Select any 1 RGB channle = greyscale, 2 = colour. HOW? NEWBIE()

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Posted By
shamilton72
Dec 2, 2005
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788
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Hi,

I’ve noticed that when I look at the RGB channels and switch off RGB and choose just one (say red), the image appears in Greyscale. The same applies to green and blue individually. Yet if I select two channels, such as red and blue, I get colour. How can this be if each channel displayed on its own is greyscale? How can two greyscale channels suddenly produce colour?

Regards,

Sarah.
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color grayscale how newbie rgb select

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nomail
Dec 2, 2005
Sarah wrote:

I’ve noticed that when I look at the RGB channels and switch off RGB and choose just one (say red), the image appears in Greyscale. The same applies to green and blue individually. Yet if I select two channels, such as red and blue, I get colour. How can this be if each channel displayed on its own is greyscale? How can two greyscale channels suddenly produce colour?

You can change the Photoshop preferences so that channels are displayed in their own color (under "displays & cursors"). Most people don’t want that, but would that solve the mystery?

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/
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shamilton72
Dec 3, 2005
I’m afraid not. I don’t understand How a single channel in a colour RGB image, when selected, is displayed in greyscale. Where does the greyscale come from and what benefit can be gained from seeing it in greyscale instead og it’s own colour channel?

Regards,

Sarah.
I
iehsmith
Dec 3, 2005
On 12/3/05 11:54 AM, Sarah commented:

I’m afraid not. I don’t understand How a single channel in a colour RGB image, when selected, is displayed in greyscale. Where does the greyscale come from and what benefit can be gained from seeing it in greyscale instead og it’s own colour channel?

The Channels palette is there to show saturation/amount of each channel’s color. Many find it more useful to view them as grey, but it doesn’t mean that they are actually grey in color. It’s loosely akin to looking at print separations.

I’m sure someone will correct me if I misspoke.

inez
BV
Bart van der Wolf
Dec 3, 2005
"Sarah" wrote in message
SNIP
Where does the greyscale come from

It could be any color or grayscale, and you can choose between 2 logical ones.

and what benefit can be gained from seeing it in greyscale instead og it’s own colour channel?

To the human eye, blue is only responsible for about 10% (or even less) of the luminance impression, so it looks dark and thus is hard to judge.

Bart
C
Chris
Dec 3, 2005
In article <BFB740FD.41ACE%>,
iehsmith wrote:

On 12/3/05 11:54 AM, Sarah commented:

I’m afraid not. I don’t understand How a single channel in a colour RGB image, when selected, is displayed in greyscale. Where does the greyscale come from and what benefit can be gained from seeing it in greyscale instead og it’s own colour channel?

The Channels palette is there to show saturation/amount of each channel’s color. Many find it more useful to view them as grey, but it doesn’t mean that they are actually grey in color. It’s loosely akin to looking at print separations.

I’m sure someone will correct me if I misspoke.

inez

You are correct. A 24-bit RGB images is comprised of three 8-bit channels (R,G,B). Each pixel in an 8-bit channel contributes one of 256 possible values for that channel to the full-color image. Each value is a step toward being fully "On." Like an 8-bit grayscale image, your coverage can be 100% "On" (black) or 0% (white). When you view the blue channel all on its lonely, black represents 100% "On" for the blue component of a 24-bit color pixel. A pure white pixel in the blue channel means no blue information is present in the full-color image.

It displays in gray (if you have your preferences set that way), because each channel is a measure of strength, and grayscale communicates that well.

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C
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iehsmith
Dec 3, 2005
On 12/3/05 1:44 PM, Chris Havel commented:

It displays in gray (if you have your preferences set that way), because each channel is a measure of strength, and grayscale communicates that well.

Especially in CMYK work where the yellow channel (yellow on white) can be pretty darn hard to discern;)

inez
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Chris
Dec 4, 2005
In article <BFB787CA.41B0D%>,
iehsmith wrote:

On 12/3/05 1:44 PM, Chris Havel commented:

It displays in gray (if you have your preferences set that way), because each channel is a measure of strength, and grayscale communicates that well.

Especially in CMYK work where the yellow channel (yellow on white) can be pretty darn hard to discern;)

inez

Excellent point. Or in any noncolor channel, such as spot varnishes or metallic inks.

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C
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John
Dec 9, 2005
"Chris Havel" wrote in message
inez

You are correct. A 24-bit RGB images is comprised of three 8-bit channels (R,G,B). Each pixel in an 8-bit channel contributes one of 256 possible values for that channel to the full-color image. Each value is a step toward being fully "On." Like an 8-bit grayscale image, your coverage can be 100% "On" (black) or 0% (white). When you view the blue channel all on its lonely, black represents 100% "On" for the blue component of a 24-bit color pixel. A pure white pixel in the blue channel means no blue information is present in the full-color image.
Actually, it’s the other way round for RGB and greyscale images. In any 8-bit greyscale image, 0 represents black (i.e. ‘off’) and 255 represents white (i.e. 100% on). This also holds when a greyscale image is used to represent a colour channel in an RGB image, so for the blue channel, 0 (i.e. black) represents no blue and 255 (i.e. white) represents saturated blue. You can see this quite clearly when you look at the channels of any coloured image. The blue areas are *white* in the blue channel display, and similarly with the other channels.

What you described above would, of course, be true for a CMYK mode image, where the numbers represent ink coverage rather than light intensity. In this case, 0 in the K (black) channel would represent no ink (i.e. white) and 255 would represent 100% coverage (i.e. black). So 100% cyan, magenta and yellow areas would be represented by *black* in their respective channel displays.

—
John
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