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HELP using channel mixer please !

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  #1  
Old 09-11-2006, 10:01 AM
alain754 alain754 is offline
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HELP using channel mixer please !

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
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Old 09-11-2006, 11:10 AM
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mistermonday mistermonday is offline
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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
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Old 09-12-2006, 02:59 PM
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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!
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Old 09-15-2006, 07:08 AM
alain754 alain754 is offline
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here are some samples

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 !!!
Attached Images
File Type: jpg channel mixer test.jpg (73.9 KB, 23 views)
File Type: jpg channel mixer test bw.jpg (67.1 KB, 24 views)
File Type: jpg channel red only.jpg (73.9 KB, 19 views)
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Old 09-15-2006, 07:51 AM
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philbach philbach is offline
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Converting to B&W

Russell Brown has an excellent quicktime.mov tutorial on techniques to convert color to B&W. Here is the Link: CLICK HERE
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Old 09-15-2006, 08:41 AM
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mistermonday mistermonday is offline
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Channel Mixer

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
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Old 09-15-2006, 02:29 PM
alain754 alain754 is offline
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attachment 3

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 !!
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File Type: jpg channel red only.jpg (71.0 KB, 12 views)
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  #8  
Old 09-15-2006, 02:47 PM
alain754 alain754 is offline
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I found the way ....

... to put a channel into a layer, but the main question still remains !!!
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  #9  
Old 09-15-2006, 03:14 PM
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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
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File Type: jpg Alain Ch Xer MM Test.jpg (94.5 KB, 4 views)
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  #10  
Old 09-15-2006, 03:52 PM
alain754 alain754 is offline
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I agree but ...

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
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File Type: jpg red channel vs rgb.jpg (58.2 KB, 14 views)
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