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Old 08 August 2008   #31
Originally Posted by moidphotos: Firstly I just wanted to say thankyou to all those who have contributed to this thread; it's been highly educational and has certainly made me re think some of my workflow.


agreed. fascinating thread. thanks.
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Old 08 August 2008   #32
Originally Posted by moidphotos: Firstly I just wanted to say thankyou to all those who have contributed to this thread; it's been highly educational and has certainly made me re think some of my workflow.

Secondly to Dtox, I think what you are asking for is called Spectral Rendering, that is rendering using a much larger energy range than visible light squeezed into the confines of RGB channels. Mental Ray supports it, but I have never had any actual experience of using it, I heard about it in a conversation with an employee of Mental Images who explained that he was helping a client create spectral shaders so that they could render images in 18 colour channels, describing the appearance of a surface as it would appear in different 'lights' for instance how ultra violet light would effect it, or what it would look like in infra red. Hope that is some use to your future research.


Well "spectral rendering" normally refers to sampling the visible spectrum (usually the range 380-780nm if I remember correctly) much more finely than your typical RGB colour representation. In other words instead of having 3 colour components--R, G and B--you might have 8, or 10, or 30.

This allows you to simulate effects such as dispersion--light "breaking up" into its "component colours" when refracted. Of course there's no reason why you can't represent "colours" outside of the visible spectrum as well, but there's very little reason you'd want to for entertainment purposes.
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Old 08 August 2008   #33
This allows you to simulate effects such as dispersion--light "breaking up" into its "component colours" when refracted.

Is that what dispersion actually does?
I'm talking about the dispersion attribute of a basic reflective and/or transparent material.
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Old 08 August 2008   #34
Originally Posted by Dtox: Is that what dispersion actually does?
I'm talking about the dispersion attribute of a basic reflective and/or transparent material.


In what shader in what renderer? Dispersion is caused by the the tendency of materials to refract different wavelengths of light to a different degree, causing rainbow-like colour effects (such as rainbows, for instance ).

There's a rather more in-depth explanation on wikipedia http://en.wikipedia.org/wiki/Dispersion_(optics)
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Old 08 August 2008   #35
In a basic cinema4d shader there's an option called dispersion in the reflection and transparency channels that's controlled by percentage.

I always thought it dispersed the actual reflection making it blurry.
That doesn't account for transparency though, so it's always kind of been a mystery to me.
The effect it gives isn't very dramatic, so not knowing exactly what it does I always avoided using it since it doesn't seem necessary.


Also, when you see the term "additive" in regard to cg lighting, what exactly does that mean?
Does it have the same meaning as when you see it as a blending method within a shader structure?
Does it refer to "additive color theory"?
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Last edited by Dtox : 08 August 2008 at 01:31 PM.
 
Old 08 August 2008   #36
Yes, basically. It usually refers to the assumption made that light combines in a linear, additive fashion, i.e. rendering an image using several lights individually then adding them together (as in A+B+C) will give you the same image as if you'd rendered an image with all the lights turned on.

There are certain situations in which this isn't true (such as capturing images on film), but it's close enough and is such a dramatically simplifying assumption that it's good to use.

I'm not familiar with cinema4d, so I don't really know what that control is doing. Sounds like it's a blurry reflection control that's just poorly named (it might be better called 'divergence').
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Old 08 August 2008   #37
Yes, basically. It usually refers to the assumption made that light combines in a linear, additive fashion, i.e. rendering an image using several lights individually then adding them together (as in A+B+C) will give you the same image as if you'd rendered an image with all the lights turned on.

Are you referring to compositing a multi-layer image together where the lights are done as separate passes?
It also allows you further control, no?

Is there a specific blending mode that's required/recommended for the math to properly come together in a situation like that?
Such as "add", which would also refer to the "additive" function I asked about earlier?

This is sort of an off the cuff question.
Would it benefit me to use real values in my light attributes even if I'm not using a renderer like Maxwell that's built that way?
For example, assume I'm lighting a pretty standard interior scene with a few normal area lights to simulate real halogen lights.
Is there any benefit to using the same color temperature in a CG area light that a physical halogen light uses even if I'm not using something like Maxwell or Vray?


I'm not familiar with cinema4d, so I don't really know what that control is doing. Sounds like it's a blurry reflection control that's just poorly named (it might be better called 'divergence').

That's exactly what it was.
A poorly named option for blur.
In the next version of cinema 4d it's just called "blurriness".
Why the hell they would even call it dispersion is beyond me.

C4D does that alot it seems.
Mostly in material parameters.
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Old 08 August 2008   #38
Originally Posted by Dtox: Are you referring to compositing a multi-layer image together where the lights are done as separate passes?
It also allows you further control, no?

Is there a specific blending mode that's required/recommended for the math to properly come together in a situation like that?
Such as "add", which would also refer to the "additive" function I asked about earlier?


Yes, just a plain 'add'. I can never remember which of photoshop's blend modes actually do that, but it's just an add node in Shake. It's the assumption of the addition combination of light that allows us to do things like render lights out as separate passes then add them together again.

This is sort of an off the cuff question.
Would it benefit me to use real values in my light attributes even if I'm not using a renderer like Maxwell that's built that way?
For example, assume I'm lighting a pretty standard interior scene with a few normal area lights to simulate real halogen lights.
Is there any benefit to using the same color temperature in a CG area light that a physical halogen light uses even if I'm not using something like Maxwell or Vray?


It depends on what you're doing really. If you're creating entirely CG images then if it helps you choose values for lights that make sense with each other, then it's probably worthwhile since it saves you the hassle of manually choosing colours according to temperatue. If you're trying to match to a plate then it's pretty useless since the exact colours recorded on a piece of film bear very little resemblance to any kind of 'real' colours.

The 'real' values in Maxwell are just a convenience really since the image is tonemapped before output anyway so the exact numbers are meaningless really.
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Old 08 August 2008   #39
The 'real' values in Maxwell are just a convenience really since the image is tonemapped before output anyway so the exact numbers are meaningless really.

So Maxwell tone maps its output.
Damn, I never knew that.
Is that a standard method in 3rd party render engines?
Or is it more of an exclusive thing with Maxwell?

I learned a bit about tone mapping at a photography board last year.
On a slightly different subject, there might be some benefit to rendering to RAW files.
A renderer could theoretically do it flawlessly because the image doesn't come from an optical device.
You could match rendered footage in the RAW workflow to footage from a camera like the Red1, and stills in something like Adobe cameraRAW.
Then you'd have an even greater amount of control over the images attributes before it is actually converted into a bitmap image.
You'd also have the RAW file for archiving.

Anyone who's used the Adobe Camera RAW workflow for digital images knows it's benefits.
Now with the Red1 there's a RAW workflow for digital video that outputs at 2k and 4k.
With RAW you're working directly with the captured data in a pre-bitmap form.
Working with them in ACR is great.
You can easily adjust the white balance, the color temperature, the light type, shadows/highlights, standard and midtone contrast, sharpening.
All before the image even gets into photoshop.

And in many cases, no further editing is necessary.
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Last edited by Dtox : 08 August 2008 at 10:15 PM.
 
Old 08 August 2008   #40
Why not just render to OpenEXR then you've got extra bits to play with? One day all film will be shot digital with direct output to linear exr. Or at least I hope so.
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Old 08 August 2008   #41
The benefit of using RAW would be that if you're using a camera like the Red1 footage can be brought in directly to the RAW workflow that the Red1 uses.

I would imagine that it could be hard to deal with even a relatively small image sequence of HDR/EXR images.
Depending on what you're doing, rendering to EXR would add time to the render with no real benefit unless the intended output supports it.
So then rendering to RAW would possibly shave some time off the render but still offer a measure of control beyond that of a basic bitmap image.

I also notice that when you open an HDR/EXR image in photoshop you don't get all the same options as with standard images.
Which isn't the case with RAW.
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Old 08 August 2008   #42
Originally Posted by Dtox: The benefit of using RAW would be that if you're using a camera like the Red1 footage can be brought in directly to the RAW workflow that the Red1 uses.

I would imagine that it could be hard to deal with even a relatively small image sequence of HDR/EXR images.
Depending on what you're doing, rendering to EXR would add time to the render with no real benefit unless the intended output supports it.
So then rendering to RAW would possibly shave some time off the render but still offer a measure of control beyond that of a basic bitmap image.

I also notice that when you open an HDR/EXR image in photoshop you don't get all the same options as with standard images.
Which isn't the case with RAW.


Rendering to EXR shouldn't take any longer than rendering to any other image format. If it does your renderer is f*cked.

Photoshop doesn't support many of its features for floating-point images but you can get them back by switching the bit depth.
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Old 08 August 2008   #43
If you switch the bit-depth, doesn't that negate the benefit of using EXR though?
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Old 08 August 2008   #44
Yes, switching the bit depth leads you back to a 'normal' image. It's best to do all the things that have advantages of a higher bit depth (like for example level control, exposure and so on) before switching the bit depth down (to enable some filters which don't work at float point or 16 bit depth).
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Old 08 August 2008   #45
big thanks to the contributors for their sharing.
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