[Sidefx-houdini-list] Linear Workflow

Peter Bowmar pbowmar at gmail.com
Tue Oct 5 06:19:14 EDT 2010


Hi David,

This workflow assumes a monitor of 2.2 gamma which is typical of all
monitors except possibly Apple computers/monitor combos. Those can be
1.8 or something like that, though I've heard they're moving to 2.2 as
well.

So, when you have a linear render you need to apply 2.2. gamma (or a
LUT) to your Mplay/IPR viewer.

Cheers

Peter B

On 4 October 2010 20:13, David Burton <daland at withatwiststudio.com> wrote:
> Does this workflow assume the monitor gamma (IE: what I would adjust in the graphic card settings)
> is set to 1?
>
> thanks,
>
> david
>
>
> On Aug 27, 2010, at 5:10 AM, Magnus Pettersson wrote:
>
>> Just to summarize for some people who might still be confused by all this
>> (most likely :P) what to actually do. This is "the ballpark" linear workflow
>> without LUTs and so on. I assume all textures and colors is already sRGB
>> (2.2) space (if stuff looks good on your monitor, it is most likely in sRGB
>> and need conversion to linear):
>>
>> 1. ALL shaders should have their input colors/textures converted to linear
>> -> apply 1/2.2 to all textures and color parameters (including point
>> colors). In houdini right now it could equal to add a multconstant node
>> after a color parameter before its piped into something (like a mult etc).
>>
>> 2. Ok now all your inputs should be correct, time to Render. Now to view
>> this "in the ballpark sRGB" put the mPlay gamma to 2.2 and this should give
>> you a good appreciation of how your render looks (if your monitor colors
>> isnt totally wacky then you should go buy/borrow a monitor calibrator!). And
>> this ofcourse means if you watch the picture in mPlay gamma 1.0 it will look
>> dark (because of your monitors gamma correction making it darker).
>>
>> 3. When happy with the render looking at it with this mPlay gamma 2.2 you
>> can send it off to comp. Like people said Nuke comp artists just take in all
>> your renders with linear space and everyone should be happy :)
>>
>> The reason we have this very confusing gamma 2.2 here and 0.454545 there and
>> some makes stuff darker (monitors) and other brighter (like mPlay gamma) etc
>> is because of the EVIL old CRT screens had a negative gamma (technically
>> limitation) and to correct for this every picture produced, every GUI
>> produced and most likely every photo taken and every webpage have been
>> having a 2.2 positive gamma applied (mostly in background without user
>> knowledge) to correct for the CRTs shortcomings. BUT now a days with our new
>> technology our LCD screens have no problem showing right colors in linear
>> (which would be the best for us in this business so we wouldnt need to
>> care).
>> But if LCD screen manifacturers from the beginning would give you a screen
>> in linear space as default noone would buy them because people would go "omg
>> this new technology is crap, everything is ugly!" so they have applied this
>> negative gamma curve to display the content of all programs and all internet
>> "correct".
>>
>> Myself would like to see linear becoming the new standard, but that would
>> require all digital content that is shown on a monitor to be chaged. So most
>> webpages on the internet would have to change its colors, all pictures
>> produced until now need to be converted to linear and so on, which you can
>> understand is a massive undertaking. Not to mention all new content made
>> needs to be made in linear (photos, programs gui, webpages etc)
>>
>> Because of a very small procentage of the people who owns a monitor or
>> creates the GUI/webpages/images found in this digital age is actually aware
>> of this linear space mumbo jumbo, so seeing a change in this in the
>> near/distant future is very unlikely so we will have to continue our sRGB to
>> linear (to sRGB) pipeline.. i blame CRT!
>>
>> Magnus Pettersson | Effects TD
>> Storm Studios AS
>> www.stormstudios.no
>>
>>
>> On Thu, Aug 26, 2010 at 9:23 PM, Andy Nicholas <andy at andynicholas.com>wrote:
>>
>>> Thanks Edward, yep, I think this is the piece of the jigsaw I'm missing.
>>>
>>> I need to go away and do some research around all this, particularly the
>>> perceptual bit. Once I set it straight in my head, I'll write a summary on
>>> my website.
>>>
>>> Thanks again,
>>>
>>> A
>>>
>>>
>>>
>>> On 26 Aug 2010, at 19:39, Edward Lam <edward at sidefx.com> wrote:
>>>
>>>> On 8/26/2010 3:45 AM, Andy Nicholas wrote:
>>>>>
>>>>> 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.
>>>>
>>>> You're missing the fact that the eye itself also has a nonlinear
>>>> response. When you put in a 2.2 value into mplay, you're now undoing the
>>>> power curve that the monitor applied, giving you linear _luminance_. Do
>>>> not confuse this with _perceptual_ lightness.
>>>>
>>>> If you want *perceptual* (lightness) linearity, then you need to
>>>> generate an approx. 0.4 power luminance curve [1], explaining why
>>>> putting an mplay gamma value of around 0.4 "works".
>>>>
>>>> Lastly, this issue has nothing to do with Houdini in particular. Take a
>>>> linear gradient in another package (say Photoshop) and you will see the
>>>> same thing.
>>>>
>>>> Regards,
>>>> -Edward
>>>>
>>>> 1. http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#lightness
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