[bjørn]

Can somebody tell me what a HDR image is. I subscribe to HDRI 3D Magazine, and in the January issue you learn to make it. But I don't understand the difference between HDRI and a usual image.

[sod]

The normal way you place many lights in your scene

The HDR way, you take a special HDRI (High Dynamic Range Image) for the lightning. The HDRI can have informations for lights brighter than #FFFFFF .

[bullet_evader]

*not sure if i should post this in this treath but....*

is their any way to create your own hdri images??? for example, i take a photograph of my room, and then make a hdri image of it.
would be cool i think!

[AverageJoe]

IIRC there is a baker kinda like the Skytracer thing.
Actualy I think there is a way to bake a Skytracer straight into an HDRI image...

[Mark]

The application you need is HDRShop

http://www.hdrshop.com/


Version 1 is free for non-commercial use. Version 2 is licensable for US$600 for 2 years...


The essentials are that you need to take a number of photographs of the same image at various exposures. HDRShop then interpolates these into a continuous range of values for each pixel - spitting out a single HDR image.

The latest issue of HDRI magazine (formerly Keyframe) has an excellent article on this very subject...

[bjørn]

Yes , I have that issue og HDRI. I understand that HDR images is used in the Reflection texture editor, but why do you need different images?

[Mark]

If you're referring to the multiple images shot to produce an HDR image, its because a regular LDR image does not contain the full amount of luminosity information to accurately reproduce the difference in brightness between pixels.

If you take the colour information out of an image, and simply work with greyscale information, then in a regular LDR image, you simply have 256 levels of brightness to work with from Black to White.

Now, if you compare real world examples of difference in luminosity, where the absolute absence of light is zero, and (say) the sun is the absolute brightest thing there is (at pure white - 255). Now, if you want to represent the light from a 100 Watt bulb, you need to put this in the spread somewhere. The sun is approximately 100,000 times brighter than a 100 Watt bulb, so in an LDR image, this comes somewhere less than 1 on the brightness stakes and is therefore not going to be visible at all since rounding will place this at zero.

Now, in an HDR image, since all values are stored as floating point numbers, there is pretty much a continual spectrum. This allows (say) the absolute absence of light to be represented again by zero. The sun this time will be represented by pure white - 1

Now, the 100 Watt bulb can be represented as 0.000001 -- Again, a very small number in the overall spread of values, but now accurately represented.

The reason for multple images is so that an interpolation can be performed for each pixel in each image to guage at what point in the 0.00000 - 1.00000 range that pixel will fall. By knowing the gamma curve of the capture device and the overall gamma of each shot (since they're all taken at either different aperture or speed settings, their gamma differs) it is possible to calculate this value.

Essentially, it all comes down to the fact that in order to have that extra information in an HDR image, it needs to be captured in some form first.

[bjørn]

Thank you, Mark. Now I understand it better: You use it to Ligten up a scene (in the reflection channel?). If so, why do you (we,I) need several HDR images? Isn't it enough with f.ex Beach_Probe found in the HDRI folder?

[Mark]

You could just use the images that are supplied with LW. However, you would find that your images all start to look very similar very quickly (for instance - take a look at all the renders that are out there and you'll very soon learn to spot kitchen_probe.pic and campus_probe.pic).

However, it's not just in the reflection channel you can use them. If you use them as an environment map (for instance by using the ImageWorld plugin, then you can switch off all your scene lights and switch on background radiosity - this will use your lightprobe image to actually provide the illumination for your scene.

The most important use for having other images is also, of course, in matching CG to real-world plates. With a lightprobe image of a real set, then you can use your probe images not only to accurately light your CG elements, but also in the reflection channel too to obtain correct reflection images as well.


--- [EDIT] ---
Oh, and I have seen some of the Dosch collections -- they really are very good

[bjørn]

I see. I'm working on the texturing part of the McLaren -the tire.
Trying to learn why Philip is doing what he does. (and I'm beginning to "see the light"). I tried out the different HDRI's in LW folder and I see the different results .

[Mark]

The other thing to remember is that for product shots (or showroom car shots), the most common setup is to have a block of four Area lights above the product and no HDRI. If you then pop a slight noise onto the reflection channel at a rather large scale that should help to get those large, bright, white shiny streaks you always seem to see...

[bjørn]

You meen 4 area lights above the objects 4 corners?

[Mark]

Pretty much over the entire object. Make them reasonably big (equal size) and leave a section like a cross between them. You really want to aim to make them almost as large as the object.

[bullet_evader]

The application you need is HDRShop

thanks for the link and explanation.
i will give it a look.