Software - - HELP using channel mixer please !

alain754 · #1 09-11-2006, 09:01 AM

I'm using Channel mixer to convert color pics to B&W, to mimic action of photographic filters (like red or yellow) , but seem to loose details in skies and faces ... picture seem to pixelise or posterize.... Is that normal ???? Would desaturate keep more details ???
any suggestion welcome !

Also, about channel mixer, is there any rule about the use of the "Constant" parameter ?

Alain

mistermonday · #2 09-11-2006, 10:10 AM

Alain, for what you are trying to accomplish, you might be better off adding a Hue/Saturation Adjustment layer and checking the Colorize box. This will allow you to desaturate, colorize, and change brightness at the same time.
The channel mixer effectively changes the contrast in the individual channels and therefore it can very easily blow out highlights or plug shadows. It can be usefull for creating dramatic B&W images but as you have discovered, can make things look bad if your mixing is in the wrong direction. Channel mixing does take some practice to get it to work well.
Regards, Murray

JayNads · #3 09-12-2006, 01:59 PM

Quote:

Originally Posted by alain754

I'm using Channel mixer to convert color pics to B&W, to mimic action of photographic filters (like red or yellow) , but seem to loose details in skies and faces ... picture seem to pixelise or posterize.... Is that normal ???? Would desaturate keep more details ???
any suggestion welcome !

Also, about channel mixer, is there any rule about the use of the "Constant" parameter ?

Alain

It's not normal. Generally what I do is after reviewing the individual channels, I add the channel mixer adjustment layer and select monotone (to make B&W). Then I use about 50-75% of the channel that I find the most "pleasing" for a B&W result. I then add in the other two channels to taste, to decrease contrast in certain objects and the like - I try to stay away from blue if it's a digital photo because that channel tends to have the most noise. Then, I add a hue and saturation adjustment layer beneath it. By doing this I can fine-tune the channel mixer by adding or removing color and adjusting the hue in the H&S adjustment layer.

If you do this, or some variation therein, you should not have any problems like you mention. If it continues, it might not hurt to post some samples of the problem.

Good luck!

alain754 · #4 09-15-2006, 06:08 AM

Thanks Murray & Jason, I send here a sample of the color pic, + sample of bw conversion.

Something I don't understand is the relation between the individuals channels as you can watch them from the Channels tab and the channel mixer, i.e. if I just look at the individual red channel, I see a pic which is really like a high key, but if I use channel mixer with R100,0,0, I expected to see the same thing ... why not ????

Other question : is there a preferable blending mode for channel mixer use ?

Merci !!!

philbach · #5 09-15-2006, 06:51 AM

Russell Brown has an excellent quicktime.mov tutorial on techniques to convert color to B&W. Here is the Link: CLICK HERE

mistermonday · #6 09-15-2006, 07:41 AM

Alain, If, while using the channel mixer, you are still in RGB mode, then the histogram you see is the composite channel and it changes as you adjust the values. If you leave Red as 100,0,0 and do not change the other channels, then the Histograms do not change and the icons in the channel palette do not change either. Moreover when I look at your attachment 1 (before) and attachment 3 (after), the histograms and color values are identical. Foregive me if I have misunderstood your question.
As for the blending mode, there is no preferable mode to change your layer to. It really depends upon what result you wish to achieve. You can very quickly cycle through all the blend modes by highlighting one and using the Up and Down Arrow keys.
Regards, Murray

alain754 · #7 09-15-2006, 01:29 PM

Hi Murray,
here is attachment 3 ... obviously, I don't know how to transform the red channel, copied, into a layer !!! that's why I send you the screen dump. So my question is why the channel as it appears is not the same thing as when mixed 100 0 0 ???

Thanks phil for the link, i'll definitely will have a look at it !!

alain754 · #8 09-15-2006, 01:47 PM

... to put a channel into a layer, but the main question still remains !!!

mistermonday · #9 09-15-2006, 02:14 PM

Alain, here is a test I did which you can repeat. Open your orig image and do Image>Duplicate to make an exact copy of your image. Select the copy and do Alt+I, A,X to bring up the Channel Mixer (or use the menus). The default will be Red, 100, 0, 0. Just click OK. Now go back to the original image, type Ctrl+1 to make active the red channel (or do it from the channels palette). Now with only the Red channel active, do Image>Apply Image. When the dialog box comes up, select as a source from the pull down menu, the Duplicate Image, choose the Red channel, Opacity = 100% and select the Difference Blend Mode. If the two red channels are identical, then you preview window will be black (as in my attachment below). That means that both red channels are exactly the same. The histograms are also identical.
So, again I appologize if I am missing something in what you are doing. If it is different from my understanding, then please provide me with a detailed description of ALL of the steps you are taking which are giving you those different results.
Regards, Murray

alain754 · #10 09-15-2006, 02:52 PM

Murray, what you describe is what i get, I mean you're right. But you do miss something. You say : "Open your orig image and do Image>Duplicate to make an exact copy of your image. Select the copy and do Alt+I, A,X to bring up the Channel Mixer (or use the menus). The default will be Red, 100, 0, 0. Just click OK. Now go back to the original image, type Ctrl+1 to make active the red channel (or do it from the channels palette). Now with only the Red channel active, do Image>Apply Image. When the dialog box comes up, select as a source from the pull down menu, the Duplicate Image,"
but I compare with RGB, not with red channel !!!
So my question is : why is Red channel only different from RGB obtained by channel mixer 100 0 0 ??? On the attachment below, you'll see the screen dump of the two images obtained with your process. They are different ! On the left you'll see red channel only, and on the right RGB channel mixed 100 0 0 .
Regards, Alain

mistermonday · #11 09-15-2006, 04:25 PM

Alain, two questions for you:
1. Was the layer opacity set to 100% prior to your application of the channel mixer? If it was not, then your results will be different.
2. Did you compare the files (or layers) prior to saving them as jpgs? If not, then do so or save the files as PSD.

I tried a number of other tests, yet I still get the same results - everything is identical. Included in the tests were checking of the Monochrome button and verifying that the resultant gray scale image had a 3 channels which were identical to the original Red channel - and they were.

So if it still does not work for you, please post a detailed description of all of the steps that you are taking in the process.
Regards, Murray

Pam · #12 09-15-2006, 05:33 PM

I believe what Alain is trying to say is that the red channel for any given image is slightly different depending on the manner in which it's created. If it's copied from the channel's palette, it's brighter than if it's created using the channel mixer. As a test, try this:

1 - Open an image.
2 - Open a channel mixer adjustment layer and use these settings:
R - 100, G - 0, B - 0, check monochrome.
3 - Open a new layer and hit Ctrl+Alt+Shift+E to merge all visible layers onto the new layer.
4 - Now turn off all layers. You should now have only the original background image visible.
5 - Open the channels palette. Click on the red channel. Hit Ctrl+A (select all), then Ctrl+C (copy). Now click on the RGB composite channel and return to the layers palette.
6 - Open a new layer and hit Ctrl+V to paste the red channel (copied from the channels palette) into the new layer.
7 - Click on the layer you created in step 3 (the red channel created from the channel mixer). Compare these two and you will find that the channel palette's red channel is different from the channel mixed red channel.

--
Pam
http://www.pbase.com/pam_r
'art is working on something 'til you like it...then leaving it that way'

mistermonday · #13 09-15-2006, 07:20 PM

Pam, thanks, now I understand what Alain is doing. Actually there is a much simpler way to demonstrate the effect. You do not need to do a channel mixer, you can simply create a dummy image in which you fill the any channel with a value (for example 20), then copy it and paste it into a new layer and the value it will read is 29 (for sRGB).
The issue is that Channels do not equal layers. Channels are always fixed mathematical values of 0 to 255 in the case of RGB and 0 to 100 in the case of LAB. When you copy the contents of an alpha channel and turn it into a layer, it needs to conform to the Color Space that you are working in.

You can see this if you repeat the example above but convert the color profile (Edit>Convert to Profile) and change from sRGB to AdobeRGB. The 20 that was in the red channel becomes 41. When you copy it and paste it into a new layer, it will measure 60. If your image was a solid shade of blue (R=20, G=60, B=100) in sRGB mode, when you change it to Adobe RGB, the values will change but the color you see on your monitor is the exact same color blue.

That's partly why there are a whole set of commands for mathematically manuipulating channels and layers.

Regards, Murray

gmitchel · #14 09-15-2006, 07:45 PM

I have a free action set that includes lots of Channel Mixer recipes. It also includes several other methods of B&W conversion. That way you can quickly and easily compare the results of different ways of converting to B&W.

http://www.thelightsrightstudio.com/...Conversion.htm

Cheers,

Mitch

Pam · #15 09-15-2006, 10:18 PM

Quote:

Originally Posted by mistermonday

When you copy the contents of an alpha channel and turn it into a layer, it needs to conform to the Color Space that you are working in.

Ahhhhhhhhh!! I have been wondering about this phenomenon for a few years now.It seems that very few people notice it and fewer still have even ventured a guess at what is actually happening. Thank you for clarifying!

--
Pam
http://www.pbase.com/pam_r
'art is working on something 'til you like it...then leaving it that way'