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Linear Workflow and Gamma Correction within Vray and 3ds Max
 
 
Posted: Jun 13, 2011
Published by: the area
Homepage: Visit the page
Software: Autodesk 3ds Max
Category: Rendering
Skill: Intermediate
 
Tutorial Steps
1Set gamma to 2.2
2Vray Linear Colour Mapping
3Specify gamma 2.2 for textures
4Render output format
5About MintViz
 

Many of you will have heard the terms “linear workflow” and “gamma correction”. It is a must for 3d rendering, especially interior visualisation because it fixes so many problems with render samples, light falloff and cuts post processing tasks in half. I am not going to go into depth about linear workflow and gamma correction as it can become a complex subject but to put it simply, this workflow will increase the fall off of your lights so that light travels further throughout your scene.

Up until now, you are used to adding in extra lights and hitting a high multiplier value in order to get light to flood through your scene and eliminate dark areas. However, this can lead to incorrect burnt out areas. Gamma correction fixes this problem and allows you to reduce the amount of lights in your scene, which in turn reduces rendering times. Here are the settings you would need to change in order to set it up correctly, along with a small explanation of each step.

Set gamma to 2.2

By default, both Autodesk 3ds Max and Vray work in gamma 1.0 (linear space). In Autodesk 3ds Max preferences under enable gamma/lut correction, change the gamma to 2.2. Autodesk 3ds Max will now have an input gamma of 1.0 and an output gamma of 2.2 for standard viewing.

Tick both affect colour selectors and affect material editor

This will drastically change the way you see your material editor and materials within the scene, this is how it should be at gamma 2.2. It will take a while to get used to but it is a necessary step for a linear workflow.

Leave input and output gamma at 1.0

Changing this to anything else but 1.0 will just confuse matters. Keeping this at 1.0 means that your textures will always come in at 1.0 and go out at 1.0. The actual Gamma change to 2.2 will happen elsewhere. A simple explanation straight from the Autodesk 3ds Max help file explains why.

“If you use bitmaps from multiple applications, and these different applications have different ideal Input Gamma values, then turn on gamma correction, but leave Input Gamma set to 1.0. Instead, for each bitmap you read, use the individual Select Bitmap Image File dialog to set the gamma value appropriate to the program that created the bitmap.”

Vray Linear Colour Mapping

In Vray colour mapping, set it to linear multiply and set gamma to 2.2. It is recommended to work in pure linear space and complete your gamma correcting in post production. Tick don’t affect colours (adaptation only). This will render out a darker version of your image. To see the actual result in gamma 2.2 space, click the sRGB button in the Vray frame buffer during or after rendering.

Once the render is complete open it in your post processing application and adjust the gamma to 2.2. The advantage of doing it this way is that it allows the Vray image sampler to see a wider range of noise, resulting in a much cleaner render.
Alternatively you can bake the gamma 2.2 in to the image by not ticking don’t affect colours (adaptation only). However, this could lead to problems when compositing render passes because the final composition may not match the RGB beauty pass due to it not being linear.

Do not tick linear workflow, it may seem like a magical linear workflow button but it is not. Instead it is a quick and brutal way of correcting Vray materials to match a linear workflow and the result can be less than desired.

Specify gamma 2.2 for textures

When adding a texture map to the material editor, in the bitmap loader, manually change the gamma to 2.2. Although they work in gamma 1.0 (linear space), digital cameras apply a total curve which adjusts the gamma to 2.2 to make the image appear more pleasing to the eye. Unless you have advanced photography skills and you are working with RAW images, this is a helpful process for the average happy snapper. It means that, as you may have guessed, the texture map JPEG you took with your digital camera is already at gamma 2.2.

You need to specify the texture map to be 2.2 gamma on entry otherwise the image will be at gamma 4.4 and over bright because you have already told Autodesk 3ds Max to be at gamma 2.2 for both colour selectors and the material editor in step 2. Please note HDRI maps, bump maps, displacement maps and normal maps do not need to be changed to gamma 2.2.

Render output format

When working with linear images and gamma 2.2, the output is mathematically correct as opposed to just looking correct. In post processing, any adjustments will give predictable results and are much easier to manage. Linear images result in a much larger tonal range. To compensate, it is best to save out as a TIF file which is at least 16 bit colour. Saving out an 8bit file format may result in some colour banding because there is not enough tonal range. Never render out JPEGs because this file format is not designed for linear images and you will lose a lot of data when saving in this format initially.

About MintViz

This tutorial is written by James Cutler who is a creative consultant and founder of MintViz. MintViz is a creative consultancy that provides tutorials, tips and resources for the CG industry.

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Posted by jpcutler85 on Jun 23, 2011 at 06:34 PM
You could save as EXR, but the file sizes tend to be rather large. I don't think there is currently a way of opening an EXR in Photoshop with the individual render elements in a stack. It requires a Plug-in, where as TIF files are separate and can be loaded into the stack in one click.
Posted by net_ralf on Jun 22, 2011 at 04:08 AM
If you are using lwf, id recommend saving it in exr, it allows you to save your elements in one container. I was told by Chaosgroup to use the tick "Linear Workflow" when working in lw. Funny, well i ll give it a try :)
Posted by Companioncube on Jun 14, 2011 at 01:35 AM
agree with ruff stuff, check out his blog
Posted by Ruff-Stuff on Jun 13, 2011 at 05:17 PM
Check out my post on linear workflow at www.ruffstuffcg.com for more in depth details on this subject.