Issues w/ HDRI (Vray & HDRShop)

[Percy_xue]

Hi, all
I read some tutorials on HDRShop to get better understand of HDR image. I have several questions as stated below:
1) Are all HDR image made up of photos that catched by a panoramic camera? Or are they all Panoromas?
2) Why is HDR image divided into four types: Mirrored ball, Angular(light probe), Latitude/Longitude, Cubic(Vertical cross)? Or say, what's their usage respectively?
Above I just follow those four terms according to HDRShop. In Vray they're: Angular map, Cubic Env., Spherical Env. & Mirrored ball. What's the exact relationship/difference between them of the two software? Most of my hdr files are marked as *_probe.hdr , *_cross.hdr & nothing else. Of course HDRShop can covert between two of the four types. Thus promotes question 3) Is the "map type" in VRayHDRI map intended to convert HDR image from one type to another, or just let me mark up which hdr map type I've chosen so that VRay will know it to deal with?
-If Vray makes convertion, does it make the same operation with that of HDRShop? I mean, if the converted image can be output, will it look same as that converted in HDRShop?
-If the "map type" is just for a mark to VRay, where can I get a hdr file of "spherail Env." type? You know the other 3 types, anyway, can be got from HDRShop Panoramic Transformation already.
4) Does the multiplier value in VRayHDRI map mean the intensity of HDR image to lighting the rendererd scene?
5) For a HDR image it is said you can use it lighting a scene for realtime rendering, e.g., from morning to evening with exporsure value changes to the same HDR image. How can I ctrl this exprosure value in VRay/MAX?
6) How to map HDR image as Env. lighting/GI in VRay? If it is mapped as general map that is assigned on a skydome sphere, how can I preview it in viewport?

Thanks for any reply,
-Percy

[jujubee]

percy- you got alot of questions lol. I'll try to answer some to the best of my ability.

1) No. Though there are panoramic cameras on the market and they come in real handy, most are extremely expensive and are probably limited to Hollywood type budgets. Spheron markets one for 50,000 US- it supposedly can knock down hours of work to just about 8 minutes. There are other panoramic camera adapters out there but none that really do all the work for you like Spheron does.

Often, you use a camera that can take pictures at different exposures, then either stitch the images together with a separate program (a time consuming process) or use a metal like ball with high reflectivity (the quicker way.) When using a metal ball they shoot the images from two separate 90 degree angles- hence the probes which you often see. Then they mask the results out in order to get rid of the tripod and camera. Each view compromises about 10 exposure levels- so a proper hdr should technically consist of at least 20 images. half layered on top of each other.

2) a) I believe the main reason is so that it can be used in different software packages.
b) I personally convert all hdr images to latitude/longitude. I find it much easier to sort that way without having to remember which each one is like.

3) Those selections are intended to convert but it really doesn't help if you don't understand what's working beneath all this- I think it's just a lazy way out unless I can be proved otherwise. It's important to note that there are two types of crosses I've encountered (vertical cross and horizontal)- one of which gives awkward background and lighting results using the cross option as vray only supports one type of cross. So I don't even bother with crosses. I always convert them to latitude/longitude.

4) Vray only temporarily uses switches for converting these four types- it does not save to your original hdr unlike hdrishop.

5) Probe generally means lighting from a spherical perspective (often seen as mirrored balls.) Cross is basically an unwrapped square box which I find to be unrealistic and not very versatile. These images can be converted between each other easily using hdrshop.

All in all, it only takes two seconds to convert from one format to another using hdrshop. It's basically just doing a 'save as' once you've made your transforms. I found latitude/longitude to be the most useful types. As these are now latitude/longitude hdrs, there are two options you may want to really use in vray- angular and spherical. Spherical will give you a real world type of result. If you're not getting the lighting you want from your hdr, you can change the multiplier or rotate the hdr in the rollout along the vvertical axis. Ocassionally (and very rarely) I will use the latitude/longitude for angular if I can't seem to be getting the spherical lighting I want right.

Hope this helps...

[Percy_xue]

Thanks, Jujubee. You really helped me a lot.
These days I try to read more mat'l on HDRI. Most are from Siggraph which tells much more about HDRI on film making. Since lack of experience/knowledge in film, something about actual applying HDRI is quite difficult to understand. (I mainly work on architectural rendering, in fact.) One of those expressive is, they say, "to make all those different exposure values captured by a singe frame to simulate realworld lighting". So seems something is wrong with me for the understanding of HDRI. So much confusing now. Still, can anyone give more replies to my questions in the first post above?

-Percy

[jujubee]

to add to my reply, the multiplier for the vrayhdri does serve to increase lighting intensities just like any other light.

As for animating an hdri from dawn to dusk, that I do not know. I know you can render out frames at different intensities manually. Whether vray supports the multiplier being able to animate is beyond me.

As for realtime renderings I am not sure. I think you might need a light baking tool if you were going to use for something like a 3d shooter. There is one out there that supports hdri called tbaker but I haven't used it.

HDR images can be previewed in the viewport just like any other environment bitmap. However, you will not see the results of the lighting from this.

I think there is alot of information on the web to get you started. That's how I learned it. Do a search for hdri vray or hdri tutorial- you will find some good links. It doesn't make sense to reinvent the wheel here by retyping all these things. You can also search the forum for other hdri related topics which relates to your questions.

[Percy_xue]

Much appreciated. Thanks. I'll follow your idea.