ACES STM2065-1 and sRGB tone response curve

Hi all,
I would like to have the explanation on Why ACES Linear image display differently compare to old fashioned Linear image in sRGB tone Response curve.

If I understand correctly There is only one in Linear image data ACES Linear and old fashioned Linear should have the same response curve
(Correct me if am using any wrong terminology)
Looking forward to learning from you all

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Welcome and thanks for your first post…I’m sure someone will respond to this shortly!
Best,

Steve T
ACESCentral Admin

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Hi @chucheep.chaipoon,

TLDR:

Friends don’t let friends view scene-referred imagery without a S-Shaped transform.

Long Story:

Basically, looking at scene-referred imagery without a display rendering transform that does some sort of tone compression is not really desirable.

As an example, let’s assume you shot outside and the scene peak luminance as measured was 20000nits and shadows about 0.0005nits. Now, you are back inside and want to display that on your screen that only goes from 0.01nits to 120nits, as you can see, there is a problem, the dynamic ranges are dramatically different.

You could naively compress everything to fit, but you would lose your imagery almost entirely as it gets pushed in the shadows. Alternatively, you could cherry pick a suitable window in the dynamic range but would clip blacks and highlights. The standard solution to that issue is a non-linear compression function that allows to fit as much possible of the dynamic range of the scene into your display dynamic range while maintaining a faithful appearance.

I have some images I can dig around if it does not make sense (but it should).

Cheers,

Thomas

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Hi,
take for example Blender: From version 2.79 there is was different tone mapping (FILMIC) the default way to display images in Blender. Before there was the standard sRGB tone mapping. The working colorspace primaries actually stayed the same (sRGB/rec.709 linear) with FILMIC, but the new tone mapping allowed to loose the hard clipping at 1.0 (same in ACES)
Before you had to try to light with lower energy no to over brighten certain areas in your render and now you can freely use higher values and get a more realistic result easier.

The underlying image data is in both cases the same, you can even switch the tone mapping method after you finished a render. So it is intended that they look different. That is the whole idea. I wrote an article about the transition from standard sRGB to FILMIC that you can find on my website www.toodee.de

ACES is also scene linear, but it uses a wider gamut than sRGB/Rec709 and therefore the result will look different depending which kind of output device transform you use and it depends on the kind of display you review your work.

I hope this also helps.

Best regards

Daniel

Thank you for your support,

For example, if I have the Arri Raw which has been captured in AlexaLogC

A- I transform the color with OCIO color management system to ACES STM2065-1 (scene linear)
And view in sRGB monitor using sRGB(ACES) response curve

B- I transform the color without using OCIO color management to Linear
And view in sRGB monitor using sRGB response curve

From my understanding,

  • A and B will look different because of the sRGB(ACES) Response curve is different from legacy sRGB response curve

  • A ACES Linear AP0 has a wider color dynamic range than B Linear image data and that make the sRGB response curve display differently.

Please correct me if any of the statement I did above are wrong and please explain.

I have a test with ARRI raw file by

  • Read The file into nuke and specify the IDT to Raw and display in sRGB color response curve and using nuke default color management (linear)

  • Read the file into nuke and specify the IDT to raw and Display insRGB color response curve and using OCIO ACES1.1 color management (ACES Linear)

The result is
1 - The image display differently in the same monitor

Why the image display differently since they both using the same color responses curve (sRGB) ?

2 - I sample the same pixel at exactly the same X,Y coordinate and they have different value.

Why the value is different since they both are the same image data reading as a raw data?

Is it because the working color space is different?

Which is mean Linear ACES working space is complete something different from
Nuke default Legacy Linear working color space ?

And what exactly is the differences between both of them?

I have to apologize if asking questions like a fool here.
I tried by best to understand Color science as an artist.

I have a homework from my boss where I need to have a better explanations About the differences between ACES linear and Legacy Linear workflow ( please suggest a suitable terminology if possible)

Best,
Chucheep


Hi Daniel
Understood, So ACES Linear image data (STM2065-1 and ACEScg) have more dynamic range than Non-ACES Linear Image data then after apply the sRGB response curve (LMT) to both of them (Non-ACES Linear and ACES Linear ) the result of them will look differences. am I understand correctly?

Hi Thomas,
Thank you so much for your time.

I’m understand that if the monitor is has been calibrate to sRGB we need to input the IDT correctly to Linear and Transform the Color to sRGB Response curve (ODT) in order to view the Linear Image data correctly in sRGB devices

But my question is Are there any differences between ACES Linear and Non-ACES Linear image data? Because I thought that both of them are the same because they both are Image data with Linear response curve.

I am so sorry that my Topic question might not clear.

Best regards,
Chucheep

Hi Chucheep Cahipoon,

no, you are mistaken. The only difference between a linear sRGB/Rec.709 EXR file and an ACES EXR (2065-1 or ACEScg) file is how it must be interpreted do be able to display it correctly. Both files just store RGB pixel- and metadata. I was sharing an article about that topic here on ACESCentral.
The view transform then makes the difference in the look of the images at the end. As you are using Nuke just open two instances and set one to Nuke-Default and the other to OCIO with a view transform set to ACES(sRGB).
I find it helpful to start playing around with a simple ramp like I was doing this here on my website and the previous page.

it might be helpful to read some more background information that you can find on the website from Chris Brejon.

Best

Daniel