HDRI is an acronym for High Dynamic Range Imaging, It is a technique to capture both the dark and bright highlights. And it has much more color detail than normal images.
The HDR images produce by taking multiple pictures with different positions at the same time. Each picture captures the discrete range of scene detail which will be required for creating HDR images. For instance, one picture is taken on a -1 exposure setting, one is at an O exposure setting, and the other is taken on a +1 exposure setting. Then combined all the images to form a single image.
HDR image technique is fruitful when taking pictures of scenes that are too contrasty. For instance, if you are positioned in front of a door during a sunshiny day. It creates a dark shadow and makes your face appear dark. If you adjust the light to brighten up your face then the door becomes very bright. It is almost impossible to achieve an accurate display for both. So, that is where the HDR can use.
It is obvious that you can't capture the whole dynamic range in one single image. So, to overcome this issue, you should capture two to three images. One image preserved highlight details. one preserved shadow detail. And one is somewhere in the middle, Just for good evaluation.
All these different exposures layered together and blend to form a single image that maintained the details from the deepest shadow to the brightest highlight.
In recent times, many advancements occur in digital sensor technology. Thereby, many digital cameras can capture good quality and detailed images in a single exposure.
Most often architectural and landscape photographers use HDR images to depict the high contrast scenery.
360° HDR panoramas
These panoramas are created by adding several images of a scene, with different exposures. With specialized pieces of equipment.
These HDR panoramas Used as a light source in 3D programs to brighten the scene. Because it creates a very natural lighting effect.
The 360 HDR panoramas are useful for 3D visualization as they catch real-world lighting of the whole environment.
Flattening the Image
An HDR image has a lot of detail and it brightens up the darker areas to reveal every smallest detail.
However, flattering the image by decreasing the contrast between the original dark and bright areas is a bad practice.
Because it makes the pictures seem less natural and not really appealing.