“HDRI” stands for High Dynamic Range Image. “Dynamic range” is the measurement of how much brightness information is contained in an image, so a “high dynamic range” image is an image that has a very large range of brightness, more than you can see on your screen in one go actually. Most photos and images, in general, are what I call “LDR” images or Low Dynamic Range images. They store 8 bits of data for each of the red, green and blue channels for every pixel. An example of an LDR image is a JPG file. The problem with LDR images is that they are limited to a relatively small range of brightness, from 0 to 255, which is not actually all that much. If you want to light a 3D scene using an image, what you really need is a format that can store more than just 8 bits of data per channel so that you can have a much larger range of brightness.

Luckily, there are several formats that can do this, the most common of which has the extension “.hdr”. Generally speaking, an HDRI (High Dynamic Range Image) is simply an image that contains more than 8 bits of data per pixel per channel. Image formats like JPG and PNG are typically 8-bit and are sometimes referred to as ‘LDR’ (Low Dynamic Range) images, whereas image formats like EXR and HDR store more data and are therefore HDRIs. However, in the CG world, we have come to use the term ‘HDRI’ to describe a 32-bit 360°x180° equirectangular image that is used for lighting CG scenes. HDRIs are often used as the only light source in order to create a very realistically lit scene or to match the lighting from video footage (using an HDRI shot on the same set as the video was taken).

HDRI map Hdr Hdri sea HDRI, environment map HDRI, environment map HDRI, environment map Environment map. Panorama of sea sunset, the view of the ocean. HDRI, environment map, Round panorama, spherical panorama, equidistant projection, sea sunset, 3d rendering.

Hdr environment with sunny sky next to an sea harbor. Driver Toshiba Satellite C655-s5307.

But of course, they are also used to complement standard lighting techniques and to add detail to reflections. It really doesn’t matter which, people generally understand you either way.