Grey level question

[mangcang]

Hey, there

A question is bugging me.
What I know:
1, Every primary color (R,G,B) has 256 grey level.
2, Grey level represent the scale from brightness to darkness in digital image.

My questions:
1, If I pick up colors of R37, R67. or R218 respectively to paint in Photoshop. Can I say these three colors have same hue, but different grey level? Do the number stand for the grey level?
2, If every primary color (R,G,B) has grey level, how can I know grey level of the combination color? (eg, what is the grey level of R87+G96 or R139+G75+B36? )

Hope I express my question clearly. Thanks in advance

PS:I read the book reagrding how to turn the pixel to dot and it mention the grey leve of pixel.

 

[JoshB]

There's no one true function for converting RGB values to a single gray value. It's a matter of perception and of the nature of your display.

Still, there is one pretty common, simple method: Try R*.3 + G*.59 + B*.11

This assumes that you have a display with an "average" maximum brightness, that it is truly black at zero, and that it gives an even response on red, green and blue sub-pixels. It also assumes that you have a linear response across value levels from 0 through 255, and that you perceive colors in the same way that most people do.

The reason this works like this is that the eye is more sensitive to some colors. Think about a deep, fully-saturated red -- basically an apple, and then consider that against a sizzling bright green leaf or blade of grass. The leaf is much "brighter", right? This seems so even if your looking at the same "value" for both colors under the same bright sunlight. Your eye just likes to notice green more.

Blue is even darker than red. Your "true blue", something like a navy blue, is so dark that it's easily mistaken for black. We usually think of blue as a bright color because of the sky. But it only appears that way because it is so desaturated.

 

[gordo]

Hey, there

A question is bugging me.
What I know:
1, Every primary color (R,G,B) has 256 grey level.
2, Grey level represent the scale from brightness to darkness in digital image.

My questions:
1, If I pick up colors of R37, R67. or R218 respectively to paint in Photoshop. Can I say these three colors have same hue, but different grey level? Do the number stand for the grey level?
2, If every primary color (R,G,B) has grey level, how can I know grey level of the combination color? (eg, what is the grey level of R87+G96 or R139+G75+B36? )

Hope I express my question clearly. Thanks in advance

PS:I read the book reagrding how to turn the pixel to dot and it mention the grey leve of pixel.

1 - The number represents the grey level. If that grey level is placed into an 3 channel RGB file then you need the grey levels of the other two channels to create a color. I.e. R 67 is not a color.

2 - If I understand your question correctly, I think that you are asking what grey level is represented by a combination of RGB values. It depends. Below is what is called a "Lagorio Chart" The black line represents where the grey tone appears equally bright to the color. It is the tone to color curve response of the eye.

best, gordon p

 

[bharatk]

mangcang

you get 256 levels of grey on an 8 bit description. The levels of grey will change according to the number of bits used to describe.

Gordo,

Where can I download the 'Lagorio Chart' from.

Regards
BharatK

 

[gordo]

Where can I download the 'Lagorio Chart' from.

Download it from this site. It's a scan from a book on photography published in 1956.
The idea was that you'd photograph the chart with panchromatic film and then compare how the film mapped tones to color compared with human vision.

best, gordon p

 

[TerryWyse]

Hey, there

A question is bugging me.
What I know:
1, Every primary color (R,G,B) has 256 grey level.
2, Grey level represent the scale from brightness to darkness in digital image.

My questions:
1, If I pick up colors of R37, R67. or R218 respectively to paint in Photoshop. Can I say these three colors have same hue, but different grey level? Do the number stand for the grey level?
2, If every primary color (R,G,B) has grey level, how can I know grey level of the combination color? (eg, what is the grey level of R87+G96 or R139+G75+B36? )

Hope I express my question clearly. Thanks in advance

PS:I read the book reagrding how to turn the pixel to dot and it mention the grey leve of pixel.

Substitue the word "step" or "increment" for the term "gray level" and perhaps it will make more sense. Gray level or levels are not talking about the color GRAY, it's just describing the number of steps in each primary channel R, G and B. In 8bpc ("bit per channel") you have 0-255 steps or *integer* values that can be described (the value 12.5 is not possible in 8bpc). Same is actually true for CMYK but we generally use 0-100% "ink" to describe CMYK.

You would not refer to a combination of RGB triplet values as a "gray level"...it's each CHANNEL that has the gray level, step, increment, whatever. The combined RGB triplet would be the actual COLOR (in the context of a color space or ICC profile of course!).

Check please!

Terry

 

[mangcang]

You would not refer to a combination of RGB triplet values as a "gray level"...it's each CHANNEL that has the gray level, step, increment, whatever. The combined RGB triplet would be the actual COLOR (in the context of a color space or ICC profile of course!).

Terry

Heym Terry,

Do You mean the gray level just exist in each channel, instead of in actual COLOR, the combined RGB triplet? How the actual color reflect the grey level if I cant name RGB values as "gray level" ? Hope you can clarify it for me thanks

 

[bharatk]

Thanks

Downloaded. Whenever I am in Canada I would really like to browse through your library.

Regards
BharatK

 

[TerryWyse]

Heym Terry,

Do You mean the gray level just exist in each channel, instead of in actual COLOR, the combined RGB triplet? How the actual color reflect the grey level if I cant name RGB values as "gray level" ? Hope you can clarify it for me thanks

Yes, the "gray level" simply defines the step or increment in each channel. The actual COLOR is described by a value or "gray level" in each R, G, B channel. Like Gordo pointed out, R=196 is NOT a color but R=196, G=0, B=0 IS a color since a gray level or value is assigned to each primary channel.

Imagine if you had three light bulbs, a red one, a green one and a blue one, and each one was on it's own "dimmer" switch or rheostat. On that dimmer switch or rheostat, it was marked off in numbers ranging from 0-255, with 0 being OFF or no voltage sent to the light bulb and 255 being maximum voltage sent to the light bulb. All the values between 0 and 255 would be varying amounts of voltage sent to each bulb causing variations in brightness for each bulb. So, to get an "additive" color combination from the three bulbs, you are simply dialing in a particular value on each dimmer switch until you get the color you want. Those "steps" or values on each dimmer switch represent the "gray level" you're dialing in for each light bulb. The combined effect of each level for each light bulb produces a color. Make sense?

Now, if you want to get REALLY technical, even the RGB triplet (r196 g0 b0) doesn't *unambiguously* define the color because that RGB triplet value can be used in a number of different color spaces and define a different color in each space. They are all likely to produce a "red" color but each color space (or ICC profile) will produce subtly (or not so subtly) different reds.

It's just like in CMYK where c0% m80% y100% k0% is likely going to produce a "red" color....but that red color is going to be different depending on the device that's used to print that color. Send that same CMYK value to an offset press, a color laser printer and an inkjet printer and you'll likely get 3 different "reds" despite the fact that it was the same CMYK value in each case.


Hope this helps,
Terry

 

[mangcang]

Thanks your guys and have a good day