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  #21  
Old 03-28-2005, 08:04 AM
hpycmpr hpycmpr is offline
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Good starts

Quote:
Originally Posted by BayCanuck
Well, to start with, you have a black and white image that's been scanned in colour. So the three "colour" channels are just what your scanner saw with different colour filters. Unless you want to colourize your image, your end product should be a one-channel image, because one-channel gives black and white.

You asked what makes a channel "good" and then answered your own question: A good channel is one that gives you the closest approximation of what you want to do with the least ammount of work.

For this image, I'd choose the red image, since it's showing the least ammount of "problems" that you have to fix, and hasn't lost very much information in the highlights compared to the green channel. Generally, for B&W images scanned in colour, the "noise" (badness) per channel is least in red, most in blue, and kind of in the middle for green.

How I determined this was to "normalize" all the channels, then look at them one by one. To normalize them, I pulled up Image->Adjustments->Levels, and used the drop-down in the levels dialog box to select each of the red, green, and blue channels in sequence. Then I dragged the small triangles right under the histogram to the "edges" of the histogram (for each of red, green, blue) and then clicked OK. This made sure that the tonal (light tone, dark tone) representation of each channel was the same.

To evaluate them, I used a keyboard shortcut: Control-1, Control-2, Control-3. This shows only the red, green, and blue channels respectively one at a time. Notice to the left of the young girl's hair how the "dirt" gets bigger and more defined as you go from red->green->blue (1->2->3.)

Since Red has the least problems, I'd select it (Control-1, or use the Channels pallette and click the eye icons so that only the "red" channel's eye is on,) then convert it to greyscale (Image->Mode->Greyscale.) Now you only have one channel to worry about, and there's no problems with colour casts.

For right now, I'd not worry too much about the "ten channels" metaphor. That becomes more important when editing colour images, not black and white ones. Since this is a scan of a black and white image, all those other channels are really combinations of the information you already have in the red-green-blue channels. Since I think the Red channel was already the best, any of the other seven channels is really some part of the (good) red channel plus or minus some part of the (bad) other channels. You can't make good better by adding bad.

Good luck with the image!
Thanks for the response. But I should have made it clear that I work with color images only, and my questions were not specifically about this b/w image.

Let me be more specific about identifying which channels to work with by a few examples:

- if I want to enhance contrast, should I blend a channel with the best contrast into a channel with the least contrast?
- if I want to remove noise, should I blend a channel with the least noise into a channel with the most noise?
- if I think sharpening a single channel will be better than sharpening a composite, should I sharpen a channel with the most (or the least?) details?

This list can extend to blending improve exposures and enhancing colors, etc. I think a list like this can go a long way to help those who are channel challenged like myself.
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  #22  
Old 03-28-2005, 08:47 AM
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Duv Duv is offline
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Quote:
Originally Posted by hpycmpr

Let me be more specific about identifying which channels to work with by a few examples:

- if I want to enhance contrast, should I blend a channel with the best contrast into a channel with the least contrast?
- if I want to remove noise, should I blend a channel with the least noise into a channel with the most noise?
- if I think sharpening a single channel will be better than sharpening a composite, should I sharpen a channel with the most (or the least?) details?

This list can extend to blending improve exposures and enhancing colors, etc. I think a list like this can go a long way to help those who are channel challenged like myself.
The answer to these and more is Yes, Maybe, No. The ultimate determinent is whether it moves the work a long in the direction you want. Maybe the image will have a better feel and look if left with some noise, or looks a little soft. Keep in mind too, that when you blend in a good channel with a bad channel, you can tone the adjustment down with Edit: Fade.

I always keep an original duplicate on screen so I can compare the changes I'm making. You don't sound channel challenged to me. You just sound like you are looking for "absolutes" and there are none. Working with channels is just one way to get to improving your images. Right now I'm "experimenting" with making adjustments thru Color Profiles and Gamma values. Just more neat stuff.

Good luck

Dave
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  #23  
Old 03-29-2005, 09:25 AM
hpycmpr hpycmpr is offline
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Still challenged

Quote:
Originally Posted by Duv
The answer to these and more is Yes, Maybe, No. The ultimate determinent is whether it moves the work a long in the direction you want. Maybe the image will have a better feel and look if left with some noise, or looks a little soft. Keep in mind too, that when you blend in a good channel with a bad channel, you can tone the adjustment down with Edit: Fade.
Kind of what I expected. Yes, Edit:Fade is one of my favorites.

Quote:
Originally Posted by Duv
I always keep an original duplicate on screen so I can compare the changes I'm making. You don't sound channel challenged to me. You just sound like you are looking for "absolutes" and there are none. Working with channels is just one way to get to improving your images. Right now I'm "experimenting" with making adjustments thru Color Profiles and Gamma values. Just more neat stuff.

Good luck

Dave
I consider myself channel challenged in a relative sense. For example, with curves I know the steep part will increase the contrast. But with channels, I almost have no clue what will happen if I blend them, etc. For this reason, I'm looking for a list to provide some guidelines instead of absolutes.

Whether I'm correcting by channels or by other tools, I find comparing results side by side (two windows, or two prints) is essential. But working with channels without any guidelines (or clues), the number of permutations is just too big. I guess I'll keep trying, and thanks for your support.
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