Tag Archives: linear workflow

Understanding Gamma and Linear workflow

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Even If I’m aware of what gamma and Linear workflow is, I’m not quite sure I’m using it always in the correct way. So I decide to dive into documentations and forums again to refresh my memory about it and at the same time closing a few gap.
Since so many people, even in the industry, still don’t know what it is and how it works, I thought I would make kind of a dairy of what I found on with my research those couple days.



Introduction


To get started, there is this great example from AEtuts+.com talking about Linear workflow in AE. It is not a the deepest explanation out there, but it will give you a nice overview with simple words and explicit example of what Linear workflow is and why it is so important !

So after that, 5 points you should keep in mind about Gamma (from Gamma 101 on mymentalray.com)

  1. Most displays have a non-linear response to pixel values.
  2. Most graphics software are written for a linear color model, i.e. they make the simple assumption that 255 is twice as bright as 128. But since the monitor is non-linear, this is not true. In fact, for most monitors (with a gamma=2.2), you need to send the pixel value (0.5^(1/2.2))*255=186 if you want 50% of the brightness of 255. The commonly used value of 128 will only produce about (128/255)^2.2 = 22% brightness.
  3. Digital cameras have a (roughly) linear response to light intensity, but since they are intended for display on computer monitors, they embed the non-linearity (gamma) in the image. (True for .jpg, whereas RAW files are just that – RAW, i.e. linear data, that gets non-linear when converted to JPG, for example)
  4. Therefore, if you input .jpg images taken with a camera into a graphics software, you need to compensate for the images gamma. (by inverse gamma. 1/2.2 = 0.455)
  5. And if you display the (linear) data generated from a graphics algorithm, you need to compensate for the display gamma. (add 2.2 gamma to the picture)



A few facts :

When creating a texture in Photoshop, you’ll see its color with 2.2 gamma applied (Because screens are big liar :p). Meaning when you think you got the good “brightness”, you actually made it twice (or more) brighter than what it’s supposed to be in real world.
When for painting, or montage it might not be import, for texture it is really important !!! Because as said above, your renderer/shader/… will assume the picture is linear and will apply math according to that.
So the only solution to bring this picture back to a “linear color space” is to set the gamma to the inverse of what the monitor shows you. As we know on PC, gamma are shown as 2.2 (I think it’s 1.8 on mac OSX). So the gamma value of your texture before saving it should be 0.455 (1/2.2).

Tips : In Photoshop, on top of your layer, add a “Level Adjustment Layer” and set the gamma value (mid-tone) to 0.455

With most today software I don’t think it is necessary to do that any more, but to be honest, this really depends on how the software you are using integrate Linear Workflow. For instance in 3Ds Max you can enable the Gamma correction in the “Gamma and LUT” tab of the preferences panel.

Because renders works in Linear space, your rendering would seems to look darker on your screen. So in case you are saving it to a 8bits type file (as JPG), you should set the output gamma parameter to 2.2. But in case you are saving  it to a floating point file (HDR, RAW, EXR, …), this parameter should remain 1.0. Because all the dynamics of your picture is saved in those raw file, you would apply the Gamma only in post process (compositing).

In the above case with After Effects, by making sure to activate the linear space workflow, it should take care of that for you, so you don’t have to change gamma to anything, just leave it.



Links :



Mental Ray Linear Workflow from chad smashley on Vimeo.





Here some nice read :

Blender 2.5 – Color Management and linear workflow

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I’ve been waiting for this since ever ! Blender 2.5 is managing linear workflow (gamma correction, … ).
Matt Ebb just did a great update where the UI (color picker, …) give you feedback of linear color and present the linear workflow.

blender_25_linear_workflow_pipeline

Here is the comment of the SVN update (revision 25065) :

Changes to Color Management

After testing and feedback, I’ve decided to slightly modify the way color
management works internally. While the previous method worked well for
rendering, was a smaller transition and had some advantages over this
new method, it was a bit more ambiguous, and was making things difficult
for other areas such as compositing.

This implementation now considers all color data (with only a couple of
exceptions such as brush colors) to be stored in linear RGB color space,
rather than sRGB as previously. This brings it in line with Nuke, which also
operates this way, quite successfully. Color swatches, pickers, color ramp
display are now gamma corrected to display gamma so you can see what
you’re doing, but the numbers themselves are considered linear. This
makes understanding blending modes more clear (a 0.5 value on overlay
will not change the result now) as well as making color swatches act more
predictably in the compositor, however bringing over color values from
applications like photoshop or gimp, that operate in a gamma space,
will give identical results.

This commit will convert over existing files saved by earlier 2.5 versions to
work generally the same, though there may be some slight differences with
things like textures. Now that we’re set on changing other areas of shading,
this won’t be too disruptive overall.

Check out the documentation on the wiki here : http://www.blender.org/development/release-logs/blender-250/color-management