Brain frazzled, EV, lux, hdr and all that stuff

[Mr Whippy]

Basically what I want to do is work out a pixels intensity value from a HDR image.

In 3D render world we can use a HDR to light a scene, so assuming the HDR is set up right (not sure specifically how that is, but I assume there is a standard exposure point), we must know the intensity the pixel throws into the scene.

Even a low dynamic range image (jpg perhaps) with known exposure value (EV) must be able to give us usable pixel information on it's intensity (if we were to point sample the reflected light basically)


I'm assuming this is quite simple but Google isn't playing ball and my brain is frazzled from gamma transforms on values (I can never figure out if reality is more contrasty or less contrasty than we actually see)...


Does anyone know of any tools that let you pick points in a HDR or JPG and tell you the reflected intensity value?

Or any maths to do the same?


Many thanks for any help!

Dave

[RobDickinson]

Not sure what you mean by reflected intensity value.

Typically in photoshop you can check each pixels level in RGB (0-255 per channel etc).

A HDR app (before mixing down to 8 or 16bit) will likely be 32bit or more per channel , but same applies as above.

[flat-planedCrank]

Mr Whippy said:

if we were to point sample the reflected light basically

You want to work out the power of the light that illuminates a surface in a photo?


Never seen any programs that'll estimate that, perhaps look though some research papers?

[Mr Whippy]

Sorry if I'm not being clear.

I found this which is quite interesting for those interested.

http://www.ee.ryerson.ca/~phiscock/astronomy/light...

Bottom of page 6 seems to start to cover what I'm interested in.


When I say reflected light value, I suppose that is the luminance value. Ie, how bright a pixel is in the photograph.


Just to give some idea of the purpose, I'm trying to balance some realistic images in a 3D engine so I can then tweak materials to get realistic responses to a real value environment.
Ie, glossy reflections, lighting tint values and so on... it just looks better and more natural than trying to use CG generated skies which always look a bit fake (for obvious reasons)




I might have to go take a few HDR images at sunset then I can pick values off the sky, sun spot, reflected light off the tarmac etc, then I can balance them all in the real-time rendering engine to make sure everything is lining up fairly closely and looking/responding right.


Thanks

Dave

[Simpo Two]

Mr Whippy said:

Does anyone know of any tools that let you pick points in a HDR or JPG and tell you the reflected intensity value?

Mk1 Eyeball? That is after all how every human judges things.

[GetCarter]

/\ indeed... e.g. Highlights are blown in the pic above - Too much intensity!

Edited by GetCarter on Saturday 26th January 07:41

[Mr Whippy]

The main problem in my case is that if every value is doubled, the final appearance might be the same, but the scene is technically twice as bright as it should be.

Thus starting with a few real values is really helpful to cascade realistic values down through everything else.


The main issue with anything on a screen is that it's LDR, about 255 instances of brightness. So we can tweak the tone mapping for what a camera might achieve, or what the eye thinks it sees and all that jazz, which is yet another variable to tweak.

Thus simply comparing HDR intensity values with HDR intensity values removes ALL those variables and guess work where mk1 eyeball is trying to gauge something in real life via a fairly low intensity screen, and just puts real values in the right places.



That image is indeed blown out, but the base texture isn't... just the tone mapping from HDR > LDR is blowing those areas out... but making tone mapping that looks natural as our eye would see it is tough because the end result can often look like these "HDR" effect images that just look wrong.

Dave