I find it serves no purpose, and typically if you're going to output the file, most printers prefer the image to be in RGB format (Yes, RGB, not CMYK) and will print accurately as a black and white RGB image than a black and white grayscale image.

Datameister
02-04-2007, 08:23 AM
You can convert to black-and-white however you want to. The methods above are all great. Generally, simply desaturating an image isn't a good way of doing it, though that has its purposes.

The Channel Mixer (under Image > Adjustments) is a great way to synthesize the various channels' grayscale information into one coherent monochrome image. I'd recommend that or simple Grayscale conversion. (Naturally, you can switch it back to RGB afterwards. But the conversion to Grayscale does nice things to the values.)

On a side note, you may want to try something to help you achieve purer blends, rattsang. Go to Edit > Color Settings, activate Blend RGB Colors Using Gamma:,and set the value to 1.00. Then try painting the same blend. You should see different colors in the middle, ones that you may find a little more attractive. They're less "muddy."

3dj
02-04-2007, 03:29 PM
I find it serves no purpose, and typically if you're going to output the file, most printers prefer the image to be in RGB format (Yes, RGB, not CMYK) and will print accurately as a black and white RGB image than a black and white grayscale image.

Hmmm ... maybe for photograghs, but not for commercial offset printing. for commercial offset printing, if you want a greyscale image, make sure it is converted from rgb to greyscale, otherwise, they printer will be forced to convert it and they may not take into consideration the options involved. Many times I will experiment with assigning a colorspace (assign, not convert) and then go to my desired color mode. I have gained alot of detail in color images doing this. I know we are talking b+w here though. Anyway, if you don't make it greyscale, you run the risk of the image being reproduced in cmyk, which will have a color shift in the values. This can be desirable, as long as you control it.

Just more of my 2 cents.
-Jim

3dj
02-04-2007, 03:32 PM
On a side note, you may want to try something to help you achieve purer blends, rattsang. Go to Edit > Color Settings, activate Blend RGB Colors Using Gamma:,and set the value to 1.00. Then try painting the same blend. You should see different colors in the middle, ones that you may find a little more attractive. They're less "muddy."

Yep, good advise, this is true. The same thing happens in lab, though I find it is better in lab. This really is noticeable when you have layers with diffrent blend modes going on. Do try both and see what works for you. lab is a very powerful and little understood colorspace. I have recomended this book here before, so here goes again:

http://www.amazon.com/Photoshop-LAB-Color-Adventures-Colorspace/dp/0321356780

Probably the best book I have ever owned on cololr theory/graphics.

-Jim

Datameister
02-04-2007, 07:47 PM
Yep, good advise, this is true. The same thing happens in lab, though I find it is better in lab. This really is noticeable when you have layers with diffrent blend modes going on. Do try both and see what works for you. lab is a very powerful and little understood colorspace.

Very true. I would simply work directly in LAB, but that disables some of the color correction features and so on. Using the gamma 1.00 feature achieves an effect which is extremely similar if slightly inferior, but without disabling any helpful features.

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