How to create HDRI images?

[steeleb]

Hi,

I have been researching on the HDRI images for a few days. I have performed several successful experimets with the HDR files that I have downloaded from Paul Debevec's web site. I also have read most of his articles about HDR images...

so basicly I know the technology and the Mental Ray usage... also have used the HDR Shop for the panorama conversion...

But I did not understand the creation process of the HDR file...

One shoots a mirrored ball... shoots the mirrored ball from two different angles to be able to remove himself from the resulting 'reflected panorama'...

But where is the 'shooting at different exposures' stage involved?

How many photos? What is the f-stop interval?

I know that this 'different exposure' shots are for getting all illumination info from different sources... But how are the different versions combined?

I have almost understood that HDRI thing... just need to figure out how to build it...

Need help.

[steeleb]

no ideas on this?

[user_deleted]

HDR shop has a feature to combine multiple images at various exposures... theres a tutorial for it on the HDR shop site...

http://debevec.org/HDRShop/

then click on the tutorials link at the top... thats the site with all the tutorials for HDR shop... I think you should find your answer there...

good luck : )

-Steve

[steeleb]

ok... tahnks...

I have read the instructions on the page before... but I was confused. So now I read again... and it seems like things are more clear now...

thanks.

but...

I have another question.

instead of using the "shooting a steel ball" technique...
what if I create my images by shooting frames in the 360 degree range and then use REALVIZ Sticher to create my panoramas?

as a result... I will get a "Latitude/Longitude" export from Sticher...

Later I will combine those "Latitude/Longitude" images in HDR Shop into one single HDR image...

Will this work?

"Shooting the steel ball" will not give you 360 degree... but combining images in Sticher will give you 360 degree perfect spherical panorama...

am I right?

[user_deleted]

hm... I think its possible. It SHOULD work, in theory ; )

and you actually can get a full 360 from the steel ball technique, you just need to shoot both sides of the ball...

anyway, good luck.

-Steve

[steeleb]

thanks but!...
...how will I connect the both sides?

one sphere is one HDR image? how will I stich the both images to get a single Latitude/Longitude HDR?

[steeleb]

here is the deal:

LIVE SHOT:

There is a room with some outdoor and indoor illumination. There is a table in the middle of the room. I will shoot a hand held camera sequence. The camera will go 360 degrees around the table.

PROCESS:

I will matchmove the liv shot inside Maya Live so I will get my CG camera behave exactly as my real-life camera.

CG:

I will model some steel balls and render the entire sequence. The result will be composited on the live-shot.

So As the CG camera will go around the table I will need to map the entire room in 360 degree HDR image inside a sphere to get the proper reflections.

Is this plan correct?

[user_deleted]

you mean how would you get shots of both sides of a sphere to become one fully 360 degree image?

well... generally speaking I use two hemi-spheres in maya and attach one HDR image to each (corresponding to the two sides of the ball)...

if you can't get that to work seemlessly, though, there should be a way to export a lat/long image of each side of the sphere out of HDR shop and then stitch it together... I have never really tried.

if this is not what you're talking about, though, then I apologize ; )

[steeleb]

umm... yes this is exactly what I am asking...
so the HDR Lat/Long image is actually not covering the entire inner surface of a nurbs sphere?

when I stich two Lat/Long images... I will get an image normally longer than an ordinary Lat/Long image generated from a sphere shot... and this longer image will perfectlycover the inner surface of a NURBS sphere?

...thanks for the help by the way

[user_deleted]

well... I could be COMPLETELY off on this, but, in my experience, taking an HDRI image generated from shooting a reflective sphere and converting it to lat/long produces an image that will accurately cover a nurbs hemisphere...

trying to cover an entire sphere stretches it out, producing an innacurate scenario...

perhaps I am doing this improperly, but, it seems to make sense, since that lat/long image was generated via a shot of only 180 degrees of an area... therefore, taking 2 lat long images, taken from both sides of a sphere, and mapping them to two perfectly lined up nurbs hemispheres, SHOULD create a full 360 degree HDR panorama...

but, again, I could be way off ; )

you could also, theoretically, create one larger/longer image from two lat/longs and map it to one sphere... I have never tried that, but it SHOULD work...

and you're welcome for the help, but, to be honest, I don't have an over-abundance of experience with HDR... just used it briefly and intend to use it again shortly...

I am really only answering cause no one else seems to be ; )

-Steve

[steeleb]

Thanks Steve,

I think you are making a very good point.

a single photo taken from the surface of a mirrored ball is 180 degree... how can it be mapped on an entire sphere?! this is very confusing...

By the way... mapping a panoramic texture on a POLY sphere / hemi sphere may also cause it to be streched...

I guess taking two pictures of the sphere is also not a solution... you will be never shure if the second shot is completely overlapping with the first one...

I guess the only way of doing this... is taking series of pictures from the position where you are going to put the CG object...

These photo sequences should be shot at different exp. and different angle... ex: 5 exp for each position... at the end the same exp images should be stiched to a panorama image in Sticher... so at the end you will get 5 Long/Lat images at different exp. covering 360 degree... (Sticher has option to export Lang/Lat panoramas)

Then those 5 images should be combined in HDR shop into a HDR image...

This is my theory... It is a theory... but I will try it...
Any ideas on this?

Thanks.