Building an HDRI library you can actually light with

Image-based lighting is the fastest route to believable light, and the collection you build is worth more over time than almost anything else in your library. It is also the asset type most people organise worst, for one structural reason: you cannot tell a warm afternoon from a flat overcast by reading kloofendal_43d_clear_4k.exr.

What an HDRI actually is

Two properties, both worth being precise about because the confusion between them causes most HDRI problems.

High dynamic range. An ordinary image clamps at pure white. An HDRI stores floating-point values with no ceiling, so a sun can be thousands of times brighter than the sky beside it. That ratio is what produces convincing light — sharp shadows come from a small very bright region, soft shadows from a large dim one — and it is why you cannot light convincingly from a JPEG panorama no matter how nice it looks.

Equirectangular projection. The full sphere unwrapped into a 2:1 rectangle, longitude across, latitude down, horizon on the middle row. The poles are stretched across the entire image width, so the very top and bottom contain hardly any real information. This is why the ground in an HDRI usually looks smeared, and why using one as a visible floor rarely works.

┌────────────────────────────────────────┐  zenith — heavily stretched
│                                        │
│                                        │
├────────────────────────────────────────┤  horizon — the useful band
│                                        │
│                                        │
└────────────────────────────────────────┘  nadir — usually smeared, often patched
 0°                180°                360°

Resolution: less than you think

The most common and most expensive mistake is downloading everything at 16K. Light contribution is low-frequency; the renderer is integrating broad regions of brightness, and it cannot see the difference between a 2K and a 16K sky in the shading of a diffuse surface.

A practical habit: keep a 4K copy as the working default and the highest resolution you own alongside it, named so they sort together. Light with the 4K, swap to the large one only for the final frame where the environment is actually in shot. Your scene load times will thank you every single day.

Setting one up, per application

The pattern is the same everywhere — a dome or environment light with the image as its texture — but the node names and the colour space control differ.

Choosing the right sky

Worth internalising, because it turns "scroll until something looks nice" into a decision you can make in ten seconds.

• Shadow hardness comes from the apparent size of the bright region. A small sun in a clear sky gives hard-edged shadows; an overcast sky is one enormous soft source and gives almost none. If your shadows are wrong, the sky is wrong — no amount of light tweaking fixes it.
• Contrast ratio sets the mood. Clear midday has an enormous range between sunlit and shadowed; overcast has very little. This is the difference between dramatic and documentary, decided before you touch a single light.
• Colour temperature separates sun from sky. Direct sun is warm, the open blue sky that fills the shadows is cool. That warm-key, cool-fill split is most of what makes outdoor lighting read as real.
• Sun elevation controls the whole feel. Low sun means long shadows, strong colour and raking light across surfaces. High sun is short shadows and flatter forms.
• Indoor HDRIs bring their own geometry. Windows become visible rectangular highlights in reflections, which is a gift for product work and a liability if the reflections do not match the implied room.

Organising a collection

Split on the questions you actually ask. You never think "I need the 43-degree one"; you think "I need soft overcast" or "late afternoon, low sun".

hdri/
├─ outdoor/
│  ├─ clear/          hard shadows, high contrast
│  ├─ overcast/       soft, low contrast
│  ├─ golden/         low sun, warm
│  └─ night/
├─ indoor/
│  ├─ domestic/
│  └─ industrial/
└─ studio/            softboxes, seamless, product
   └─ product/

Then name for the light, not the place. kloofendal_43d_clear.exr is a location; outdoor_clear_lowsun_warm_4k.exr is a lighting decision. If you keep the original name for provenance, put the descriptive tokens first so sorting groups by character.

And keep resolution variants adjacent — ..._4k.exr and ..._16k.exr sharing a stem — so a tool that groups by stem shows you one entry with two options rather than two unrelated entries.

The preview problem

Everything above still leaves the core difficulty: you cannot see your HDRIs. Windows has no thumbnail handler for EXR or HDR, so a folder of skies is a grid of identical blank icons. HDR data also needs tone-mapping to become a viewable thumbnail at all — a naive conversion turns every sunny sky into a white rectangle — and the 2:1 aspect gets mangled by anything that assumes square tiles.

This is why HDRI collections rot faster than any other asset type. You download 200 and then use the same four, because those four are the ones you can remember.

How Kiosk handles HDRIs

HDRIs are one of the three built-in categories, and they get specific treatment rather than being filed as images.

Previews are tone-mapped rather than clamped, so a bright sky reads as a bright sky instead of a white rectangle, and the 2:1 aspect is preserved so the horizon and sun position are visible in the tile. That alone converts an unusable folder into something you can choose from at a glance.

On export, sending an HDRI to a live session creates the correct setup for your renderer — a dome light, environment light or sky light with the image loaded, colour space set to linear, ready to rotate. Send a second HDRI with a light selected and it retargets that light instead of stacking another, which makes comparing skies a matter of clicking through the grid and watching the viewport change.

The Poly Haven library is browsable inside the app, so downloading a new sky and getting it into your scene is the same action. Anything you already downloaded from the website indexes as an ordinary folder.

Questions