Shooting lighting references

Take reference stills

You should use a digital stills camera to shoot 2 types of lighting reference for each set up. The computer uses data from these reference shots to replicate the lighting conditions in the digital environment.

Shooting HDRI for Cgi

High Dynamic Range Imaging (HDRI or just HDR) is a series of digital techniques allowing the capture and subsequent editing, of the light sources and reflections within a scene.

Producing a much greater dynamic range between the light and dark areas of a shot than in normal imaging, the intention of HDRI in cgi is to produce a set of radiance files enabling the cgi lighting person to accurately replicate the ‘real’ lighting conditions of the scene as it was shot.

Basically the technique is to shoot a series of reference shots with a chrome ball or better still a 180-degree fish eye lens. These images are then used primarily to achieve four things;
 

1. Replicate the lighting conditions within a shot.
2. Replicate the light sources within a full cgi environment
3. Control the lighting conditions within a shot.
4. Create a real reflection map to put reflections on your CGI objects

Most commonly HDRI is used to replicate the lighting conditions within a shot to say add a cgi creature or object within a live action backplate.

Quite often, just using a single HDRI (Radiance) file will get you where you need to be, but normally HDRI will get you about 90% of the way, you then need to add an additional light or tweak a shadow in order to get it to perfectly match the shot.

If you shoot JPEGs for your HDRI reference, your exposures are critical. If shooting in RAW, then you have more latitude to adjust later.  The RAW file on most digital cameras includes a jpeg anyway.  At the same time, try and get a perfect colour balance by using the custom white balance feature in your digital camera. This makes life much easier in the post-processing stages and avoids any additional unnecessary corrections .

How many shots do I need to make an HDRI (Radiance) file?

Technically just one, but it will not provide much lighting information. Typically it will be 3 exposures bracketed by -2 and +2 stops as shown below.

Once combined these three images will provide the controls necessary to replicate the information required.

Grey ball reference

In addition to shooting the HDRI stills it’s essential to shoot a light grey diffuse ball (see pictures below), which will act as your primary key-light reference.

For more advanced information on HDRI go to http://www.hdrlabs.com though this site is primarily aimed at the photographer