How to use HDR image or HDRI image

The Twilight Render Team shares tips, ideas, helpful hints, and more on using Twilight Render
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Fletch
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How to use HDR image or HDRI image

Post by Fletch » Sat Mar 05, 2016 2:51 pm

HDR / HDRI - High Dynamic Range Images contains all the lighting information needed to light your 3D model / scene. This is because within the image there can be brighter and darker pixels than can be physically displayed on a screen/monitor. This extra information is used to add light to your scene as if this image is shining down on your model from a surrounding sphere. HDR images are designed to replicate exact lighting situations (note that an HDR image is not simply a spherical image; if the file doesn't end in .hdr or .exr, it's not an hdr image!). Work free with HDR using Picturenaut.

Critical info about HDR images:
  1. HDRI come in different shapes and styles: called "projections"
  2. HDRI come in different sizes: called "resolutions".
  3. HDRI come in different quality: called "all HDRI are not created equally"
  4. HDRI can be very large files: large HDRI can fill the RAM very quickly when rendering and can therefore cause very slow renderings, or crashing.
Image

How to use HDR with Twilight Render:
  1. Uncheck the Sun so that it's not shining in the scene.
  2. Leave all other sky options at default.
  3. Change sky type to the correct sky format this will usually be "Spherical Sky" or "Sky Probe" and browse to load your HDR image as the spherical sky. An HDR Probe image usually has a circular thumbnail, if not, then it's a normal "spherical" map.
  4. If a quick test rendering using Easy 01 or Easy 02 reveals that the sky is way too bright or dark: increase or decrease the intensity of the sky... many HDR need intensity set to "0.1", while most are best at the default of "1.0". (see point 3 above)
  5. Use a Low Resolution Version (sometimes called a blurry HDR) of the HDR image for very quick and high quality results. Open the HDR image in an image editor that can handle it, and resize the file to be only 400px wide maximum and save-as to a new name like "MyHDRLowRes.hdr" or "MyTinyHDR.hdr" - using this
When to use HDR with Twilight Render
  1. Product Shots such as cars or jewelry or other single objects on a stage are perfect for HDRI lighting. You can find many free Photo Studio HDR available online.
  2. Whenever photorealistic lighting with very soft shadows are desired.
Can I use Sun combined with HDR image?
HDRI supposedly have all the light for the scene already contained within them, however, if sharper shadows are desired you can turn on the sun. Setting the strength of the sun to 4 instead of default 5 or even lower, to perhaps 3, may look better, but also, reducing the intensity of the hdr image simultaneously while adding the sun may be required.
This is partially up to the artist, and partially to avoid rendering artifacts. Rendering artifacts are things like fireflies or light leaks or flashes which result from too much light in the scene.

Why is it so BRIGHT when I render with HDR?! Why is it so DARK when I render with HDR?
Increasing exposure in the tone mapping panel of the Twilight Render dialog is the best way to increase light in the scene. This, in combination with balancing HDR strength/intensity with Sun strength/intensity is required for correct expsure with HDR images.

Where can I get some HDRI to use and more info about HDRI?
Download Free HDR images from Kerkythea.
Look closely at Rafa's great tutorial on HDR and sphericals, globals, etc.
Subject: Scale an HDR Spherical Image to Match Your Scene (How To)
Subject: Background Image / Picture Using Alpha Mask
Subject: How to use Alpha Mask render pass in Post Production
Subject: Loading heavy (high resolution, large) hdr
Subject: Create your own spherical sky...
Subject: HDRs causing random surfaces to emit light?
Lots of great free studio HDR images here.
Alvydas at Kerkythea's forum wrote here about which are his favorite:

If I use a Blurry HDR for lighting, how do I get the background to look good?

Plan to insert your sky for the final image using the Alpha Mask Pass from Twilight and a high resolution background image in a photo editor.
Subject: Background Image / Picture Using Alpha Mask
Subject: How to use Alpha Mask render pass in Post Production
Subject: Force background image to render unaffected by lighting

How can I resize my HDR?
Subject: Scale an HDR Spherical Image to Match Your Scene (How To)

Subject: Maximeroz.com 6 free interior HDRI images

The Apartment probe has never failed me as studio lighting.

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