Radiance HDRI for Cheetah3D

The title is a bit misleading: this applies to any 3D package that can use a 32-bit image for HDRI. It should probably be called “Making HDRIs in Photoshop”, but I digress…

Straight to it

The aim of this technique is to allow you to use Cheetah3D’s HDRI tag to the exclusion of all other lighting to produce results something like this:

The objects in this scene are a simple sphere and a disc (6-sided – is that still a disc?) and there is no light. In addition, the 2 objects use the same grey, reflective material in all examples.

The process

Light and colour

The first thing I did was create a 32-bit document in Photoshop, 1000px by 500px:

I then added a guide dividing the image in half across the middle. I selected the bottom half of the image with the marquee and filled it with a gradient from black to white (or dark grey to light grey, we’re not too bothered):

I called this layer “floor”. Then I created another layer, underneath the floor layer – so the floor can mask the image at halfway up, no matter what you end up doing on this neither layer, which you could call “ceiling” or “sky” depending how you feel about the open air. In this new layer, I also created a gradient, going from black just below halfway to white close to the top (this is with the floor layer switched off):

OK, switch your floor layer back on, then get creative! First up, a very simple one: add a Photo Filter Adjustment Layer above the ceiling, but below the floor (work that one out!) Try a Warming Filter, for example. This already gives you something that would light your scene quite nicely – without any details in the reflections, which is often what you want. It’s a BIG, WARM, SOFTBOX! (Does that sound rude to you as well?)

What you could experiment with here is adding a Layer Mask to this filter with a gradient of its own. The next thing I did was make another Photo Filter with a Cooling Filter on it. Obviously, it’s the same effect, but blue instead of orange. Here’s a cool thing though: switch them both on for a nice magenta sunset/sunrise effect. Of course, I could of just chosen magenta as my filter colour, but we’re experimenting, OK? The beauty of doing it by mixing the filters is that it’s a lot quicker to use this Photoshop document as a template for different feels – everything can be done just by hiding and showing layers. You could also make red, green and blue filters, and play with their opacities, but enough, you get the idea…


This is OK, we have some nice light with any colouring we like, with whatever gradients we like using the layer mask for the filter. Let’s add some detail though.

On a new layer, between floor and ceiling, I started with just a white brush, something like 10 pixels wide, and fairly soft-edged (about 50% hardness). Then I spotted it around the image, a dot here, a dot there, changing the size slightly as I went (don’t forget: ‘[‘ and ‘]’ on the keyboard to do that quickly) to make something like this:

You could use a regular pattern, you could use harder-edged dots, you could use stars, whatever you like. To make it more interesting, though, add Layer effects (“Blending Options”) in which you can go crazy and add diffuse drop shadows, glows (inner and outer), overlays (color and gradient). Whatever you like. For the examples above I used mostly drop shadow, outer glow and a gradient overlay:

What you’ll notice is that the “horizon” is preserved, by having the floor on top of everything else – hopefully that makes more sense now! You might not want the horizon preserved, but we have to start somewhere.

Other things I used in the examples are more complex gradients for the ceiling and adjustment layers (levels, gradient fill) for the floor. Now all you have to do is save your environment. (Save The Environment – good advice.)

I hope you already saved the whole thing as a PSD, but now you need to save as Radiance format each time your layer experimentation comes up with something you want to keep. Just Save As… then select Radiance. The dialog should force you to save as a copy (as Radiance can’t have layers) and you’ll give it a name with an .hdr extension.

Bug alert: I think I’ve come across a Photoshop bug in that if you have an adjustment layer selected when you Save As… you can’t select Radiance as your format. I haven’t checked to see if this bug has been reported already.

Now you have your .hdr what do you do with it?

Cheetah3D (or your choice of HDRI-capable renderer)

I use Cheetah3D which I can definitely recommend. I only don’t know how I hadn’t noticed it earlier. Great features and interface, and if like me 3D isn’t your main professional angle, it’s still a realistic proposition, price-wise.

I’ll not go into too much detail on how to use the HDRI as it’s been covered elsewhere and it’s also extremely simple. So, to test your environment, make a new scene and add the HDRI and Radiosity tags to your camera. Switch the camera light off as well. In the HDRI tag’s properties, load your new Radiance file. For the examples, I left both HDR and Radiosity on their default settings. Ambient occlusion is faster than radiosity as the type and usually will give you what you’re after.

A simple sphere as in the examples is as good a place to start as any, being fast to render and it shows all angles of your environment nicely. Add a material with some reflection on it to get an idea of what’s going to come back from the HDRI and have at it with the renderer. And that, as they say, is a wrap.

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