High-Dynamic-Range imaging (HDRi) is an imaging process used to help blend a Computer Generated (CG) object or character into a scene. It is a very helpful tool and is essential to get that professional quality of VFX. A HDRi is a 360 degree image which is wrapped around the VFX element to get the correct lighting environment. When an object or green screen character is placed into a scene they will look out of place unless a lighting setup can be exactly replicated in a green screen studio. This is not always possible since the lighting of a green screen itself can be a delicate operation.
Often the best thing to do is to make a HDRi on the day of the shoot. The first thing needed is a large completely reflective sphere or mirror ball, one the size of a football would be ideal but smaller will do. They need to be completely reflective and blemish free. This is then placed on location and is taken the same day as the shoot, preferably as close in time to the shoot as possible so that the lighting does not change from cloud cover, for example.
HDRI image creation - Yusuf
You start by taking 7-8 images each of the ball from 3 different angles leading to a total of 21 or 24 images. You take the 7-8 images at different ISO settings to give the full range of lighting (darkest to brightest). Each set is averaged out to get the most accurate lighting.
You are now left with 3 images from different angles, when these are all stitched together they create the full 360 degree image, rather similar to a full panoramic.
HDRi image creation - Different ISO
This is then used in the 3D software (3ds max. Maya, Blender, cinema 4d etc) to recreate the lighting from the day of the shoot. It’s not just the lighting and the shadows which are important but the colours and reflections. For example if you are placing a 3D model of a smartphone or even a product you have not yet created in real life into a scene you would need a HDRi to show the correct reflections and colour tones on the screen to make it believable.
Even when working on a completely CG piece it is good to use a HDRi to help get the most realistic lighting and shadow effects whether it be in an online video, presentation or your corporate video demonstration for a new product or service. On our most recent project; the Circles Music Video, we created our own HDR images and documented the process. It was a huge help to us in making the most believable scene possible.
HDRi image creation - Ray