dvmorris

Normally I would spend a few hours googling a topic like this until I became well-versed enough not to need a forum post, but I am googled out for the day.

I am looking for a way to analyze light levels in an HDR image, and I've seen it a million times in magazines, the little heat map image with red to blue gradients showing light intensities, but I really have no idea exactly how "they" do it. Any ideas?

I looked at this tool called Radiance, which baffles me because the website looks like it's from before the internet was even invented, and the instructions for installing are far from understandable. It seems to me that program is only for rendering hdr images from 3d scene data, and not really for analyzing an hdr photo. any insight would be appreciated. thanks,

dave

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John Dollus

Are you wanting to analyze lighting levels from an existing photograph of an actual place or analyze lighting levels in a 3d scene you have created in Max?

dvmorris

an actual photo of a place

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Andrew Schroeder

I'd think your best bet would be to take your custom HDR and meaure at the time of HDR capture the light levels of one of the lights in the final HDR (measured not at the light, but at the HDR capture location). You could do this with a meter and by turning off all other lights in area, if it's outdoors just measure the values in direct sunlight (there might be some way to calculate what the ambient light levels are and remove them from the reading)

Once you have the values of a particular light, I suppose it'd be possible to map all the other light source-levels by applying a inverse squared formula to their HDR values. Of course for this you'd need to know the distance each light was away from the HDR collection point.

In short, I have no idea how you'd use HDR for lighting analysis, and would probably suggest not doing it in the first place, since the results would be questionable at best (imho).

On second thought, you might be able to get somewhere by using one of the HDR to max-light converter scripts that reads the light locations in the spherical HDR and creates light objects casting towards the center. The script would have to assign light values/colours to each light, so that might work as a starting point

Good luck.

Andrew

dvmorris

hmmm maybe I didn't really explain what I wanted to do very well, but that's probably because I don't really know what I want to do.






what do the colors in these images represent? I thought there was a way to do this with an HDR photo that was merged from multiple LDR exposures. I know I've seen something like that before, and it doesn't have anything to do with the distance each light is from the surface or the camera or whatever.

I could have sworn I saw a professor do a presentation on this using a program called Radiance, but everything I read about Radiance is about 3d modeling, and not about analyzing a real photo.

I hope this makes sense, this whole subject is mind boggling to me right now. I just can't find any good documentation on it. thanks for the help,
dave

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SunGlare

Those images was rendered with accuRender, and it seem that it was long ago. Using that software You're able to generate light intensity map, spreaded on the 3d surfaces. Map was based on the light sources You have provided in the scene. You could scale those colors(gradients) in lux units, or by light color temperature in Kelvin units.

But to get this kind data from hdr map thats another story. Hdr has all the info mixed in color composite, it is take from reality, there are no channels turn of/on. Its a pity

Maybe garayscale of 16 bit hdr in to a gradient map in photoshop.

I'm not sure if this is what You lookin for?

drapp@frontiernet.net

Those look exactly like a pseudo color image rendered from MAX with radiosity turned on.

You could map your HDR image to a flat plane and render from the top view using the radiosity stuff found in the render scene dialog box. I use it like 2x a year, so I just claw my way around in the dark until I get it to work. Hope that at least points you in 'a' right direction.

Arnold Gallardo

The 2nd photo looks like its Lightscape. The colors in the Lightscape image shows the computed 'Illuminace' or 'Luminance' levels and based on the dialog box it is "Illuminace' which can be shown in Linear or Logarithmic scale.

One thing you have to remember is that most renderers do handle 'Tone Mapping' differently. Tone mapping is the process of making the computed luminance/illuminance visible to a display device (like a monitor) and if a 'averaging' or 'weighted' mapping is used it would never shows all of the computed luminance/illuminance only those 'deemed' significant or important and how this is done can be eitehr 'dumb, blind or stupid' in the decision making. I think this was the reason for having HDRI anyway to sort of do a smart 'image based rendering' reverse light level extraction base don captured series of images showing the extent of light level present that is 'perceptible'.

As for the HDRI side have you looked at Paul Debevec's website where it all started?

http://www.debevec.org/

I hope this helps

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Arnold Gallardo
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