Physically Accurate Linear Workflows - Part 1

Over the past few years, 3D artists have gradually been incorporating linear workflows into their pipeline after being convinced of the benefits but, in my  opinion, a linear workflow is only part of the final solution. What I'm referring to is the use of physically accurate lights and cameras and crucially, how to deal with linear images in post-production.

There are plenty of tutorials and explanations on the internet about linear workflows, but very few that explain the whole process from start to finish. This two-part guide will explain the settings I use to keep everything physically accurate, from loading images as maps and materials inside 3ds Max right through to the final render out of After Effects. I will be using one of my scenes, shown below, as an example.

Phase 1 - 3ds Max gamma correction

The first thing we need to do is tell 3ds Max how to handle gamma. (For an explanation of why it is important, check out my article on linear workflows and gamma correction.) Then navigate to Rendering > Gamma / LUT and input the settings below. This article is more of a 'how to' rather than a 'why', so if there is something you don't quite understand, please feel free to leave a comment.

Phase 2 - V-Ray

Now that 3ds Max is working linearly, we need to adjust a few settings in the V-Ray Colour Mapping controls, as seen below.

Next up: the V-Ray frame buffer. If you're still using the 3ds Max frame buffer, I'm afraid it's time to switch, but I guarantee you'll never look back. The buffer is enabled by checking the option in the V-Ray Frame buffer rollout menu, as seen below.

Once that option is enabled, the next time you render, you'll get a new buffer with lots more controls, but the one we are interested in is the sRGB switch located at the bottom.

This option is turned off by default and, as you can see, the image here is rather dark - that's because it's being displayed linearly. Just as we set 3ds Max up to work linearly but display in 2.2, we also need to do the same for the V-Ray frame buffer. This is as easy as enabling the sRGB button. Once enabled, you instantly get an image that looks correct, with lots more light spread evenly over the scene.

This is all that is needed to configure both 3ds Max and V-Ray for a linear workflow but, like I said previously, it shouldn't end there if you want a really accurate solution.

Phase 3 - Physically accurate lighting

When working with outdoor scenes, such as the one above, I see no reason why you shouldn't use the V-Ray Sun and Sky system. I usually put them inside a daylight system so I can use precise data about the sun's annual position.

When setting up the daylight system, ensure the Sunlight parameter is set to V-Ray Sun. Once this option is selected you will be prompted to add a V-Ray Sky to the environment channel (shortcut key 8); accept this prompt. As we are now using the V-Ray sky for our environment light, you'll also want to make sure the daylight system skylight is turned off.

For indoor scenes, it is highly recommended that you use real world lights in the form of IES data files. These data files are available online from lighting manufacturers - Google 'IES data file', find one that suits your needs and download it. Once you have an appropriate light type simply load it into a V-Ray IES light and let it light your scene.

For every light that you come across, there is probably a corresponding IES data file available for download. So, if you see a light that you like (whether it's a particular falloff type or colour) make a note of the manufacturer and search online for its IES file.

Phase 4 - Physically accurate cameras

When working with V-Ray, I would recommend you use a V-Ray Physical Camera due to its ability to use real world camera settings such as shutter speed, f-stop and colour balance. The importance of understanding how cameras work is essential for producing quality images that are properly exposed and have that photographic quality. Even if you are working linearly and have set up real world lights, your scene will still look awful unless you dial in the correct camera settings.

If you don't own an SLR camera or are unaware of what all these settings mean, it can be quite daunting but Jigsaw (with the help of Wikipedia) are here to help. Have a look at this page for an explanation of exposure value (EV) and then scroll down to Table 1 to see the relationship between shutter speed and f-stop.

Now let's take the outdoor scene from earlier. It's a pretty clear sky and since I've set the scene up with a daylight system I know that it should represent light in about 10am in early February. Taking all that into consideration, you would say that the scene is pretty bright, but definitely not at its brightest point of the day or year.

To set the camera up correctly, we need to decide on an appropriate EV number. To do this, take a look at Table 2 on the Wikipedia page linked to earlier.

I reckon the scene we have is a 'typical scene in full or slightly hazy sunlight' which corresponds to EV 15. Now that we know the EV number for our scene we can go back to Table 1, look along the left hand column and find EV 15. Just to put this number in context, there is a rule called the 'Sunny 16' which as you might imagine is the EV number for a sunny day, numbers lower than this correspond to days with less light in the scene.

The next step is to choose an f-stop number. If you don't know anything about f-stop or aperture size, try to stay within the range of 4.0 to 8.0, but just remember that the lower the f-stop, the shallower the depth of field will be. For 3D users, this is your only real consideration when choosing an f-stop number. For our scene, we will be using an f-stop of 5.6 - a fairly standard value.

Now that we have our EV and f-stop number, we can find out our shutter speed setting. In this case it is 1/1000, as shown below by the highlighted red column and row. An f-stop of 5.6 and a shutter speed of 1/1000th combine to give EV 15. Other values that also combine to give EV 15 are: f8 and 1/500th and f4 and 1/2000th.

With our f-stop and shutter speed settings decided on, all we need to do is configure our V-Ray Physical Camera accordingly. Open the modifiers tab and enter 5.6 for the f-stop and 1000 for the shutter speed.

The only other thing that you may also want to configure is the white balance. There is a daylight preset for daylight scenes, so for this scene we will choose that.

Now you can be sure your renderings will be properly exposed and hopefully more photorealistic. Sometimes, despite taking the time to figure out the correct camera settings, you feel your render isn't correctly exposed. This is where the power of the V-Ray frame buffer really makes a difference. The frame buffer gives you the ability to effectively change the EV number of your scene, even after it has finished rendering. What I mean by this is that you want to tell V-Ray that there is actually more light in the scene than you originally thought. 

To do this, enable the corrections control and the exposure correction buttons located at the bottom of the V-Ray frame buffer, highlighted in red in the below screenshot. A new window will appear that will allow you to change the exposure value of the render. So, lets say that we want to tell V-Ray that there was less light in the scene, all you would need to do is to slide the exposure control to the left.

By lowering the exposure value, you are actually lowering the EV number. Conversely, if you need to tell V-Ray that there was more light in the scene, move the slider to the right, as demonstrated in the next image.

That pretty much concludes all the considerations you need to make when setting up 3ds Max and V-Ray for a physically accurate workflow. If you want to go a step further, you can use real world reflective and refractive values of a given material, which can be found at vrayinfo and on Wikipedia, or use a colour calibration device, such as ColorMunki Photo to measure real world diffuse and reflectivity values of any given material you may find lying around.

In part 2, I'll run through how I render out individual elements, how to save that file and how to work with those files inside After Effects.

If you have any questions about V-Ray for 3ds Max settings, or have any tips of your own, please feel free to leave a comment, email us at 3D@Jigsaw24.com or call 03332 409 309.