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Setting your Mid-Grey Value

Jahirul Amin|07.11.2022
Here we will look at how to go about setting your mid-grey value and I can tell you in advance, it ain't gonna be 0.18!

Introduction

Just a quick note that although this document is hosted under ‘Surfacing’ (Texturing, Look Development and Groom), it is very much relevant to Grading, Lighting and Compositing too (any department that deals with colour really).

This particular lesson is a snippet from the following course:

The mid-grey value allows us to set a standard in which we can go about neutralising our images, and just as importantly, it allows us to take a series of Macbeth charts, all shot with different cameras and under different lighting conditions, and align them. This in turn will allow us to more consistently grade and expose shots, textures, and shaders.

Before we begin, I highly recommend you read through the following conversation:

If you want to test this yourself, you’ll need the following data:

The above data has been provided by the wonderful team at Colour Science, so a huge thanks to them. If you can, I highly encourage you to support them, so they can provide us with even more fancy tools and training. There is a reason why so many colour-related headaches have been reduced over the years, and they, along with ACES Central, have been instrumental in pushing for consistency and standardisation.

…and you’ll also need a HDRI, of which I am using the following:

..but again, you can use any HDRI, or even stick with the Sun and Sky setup.

If you are looking for a more detailed step-by-step breakdown on all things onset related, you can check out our courses on the topic here:

Setting up your Applications

Before we begin, you’ll need to ensure your applications (Maya, Nuke, etc.) are all working within an ACEScg colour space. For guidance on setting this up, please take a look at this document (or check out the intro portion of the main video on this page):

Defining the Mid-Grey Value

The first thing we need to do is set a mid-Grey value as our base on which we can ensure consistency across the board. In your studio, you might be sticking with 0.18 or 0.21. For us at CAVE, we are going with 0.19, and here is why…

If we load the X-Rite Colorchecker Classic 2005 GretagMacbeth v2.0 script in Nuke, we can pick from a selection of Macbeth charts that have been calibrated. And when hovering over the mid-Grey swatch (swatch 22), we can see that the result is a value of 0.19. It is also important to note that the mid-grey value is the result of both diffuse and specular reflection. The sum total. Therefore, in your shader, your baseColour value may indeed be slightly lower (say back to 0.19 once you enable the spec).

What you should NEVER do is hold up a Macbeth chart in your current environment against the image you see on the screen.

Why? Because, on most occasions, the Macbeth chart will have been shot under a specific lighting setup (the image you are seeing on the screen), and you will be existing in a different lighting setup (whevever you are in the real-world at that specific moment in time). So they will never match.

On the odd occasion, it could be that you are lit in the exact same lighting environment as the image of the Macbeth chart (especially as we head into more immersive experiences and shoot conditions) but until then, DON’T DO IT!

Lighting is key to everything. Please remember that.

Testing in Maya

Now that we’ve defined our mid-Grey and we have a ‘standard’ we can always refer to. Let’s test this in Maya (and you  can use any application you like that supports an ACEScg colour space). Start by creating a simple polygon plane and apply s Surface Shader (essentially a ‘constant’ shader where lighting does not affect the results).  The apply the following exr into the colour input of the shader: ACEScg_Labels_ColorChecker2014.exr

If you render this out, you should see something like so. You won’t need any lights as this is again, just flat colour. If you review the values of swatch 22, you will see that you get a value of 0.191. Whoop! Whoop!

Creating a Grey Ball

With the Macbeth chart up and running (and we’ll switch the shader so it responds to light later on), let’s get a Grey ball created. Do this by creating a sphere. Easy 😉

Apply a Standard Surface material and call this material grey_MAT. Then set the following values:

  • Base Color (HSV): 0, 0 , 0.191 (make sure to do this in Rendering Space)
  • Base Metalness: 0
  • Specular Weight: 0 (we will turn this on later)

Next, create an Arnold SkyDome (Arnold > Lights > Skydome Light), which will be a 360 white dome of light, and hit render. You should see something like so, and again, when reviewing the values, all should be 0.191.

So all good and as expected. Now, please do remember that the mid-grey value  of 0.19 should be the result of both diffuse and specular reflection. So, we will modify the baseColour value to compensate for this.

Comparing the Renders in Nuke

If you export the renders (as EXR and JPG – just for the fun of it), and load them into Nuke, comparing the values, all should remain consistent.

Modifying your Grey Ball and Creating a Chrome Ball

Currently our Grey ball has no specularity, which is not how things would be in the real-world. So tweak the following parameters on the shader:

  • Specular Weight: 1
  • Specular Color: 0.4 (or something similar)
  • Specular Roughness: 0.6

You will want the above values to match your physical real-world grey ball. If you do not have one (maybe you are working on a full CG project), then the above values should do the trick. Or else, look at some real-world paint reflection values.

As we’ve now added some specularity to the grey diffuse ball, if we were to sample the ball, it will more than likely be higher than 0.19, therefore, we recommend you dial down the baseColour value until the value in render equals 0.19.

For the Chrome ball, create another sphere, and apply a new Standard Surface material and this time, call this material chrome_MAT. Then set the following values:

  • Base Color (HSV): 0, 0 , 0.6 (make sure to do this in Rendering Space)
  • Base Metalness: 1
  • Specular Weight: 1
  • Specular Color (HSV): 0, 0 , 0.6 (make sure to do this in Rendering Space)
  • Specular Roughness: 0.06

Now, I do have a reason for 0.6 for the chrome sphere, however, I need to dig out my notes to better explain why. Once I’ve dug them out, I’ll share more info on the reason why. Or if anyone else has the knowledge, please do share your thoughts in the comments.

Doing a quick render, we get something like so. The mid-Grey value may change slightly. That is all good and expected as we’ve added some specularity to the shader.

Grey/Chrome Response to Lighting

Currently, things look a little flat. And that makes sense as we’ve surrounded the spheres with constant white light. We are in The Construct! Sorry, Matrix reference. I hope you got that.

And that is how the spheres should look in such an environment. Now add a HDRI to the Skydome and check out the results. Again, I am using the following (but you can use any HDRI):

You can see from the result that the spheres are very dark but the Macbeth is constant (due to the shader applied). We’ll fix the Macbeth shader but DO NOT change the sphere shaders. We have set them to be correctly calibrated.

What you need to change is the lighting. Our shaders for the spheres should stay consistent. And now we can see our balls! Great. But now you might say:

“The ball has gone ‘blue-ish’, and the Macbeth is looking correct. Aggghhhh!!!”

Don’t worry – this is as expected. The grey ball looks ‘blue-ish’ becuase the light has blue in it. It is still grey. The Macbeth chart looks as it does becuase the current shader does not respond to light. So apply a Standard Surface to the Macbeth chart geo, plug in the texture, and waa-laa, you should have something like so:

And now you might start crying and go:

“Aggghhhh!!!! Now it all looks blue-ish”

But fear not. That is how it should look. Because now the Macbeth chart and the balls are responding to light. You shine a coloured light on something, you get a different result. That is the magic of the universe! Don’t believe me? Then watch the film, In the Valley of Elah with Tommy Lee Jones and Charlize Theron, and tell me what colour the car is (drop your answer into the comments).

Experiment with changing the HDRI or play with the Sun and Sky setup to see how the reference objects respond to light.

What is the Deal with Photoshop

So if you take your image into Photoshop, you’ll find that the result for mid-Grey will result in a value of around 95-ish. Not around the 128, 128, 128 mark (for RGB).

This is because in Maya and Nuke, we are using the ACES 1.0 SDR-video (sRGB) View Transform. But if you flip this to Un-tone-mapped (sRGB), you should be good to go.

…and this should bring you more in-line with what you expect. Nice 😉

Conclusion

So in a nutshell, at CAVE Academy, we’ll be setting the mid-Grey value across the board on all shaders/applications in ACEScg colour space as 0.19.

Considerations

Upon doing further research, I found that many grey balls will be painted to match the Kodak grey (which is 18%) , so is it best to set your grey ball shader to 0.18 or 0.191? Hmm, something to think about. At CAVE, we’ll be going with 0.19.

If you are looking for a paint closer to the mid-grey on the Macbeth chart, you could use the Munsell NG Neutral Grey Paint:

The best thing to do is actually measure the reflectance of your grey ball and then create a digital shader to match that. This is something we’ll document later on.

Onset Training

For more information on all things onset related and look development, you can check out our courses here:

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If you are a fan of mixing science with art, or maths with creative thinking, then join our Connection Programme, where we’ll be exploring VFX, animation and games from the core fundamentals:

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Comments 2
  1. Rachel on 16.08.2023 at 10:47 BST

    Do you know where is best to buy VFX chrome and grey ball kits in the UK.

    Reply

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