[Sidefx-houdini-list] Linear Workflow

Andy Nicholas andy at andynicholas.com
Thu Aug 26 03:45:36 EDT 2010


Hi Craig, thanks, yep, that's what I'm doing already.

You know, I thought I understood all this linear colour workflow business. What endlessly confuses me is when people talk about gamma and gamma correction, as I'm sure these terms are often used interchangeably in articles when they shouldn't. One puts the colour to the power of the value, while the other puts it to the power of the reciprocal of the value, and I can never remember which. (Jordi did send me a pretty good link to a GPU Gems article that seems to explain it, so I guess I need to commit that to memory)

What also didn't help my understanding was by putting Houdini's colour correction gamma setting to 1/2.2 it made a pure linear ramp (defined by a shader) in Houdini look correct in the render (i.e. an even distribution of black through white), but using a value of 2.2 made it look anything but linear and over exposed the dark areas. I still haven't honestly figured out why 2.2 is the correct value to be using, except that it matches our existing workflow in XSI which we've been using for ages. If anyone has a good explanation of that, then I'm all ears.

A



On 25 Aug 2010, at 20:10, Craig Hoffman <craigleehoffman at aol.com> wrote:

> Is your monitor set to Linear space too?  If not, you should apply a gamma at the end of the chain to visualize the image properly.  
> 
> Linearizing generally is something you do to textures before the render so the lighting math all works out correctly and then you gamma at the end of the render or composite to look at it- unless you have your monitor set up in linear space (but then everything else will look like garbage usually and life is generally painful).
> 
> -Craig
> 
> 
> -----Original Message-----
> From: Andy Nicholas <andy at andynicholas.com>
> To: sidefx-houdini-list at sidefx.com
> Sent: Wed, Aug 25, 2010 11:52 am
> Subject: [Sidefx-houdini-list] Linear Workflow
> 
> Hi guys,
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> 
> We've just started a job in Houdini and we're trying to use a linear
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> colour workflow. With the test I'm doing at the moment, I don't have any
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> textures, it's all pure lighting and shader models. I've got the Color
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> Correction gamma setting at 2.2 which I believe is the correct setting as
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> it visually matches what we do in XSI when we enable colour management.
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> The problem is that we're having a few issues with generating glossy
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> reflections because noise that wasn't visible before, now shows up a
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> treat, even with a large number of samples.
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> This is partly because the objects I'm reflecting are quite small, so in a
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> way, I'm not surprised I'm getting some noise. The issue is that before
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> using colour management, this wouldn't have been visible. On a black
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> background typically some of the noisy pixel values are only at around
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> 0.0002, but they still show up as little grey splats on the image.
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> I've been doing a few tests, and while I get noisier results in mental ray
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> (using XSI with colour management enabled), it generally isn't as bad as
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> I'm seeing in Mantra.
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> So my simple question is, have any of you guys run into similar problems,
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> and if so, what strategies did you use to work around this issue? (or
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> alternatively, are we just doing something stupid?!)
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> Thanks,
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> Andy
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