Better HDRI Maps for Final Gather (General Technique)

Prerequisite knowledge

  • Basic knowledge about rendering techniques.

What you will learn

  • How to process an HDRI map for final gather rendering
  • Learn about a free tool to do HDRI processing  that would normally require very high end software (e.g. nuke)

What you will need

  • Picturenaut (free, and a mac version is on its way )
  • Banty’s Toolkit
  • An HDRI image to process. A good demo set can be found here for free.
  • Windows PC to follow the tutorial.
    Note that the information can be applied to any set of tools, so don’t be discouraged  if you are on a mac.
    If you are aware of mac substitutes for this tool, let me know so I can add it to the  tutorial.

The problem to be solved

Generally speaking final gather works by sending out a ray from the camera  to an object and then bouncing that ray to the environment to get an approximation of the indirect lighting contribution.

HDRI maps are inherently problematic as  you could have  pixels  that are extremely bright and extremely dark next to each other. If a ray hits a bright pixel on the  environment map and the ray next to it hits a very dark pixel, you will get splotches in your render.

The only solutions to this are:  brute force sampling (bad)  or processing your HDRI image to resolve the problem (better)

 

The solution

Create 2 HDRI maps.

The high resolution one is used for  reflections.

A secondary HDRI map is generated for use as your environment map.  This is done by reducing the resolution and applying a diffuse convolution filter onto your HDRI map. This may  appears similar to a gaussian blur but the key difference is  that it preserves the edges of the map and appropriately approximates the indirect lighting contribution within the HDRI map.  Your final file will will be of a lower resolution and will give you better results with less samples.

 

Setting Up Picturenaut & Banty’s toolkit

  • Download the appropriate version.
  • Run the installer
  • If you are requested to, download and install the .net framework 3.5
  • Download the Banty toolkit
  • Extract the contents of the plugins folder within the zip to \InstallDirectory\Picturenaut\plugin-ins\hdrshop\

Processing an image

  • Open Picturenaut
  • Click File > Open File of your choice

hdr_before

  • Click Filter > HDRShop > Diffuse SH
  • Run the filter when it pops up
  • Save the new image and load it as your environment map. It should now look a bit like this:hdr_after
  • As a proof of the technique, you can take a look at this link
  •  file containing spherical harmonic coefficients is also created. if you have the appropriate tools, you can use this information to approximate indirect illumination in real time,  but that is  way out of the scope of this tutorial.

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  1. Process HDR Maps for Better Final Gather Renders | LESTERBANKS - January 29, 2014

    […] environment map for lighting scenes rather than using a typical standard HDR image. Shane shows how you can get better and faster Final Gather renders by processing the HDR images by applying a diffuse convolution filter to […]

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