Inspiring

Answered

Calculate Hue from RGB?

Forum|Forum|4 years ago
December 18, 2021
5 replies
13951 views

Hi there --

Do any of you know the formulas to calculate hue from RGB values? I know I can plug the RGB values into the color picker, which will show the HSL values. That's a tedious process, and I want to do it in an Excel spreadsheet. I have a grid with blocks of solid colors in Photoshop, and I want to arrange the blocks in hue order (0-360). There are different formulas based on red, green, and blue light ratios:

R > G > B

R > B > G

B > R > G

B > G > R

G > R > B

G > B > R

Scott

This topic has been closed for replies.

Correct answer Stephen Marsh

I'm not in Photoshop. I have a list of RGB values in an Excel spreadsheet.

By @scotwllm

So this sounds like it is more of a MS Excel question than a Photoshop one at this point...

https://www.mrexcel.com/board/threads/rgb-to-hue-formula.559852/

=IF(180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2))<0,180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2))+360,180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2)))

To round the result of the formula down to 161 to match Photoshop sRGB, apply the following formula in cell E2:

=ROUNDDOWN(D2)

T

Test Screen Name

Legend

In general I say BEWARE of colour conversion information on the internet. Much of it is written by people who know nothing about colour. However, this is one of the safe ones. RGB is (as you may know) not an exact reference of colour: two people may see the same RGB colour as wildly different. However, HSL is a simple conversion of RGB. This means of course that HSL is also not an exact reference of colour and may look wildly different to different people.

Ok, warning aside, this page has both a converter and the actual formulae it uses:

RGB to HSL converter | color conversion (rapidtables.com)

Don't necessarily trust the other converters on this site though, it has the common nonsense of a "CMYK to RGB converter" as if this even exists.

scotwllmAuthor

Inspiring

I understand about output looking different on different screens or when printed. I doubt the people I share these files with will have a monitor as good as mine. All I am trying to do is to sort the colors.

Stephen Marsh

Correct answer

Community Expert

I'm not in Photoshop. I have a list of RGB values in an Excel spreadsheet.

By @scotwllm

So this sounds like it is more of a MS Excel question than a Photoshop one at this point...

https://www.mrexcel.com/board/threads/rgb-to-hue-formula.559852/

=IF(180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2))<0,180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2))+360,180/PI()*ATAN2(2*A2-B2-C2,SQRT(3)*(B2-C2)))

To round the result of the formula down to 161 to match Photoshop sRGB, apply the following formula in cell E2:

=ROUNDDOWN(D2)

scotwllmAuthor

Inspiring

It's a Photoshop project that uses Excel a bit. It's a reasonable assumption that Photoshop users would be more versed in everything to do with light than your everyday Excel expert who's probably more interested in macros to create invoices.

I tried this formula earlier today. You have to change the semi-colons to commas, and it still doesn't work.

Stephen Marsh

Community Expert

It worked for me in LibreOffice, it also auto converted the semi-colons to commas when I pasted in the formula. The screenshots in my post shows that it works (in LibreOffice at least) and I tested and verified in Photoshop with multiple values before posting this as a proposed solution. I have updated my previous post to use commas.

EDIT: It also works in Excel and Apple Numbers. I have attached the spreadsheet.

rgb_to_hue.xlsx

Semaphoric

Community Expert

If you search the web for "rgb to hsl algorithm", you'll find lots of info about this. Mostly differing in presentation,some more general, some aimed at coders.

Here's a site that has description of how it works.

TheDigitalDog

Inspiring

First, you must define which RGB color space. That defines the scale of the RGB values. IOW, in sRGB, R234/G56/B89 isn't the same color as the same triplets in Adobe RGB (1998).

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

scotwllmAuthor

Inspiring

At this phase of the workflow, I'm not in Photoshop. I have a list of RGB values in an Excel spreadsheet. I need to sort them by hue. Then I use the colors in that order in Photoshop.

TheDigitalDog

Inspiring

Eyedropper tool. In this instance, I screengrabbed color samples from an Adobe pdf document.

@scotwllm wrote:

Eyedropper tool. In this instance, I screengrabbed color samples from an Adobe pdf document.

And that's based on a color space. What is it?

Oh and a screenshot doesn't account for the source color space.

Even using Apple's Digital Color Meter utility** or similar to 'measure' color. They can't tell you anything about the actual color numbers of documents inside say Photoshop. This is a Photoshop forum and if you don't care what PS now does with the numbers fine. PS does however. Sorting RGB and calculation from RGB in PS, big difference.

**This is not measuring anything. It is taking two or three bits of information:

1. The color that an app is actually outputting to a pixel. i.e. an RGB level.

2. The colorspace that the app says should be used for that pixel for ColorSync to correctly display it (defaults to sRGB if the app doesn't specify).

3. The ICC profile associated with the display.

For this reason, a mismatch in numbers is expected.

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

Kukurykus

Legend

with(new SolidColor) rgb.hexValue = '00ff00', hsb.hue

scotwllmAuthor

Inspiring

Thanks for the speedy reply. Kukurykus. What do I do with that code?

Scott

Kukurykus

Legend

Save as hue.jsx to Presets/Scripts folder of your Photoshop. After relaunch go to File/Scripts.

The '00ff00' is hex value you may change to other while end of code change to alert(hsb.hue).

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