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a simple question of colour ?


felixakulw

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i have a very simple image consisting of two boxes filled with two different colours

how do i adjust these so that they are of equal tonal value ie if they were grayscale they would look identical

sounds simple but i've spent two hours trying to do it and eyeballing is the best answer so far

thanks - i expect its that i am thick!!:banghead:
 

Tom Mann

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Nope, I guarantee you that you are not "thick", at all. In fact, this is a fairly subtle point in color theory.

The problem you are most likely running into is that the same two colors may have equal "brightness" in HSB coordinates, but different "value" in HSV, and different "luminosities" in Lab coordinates. There is a good discussion of the intricacies of conversion between HSV and HSL color coordinates on Wikipedia: http://en.wikipedia.org/wiki/HSL_and_HSV.

Because of this, if you believe that you have two color of equal brightness, but you adjusted their their brightness using a method that is different from the internal representation of color in PS, Lab coordinates, if you desaturate the two colors in PS, there is an excellent chance they will wind up with different tonal values.

The only way to ensure that you wind up with the same tonal value is to change your working color space to Lab. Then, if you select (or make) two colors have the same tonal value, and then desaturate them, they result will be what you expect: the same tonal values. Here's a simple example of what you get if you work in the Lab color space:

1. My starting colors - fully saturated yellow and blue:

My_two_starting_colors.jpg


2. Next, I placed a mid-gray adjustment layer above the previous layer, and change its layer blending mode to luminosity. This is one way to guarantee that all areas of the image will have the same luminosities. As expected, one gets darker, more muted colors, and the black background turns to gray:

Result_of_putting_a_solid_gray_layer_set_to_Lum_blend_mode_above.jpg



3. Next, I desaturate the previous layer (...I happened to simply slide the saturation slider to zero in a Hue/Saturation adjustment layer, but any method of desaturation would work equally well). As expected, if you have been working in Lab color, the two colors will have exactly the same gray tone. However, if you were working in one of the RGB or CMYK spaces, the two tones will almost always be different.



Result_of_desaturaing_previous_colors.jpg


HTH,

Tom M
 

thebestcpu

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Hi Felix - Here is an alternate approach

Yet first note that trying to get tones to match on just your monitor (let alone when you share the image on a different monitor) would require the monitors to be calibrated with a product from the likes of Xrite or Datacolor and the viewing done with software that is color managed (not all software is color managed including a number of browsers).

So, whatever technical accurate approach is used (including the one I mention below) will have the above mentioned limitations. So don't be surprised if the technical approaches don't look better than your eyeball approach for the above reasons. With that said here is an alternate approach:

- Instead of jumping to Lab space, just place color sampler tool spot on each of the two colors of interest and in the Info panel, right click on the numbers and change to Lab mode for each. The L channel will show you the "official" brightness according to the data numbers (which may or may not match your visual experience depending on monitor calibration)

- The using your preferred selection tool, select the color you want to modify and then add an adjustment Layer of your choice (e.g. Hue/Sat, curves, levels etc).

- As you make changes to the adjustment Layer (masked to change just one of your colors), monitor the L value between the two color sample points and when the L numbers match, the mathematical tone is the same.

That is another way of mathematically matching the tones in your present working color space, as working in Lab mode is not every ones cup of tea.

Hope any of the posted approaches help you out.
 

Tom Mann

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Hi Felix -

After re-reading your original question, "...how do i adjust these so that they are of equal tonal value ie if they were grayscale they would look identical...", I am worried that you have not factored in the fact that there many ways to turn a color scene into a B&W image, both in Photoshop, as well as in the real world.

For example, if you were using orthochromatic film (or trying to imitate a very old-school look in PS), in my example of a yellow and a blue square, you would need many fewer photons of blue light compared to the number of photons of amount of yellow light compared to shooting with one of the many variants of panchromatic films (or software equivalents) to obtain "equal tonal value". In fact, even the number of photons is not a perfect, one-size-fits-all basis on which to evaluate whether the two squares have equal tonal values. The same is true for trying to use the power of the light at the two different wavelengths because blue photons carry more energy per photon than red photons. If, instead of physical properties, you switch to using physiological responses to different wavelengths of light as the basis of comparison of "equal tonal value", then you get into all of different coordinate systems that we mentioned in previous posts, e.g., HSB, HSL, HSV, Lab, etc.

The bottom line is that there is no single "right" answer to your question. However, if you told us the context of your question, or a bit more about exactly what you are trying to accomplish, we could almost certainly give you pointers that would help in your specific situation. For example the approach suggested by @thebestcpu (ie, staying in whatever color space you are most familiar with, but reading out the coordinates of interest) could easily be modified to work to equalize two "B" values in HSB, two "L" values in HSL, two "V" values in HSV, or even the results of a custom setting of a "Black-white" adjustment layer in Photoshop.

Just let us know.

Tom M
 
Last edited:

MrToM

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I haven't got to 'photon' status yet so here is my simple man's interpretation based purely on your initial post.

Should be fairly self explanatory, but no guarantee its any use to you.
(To be honest I'd made this before the other posts and although not on the same level I was darned if I going to waste it...:cool2:)

Essentially I put a 'color overlay' on the 2 squares 'Y' (Yellow) and 'B' (Blue).
Whilst setting each colour you can use the colour picker to monitor the RGB value in the info panel.
The good thing here is that you can adjust the brightness value with the mouse wheel at the same time as hovering over the image giving you 'real time' feedback of the value.

[video=youtube_share;qj6zY4XX9gc]http://youtu.be/qj6zY4XX9gc[/video]

Regards.
MrTom.
 

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