What is HDRI?

The acronym HDRI stands for High Dynamic Range Image, essentially, HDR Images are images that have a dynamic range far greater than the 0-255 range that we are currently used to using in our traditional computer based images, this means that images using the 0-255 range cannot display the dynamic range accurately (The contrast ratio between darkest and brightest regions). 
 

An HDR Image has an important property in that their pixel values are proportional to the amount of light in the world corresponding to that pixel, unlike most regular images whose pixel values are nonlinearly encoded, HDR pixels use floating point numbers, which are capable of representing light quantities of one to a million and beyond.  As mentioned earlier, Low Dynamic Range Images usually represent pixels using only eight bits per channel, with the pixel values ranging as integers from between 0 and 255 respectively.

Because of the greater range capable of within HDR Images, these types of images are able to store accurate lighting values, and by using these images in conjunction with Global Illumination, Final Gathering, etc, you can produce extremely realistic and warm looking images than that which could be created with a low dynamic range.

For more information on HDR Images, see the following website : - http://www.debevec.org

Maya / Mental Ray HDRI Tutorial

Ok, If you've read on using Final Gathering with Maya / Mental Ray, you'll already have an idea of some of the concepts behind using Mental Ray within Maya and how to effectively set up a scene.  In this tutorial, we'll be utilising the scene from, which you can download from here (Maya 4.5 only) : - Scene File (Zip File - 39kb), once you've downloaded the zip file, extract the contents, we're going to be adding some extra's to this scene during the tutorial.  Now before we continue, I'd recommend first downloading a nice HDR Image, there's a good collection which can be found at this site : - http://www.debevec.org/Probes/ , for the purpose of this tutorial, let's go for a nice warm looking image, download the Grace_Probe.hdr, as shown below : -

Ok, now that we have our HDR Image, it's time to load up the scene.  You should now have something similar to diagram below within Maya if you've downloaded the scene : -

Now if you do a quick render, you'll probably see something like this : -

The quality is a little blotchy, but that's not a problem right now, we can 'up' the final gather rays in the final render.

We are now going to add an extra object to the scene, a half dome, select Create > Polygon Primitives > Sphere, a sphere should appear within your scene, select the sphere and increase the radius to 10, so that it encompasses the scene, now, in the front view, select the polygons just below the half way mark, but not at the half way mark and delete these faces : -

You will now be left with a dome shape or half sphere, now the important part, select the sphere, and then go to Edit Polygons > Normals > Reverse. The reasoning behind doning this is so that when we map our image onto the dome, the normals are pointing in the right direction for illumination to be correctly calculated.  Create a Lambert Shader, and assign it to the dome, now open it's Attribute Editor, and click on the box in Lambert tab next to Ambient Colour, in the 2D Textures section, select File and then find the .HDR Map we downloaded earlier, if you cannot view it inside Maya, select Best Guess(*.*) In The Files Of Type section in the open dialog box.  Once you've done this, do the same as before, only this time add it to the Colour channel of the Lambert node.

Now select your dome shape, and go to Edit Polygons > Texture > Planar Mapping, go to the options box, set it to Y axis and then ok, with the object still selected, open the UV Editor, leave it open, in the front view, select the following UV points : -

Now, scale and then deselect the UV's in their rows, until you get something similar to this in your UV Layout : -

Select ALL Uv's now and then scale them so that they fit exactly into the 0 to 1 UV Space (the grey square in the diagram above), what we've essentially done is made sure that our UV layout is utilising the whole of the HDR map correctly, by relaxing the area's where the map would've been stretched.  Now the beauty of HDRI is mostly found in the lovely reflections it's produces as well as it's lighting, in this case, we will now convert the plane the red ball sits on from a Lambert to a Blinn, this is easily accomplished, select the plane, open it's Attribute Editor and in the Lambert tab, change the type to Blinn, now with added new specular attributes, set them up as follows (similar to the red ball's attributes) : -

I haven't mentioned this yet, but it's worth taking note now that Maya will not actually display the HDR map, Mental Ray recognises the map, but Maya doesn't hence it not displaying.  Ok, we're about ready now to do a test render, but first we must make a few small changes to our scene, you'll remember from the final gathering tutorial that the Min and Max samples are based on the scene size, well this has changed since we added our dome, so recalculate for the scene, I come out with 20 units, so that's 2 for Max and 0.2 for Min, so open the Mental Ray Render Globals and in the FG section, make the necessary changes.  Change the final gather rays to 500, just for viewing purposes too.  Now delete the white plane object from the scene that is currently illuminating it, as we want to just concentrate on the HDRI effect.  And perform a test render : -

Ok, not bad, but not great, how can we improve the look?  It's blotchy, but we can increase the samples later to fix that, in the meanwhile, decrease the plane's reflection value to just 0.3, it's also a little dark, so select the dome and within the Hypershade, graph the material : -

Select the file node and open it's attributes, and within the Colour Balance section, click on the Colour Gain's 'colour' box : -

Once you've done that, increase the V value in the H S V section to 2 : -

Now, let's try another render : -

Now, looking a bit better than before, let's try a different HDR Map that will show off the scene a bit more, in this case, the HDRI map being used here is already quite bright, so I set the Colour Balance back a little - HSV Value V to 1.5 : -

Now, that's looking quite nice, the HDRI map has helped to achieve a fairly good, very realistic looking scene due to the lighting range available.  And in this final test, I've increased the size of the plane so that it overlaps the dome, just to fix the reflections a little, I've changed the plane to white and changed it back to a Lambert, and the ball has had it's reflection value upped a little bit more too, and I've increased my render time a bit more by upping the final gather rays (1500), and within the globe's attribute editor, I've changed the shape nodes render stats as follows : -

Note Switch off 'Primary Visibility' if you don't want the globe to appear in renders, but still affect the scene : -

And the final test render : -

I'd agree, it's not the best quality in the world, and the render time is way too high, (above 10 minutes for the still above) so we now need to look at different ways to improve the final image and render time.  One way is : - let's not rely on the final gather rays to correct the blotchiness within the final image, but instead, let's mess around with the Min and Max radius, I've set them to Max = 8, and Min = 0.8, and this is the result that produces : -

This still rendered in roughly only 2 minutes, and is a vast improvement over the previous one's blotchines, now experimenting again, I set the Max value to 16 and increase the FG Rays to 2000 : -

As you can see it's improving, this one took just under 3 minutes to render, whereas it's not an exact science, and values can't be given as defaults for each and every scene, and most of the time the rules seem to be broken for each and every scene, as in the case of Min And Max values formula we came up with within the FG Tutorial, which does not seem to be the case in this scene, all I can end on is saying, experiment and experiment until you get the 'look' your after, I just hope that within this tutorial, you've been able to gain a little insight into working with HDR Images.

Final Test Render - FG Rays set to 3500, Max Radius set at 24, Min Radius set 2.4, 4 Minutes 45 Seconds rendered

LAST MINUTE ADDITION - VERY IMPORTANT

Ok, based on my previous FG Tutorial, there was a way of calculating the FG Min and Max samples using the scene size in Units and then taking 10% if that.  However, that was for a flat scene, now again, this is probably not the way to do this, but I came up with another idea, when a scene has more depth, i.e. height and width, measure them both, as in,  I get (using Maya's measure tools) 20.57 (rounded up) for my width, and for my height I get 11.57 (rounded up).  Now, use the formula Height * Width, and I get 237.9949, let's say, 240 , for arguments sake we've rounded it up.  Now using the formula from the first scene, i.e. Max radius is 10 % of the scene in units, that would make the MAX Radius should be set at 24, and again the MIN Radius should be set at 10% of that, and so 2.4 is the value there, now look at the final render test I did, the final values I came up with before the scene actually started to looking presentable, wow, that's some co-incidence!  Now, like I said, they may be nothing in it, but the results seem to speak for themselves.  Remember though in the scene above, which was fairly square, the formula may change if the depth was the longer value in the scene.

If your wanting to view and edit HDR Image maps, I'd also recommend downloading HDRShop, available from : -

http://www.debevec.org/HDRShop

Thanks for reading and I hope you find this tutorial useful.