[Sidefx-houdini-list] Linear Workflow

David Burton daland at withatwiststudio.com
Mon Oct 4 23:13:36 EDT 2010


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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