Starting in the 1980s, a number of rendering researchers worked on establishing a solid theoretical basis for rendering, including physical correctness. PBR principles may be implemented in real-time applications using Shaders or offline applications using Ray tracing (graphics) or Path tracing.
Photogrammetry may be used to help discover and encode accurate optical properties of materials. Feasible and quick approximations of the bidirectional reflectance distribution function and rendering equation are of mathematical importance in this field. Many PBR pipelines aim to achieve photorealism. Physically based rendering ( PBR) is a computer graphics approach that seeks to render images in a way that models the flow of light in the real world. Tessellation is used to generate an object mesh from a heightmap and normal map, creating greater detail. Even though this is a rough, opaque surface, more than just diffuse light is reflected from the brighter side of the material, creating small highlights, because "everything is shiny" in the physically-based rendering model of the real world. However, there are many sites providing HDRIs for free, usually in reduced resolution.An image of bricks rendered using PBR. All that difficulties are the reason why quality HDR images are quite rare and expansive. Exposure bracketing and using filters is essential. I think, there is no camera capable to capture such dynamic range in a single shot. Of course, capturing a 360deg panoramic environment map with such a high range of exposure is not an easy task. OpenEXR is often preferred in Blender, because it enables to store renders with multiple layers and passes for post-processing and also seems to be more memory effective, but HDR files should work fine as well. There are two most common file formats: Radiance HDR (.hdr) and OpenEXR (.exr). That's simply essential for capturing real lighting conditions in outdoor environments, where the difference between the luminance of sun and the rest of the scene might be up to 20 EV (exposure value numbers) or even more. Thanks to that, they are capable to capture extremely high range of luminance.
HDRI (high dynamic range images) are image files with extended bit depth (usually 32bit per color channel).
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