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Trulsi
12-23-2009, 03:33 PM
Hi guys!

I am trying to light a scene with a sky I generated in Vue. It's the first time I try any HDR stuff on my own and I am running across a few problems.

First of all, Lightwaves image editor gives me an analysis of the HDR image. It says that the black point is somewhere around 90%, and I am unsure what that means. Does it mean that nothing is darker than 0.9 in the image? The sky is quite dark, and the sun is low, so that doesn't seem an entirely correct analysis.

What really bothers me, however, is that the whitepoint according to this analysis is around 3.5. From the HDR theory I've read, it should be many orders of magnitude brighter than the sky (look at http://gl.ict.usc.edu/skyprobes/ms_thesis_stumpfel.pdf for example. In the bottom paragraph on page 30 they got upwards of a million in luminance).

The white point analysis looks like it could be right. If I use the image as it is to light my scene, I get a very low contrast light. There is a slight hint of light direction, but it is much weaker than what I would expect.

Does anyone have any experience with Vue renders in this respect? I mean, I suspect that the sun is really a lot stronger and that I should create it with a light in LW instead. I am just a bit unsure, due to little experience with this - I might have done something wrong, forgotten to turn something on in Vue for example.

The other major problem is with the exposure of the Vue image. Rendered out as an HDR image it should not be biased in any way, right? So if I use it as a backdrop in LW, render out an image and apply gamma 2.2 (I am on a PC), the render should look just like it did in Vue. But it doesn't. It looks more washed out. Even when I turn on an automatic HDR exposure filter in the image editor in LW. I don't know what I'm missing here - maybe some sort of incompatibility in what LW expects of an image and what Vue delivers? Punching up the black point percentage (although I don't know what this really does) rectifies it to some extent, but I need to know what I'm doing.

Have been googling and reading a bit, but I haven't found anything dealing with the Vue -> LW HDR lighting case.

toby
12-24-2009, 12:16 AM
First of all, Lightwaves image editor gives me an analysis of the HDR image. It says that the black point is somewhere around 90%, and I am unsure what that means. Does it mean that nothing is darker than 0.9 in the image? The sky is quite dark, and the sun is low, so that doesn't seem an entirely correct analysis.

What really bothers me, however, is that the whitepoint according to this analysis is around 3.5. From the HDR theory I've read, it should be many orders of magnitude brighter than the sky (look at http://gl.ict.usc.edu/skyprobes/ms_thesis_stumpfel.pdf for example. In the bottom paragraph on page 30 they got upwards of a million in luminance).

The white point analysis looks like it could be right. If I use the image as it is to light my scene, I get a very low contrast light. There is a slight hint of light direction, but it is much weaker than what I would expect.

Does anyone have any experience with Vue renders in this respect? I mean, I suspect that the sun is really a lot stronger and that I should create it with a light in LW instead. I am just a bit unsure, due to little experience with this - I might have done something wrong, forgotten to turn something on in Vue for example.

The other major problem is with the exposure of the Vue image. Rendered out as an HDR image it should not be biased in any way, right? So if I use it as a backdrop in LW, render out an image and apply gamma 2.2 (I am on a PC), the render should look just like it did in Vue. But it doesn't. It looks more washed out. Even when I turn on an automatic HDR exposure filter in the image editor in LW. I don't know what I'm missing here - maybe some sort of incompatibility in what LW expects of an image and what Vue delivers? Punching up the black point percentage (although I don't know what this really does) rectifies it to some extent, but I need to know what I'm doing.

Have been googling and reading a bit, but I haven't found anything dealing with the Vue -> LW HDR lighting case.
Your sun probably doesn't have much effect on the scene because it's relatively small in the hdr. Radiosity rays have to hit the sun in order to read the intensity from it, and if there's only 250 rays (Rays Per Evaluation) the odds of hitting it are very small. A hot sun would also tend to leave super-bright pixels when a ray does hit it, pixels so bright they can't be anti-aliased. The solution for both problems is to blur the HDR quite a bit, 25-50 pixels or so, depending on the maps size, and use a cg light for the sun, because you'll have a hard time getting any direct light from an HDR. The map doesn't need to be full res once you've blurred it, it can be scaled down to 500 pixels or so, and save texture memory.

Since this would make your reflections blurry, you'll want to put the original copy of the HDR on a dome, and turn off all radiosity flags for it.

The HDR probably has a gamma curve applied already. If you add a 2.2 gamma and it looks washed out, that generally means you don't need it.

Trulsi
12-24-2009, 12:52 AM
Hi Toby!

Thanks a lot for your reply:D I see, then if that is the standard way to go, that is what I will be doing as well:) What you're saying makes sense to me.

I guess the "full precision gamma" with 0,455 can be used to linearize the Vue-generated background then, without reducing it to an LDR.

If I have the full size hdr on a dome, and the blurred on as a backdrop texture, won't the dome block out the backdrop texture if I try to do anything but "backdrop only" radiosity?

toby
12-24-2009, 01:40 AM
Oh you wouldn't want to linearize the HDR unless you're going to do the same with every color in the scene, including texture maps, shaders and lights. Render the HDR environment and see if it looks right the way it is.

I hope that helps!

Trulsi
12-29-2009, 02:35 AM
Oh you wouldn't want to linearize the HDR unless you're going to do the same with every color in the scene, including texture maps, shaders and lights. Render the HDR environment and see if it looks right the way it is.

I hope that helps!

Thanks! Things are looking better now!

I'm going linear, by the way! I noticed that I was having problems because the "auto exposure"-settings in Vue were tied to the camera settings, and therefore Vue wrote a gamma corrected file. So now I've got it linear out of Vue.

Even though I haven't installed a linear color picker and such yet, I am eyeballing a compensation when I choose colors etc.

BTW: I found the answer for my question regarding "won't the dome block out the backdrop texture if I try to do anything but "backdrop only" radiosity". I found the "not seen by radiosity" object property which I could check on my background dome.

Also, it seems to be far too taxing to render a full size background dome texture out of Vue, so I guess I have to try a camera sync-thing. Will have to try and carry rotation and zoom values over... I know some people have been writing about this in the forums, so I will have a look and see what they have found out:) There was, if I remember correctly, some issues with getting the focal length to match between Vue and LW.