Simply put HDR is taking several images and combining the exposures so that no pixel in the resulting image is either 100% black or 100% white and using an algorithm that is referred to exposure fusion. This gives an image that has a tonal range that more closely resembles that of the human eye. The reason is fairly simple; a photographic image is static, where as the human eye is constantly adjusting as the viewer moves from dark to light areas of the scene and is processing a “live” image that is constantly being updated. This allows us to see the full range of tones and brightness. Using HDR allows the photographer to combine the “best parts”, as it were, of several or more shots to try and achieve this same range. That is HDR is all about, but was is often shown as HDR images on the net are really not the result of just a straight HDR image, but rather are the result of further enhancements with some sort of Tone-Mapping post-processing.
Understand this is not a judgment, good or bad, of the Tone-Mapping process, rather its just that those types of HDR images seem pervasive currently and the concept of what HDR actually is gets lost since these images are displayed as HDR images, when in fact they are post-processed Tone-Mapped images, whether they are based on an HDR image becomes irrelevant at that point, in fact you can make a Tone-Mapped image from a single image file. Tone-Mapping is merely a post-processing technique that can be applied to any image to enhance tonal contrast, color contrast, soften edges etc. It is easy to get carried away with it, and that is probably its biggest draw-back one gets sucked into the Wow factor of making surrealistic images very easily and the result if what we often see on the net.
Here are a few examples I created to try and exemplify the differences all based on a single scene.
The first is a straight conversion from the middle “neutral” (ev +-0) image of a set of three bracketed images. No post processing, other than what the software does in converting the RAW image to jpg. Note the shadows (esp. around the hull of the boat).
Now here is one that was done in HDR using the three shots from the bracketed exposures of the scene above. Keep in mind this is a “straight” conversion using a command line driven program in Linux that simply creates a single image using an exposure fusion algorithm with no further processing (see first article below). Again note the shadows and compare them to the one above. Note to that the contrast is somewhat reduced giving a more even overall tone. This to me is one of the several uses of HDR, probably not so much used as it is for bringing detail out of shadows that would otherwise be in total darkness or subduing over blown highlights.
Now here is the first image above rendered with a Tone-Mapping program. Keep in mind this was using a Single image Not the HDR image of photo number two
And going all the way, note how the halos have formed around areas of higher contrast and the surrealistic nature of these two last images.
So the bottom line is (IMHO) HDR can be used to enhance a photo, esp ones where there is a high range of contrasts, bright or very dark areas where a single photo simply can not capture the range of tones Without the need to do any other post processing, it simply a matter of what the creator has in mind.
Here are a couple of links of interest on HDR and Tone Mapping for those how care.
From http://www.linux.com/archive/feature/127062
If you read carefully, you might have noticed that my description of the algorithm did not refer to the original scene at all. That distinction is what makes Enfuse so fast. High dynamic range (HDR) based tone-mapping apps always start by combining the input images into a reconstruction of the original scene, usually using the exposure information in the EXIF tags to properly arrange the input stack darkest to lightest. Only then can the HDR image be tone-mapped down into a regular TIFF or JPEG.
With exposure fusion, none of that is necessary. Each pixel of each image is graded on its individual merits alone. “
From http://www.secondpicture.com/tutorials/ ... pping.html
“ Tone mapping is the process in which the colors of an HDR image are mapped to a normal image (LDR). In other words the dynamic range of an HDR image is reduced to fit into a normal image (LDR).
The acronym HDR comes from the words High Dynamic Range. In a photo, dynamic range means the difference between the lightest and the darkest color value. In other words, dynamic range means the contrast of a photo. HDR photo means a photo which has larger dynamic range than an ordinary photo. HDR photos have larger dynamic range than today's digital consumer cameras are able to capture.
Some people might think that the purpose of HDRI is to produce unrealistically colorful, flat, or artistic photos. In my opinion the real purpose of HDR photography is to produce a photo, the quality of which is higher than that of a normal photo(LDR). The purpose of HDRI is to overcome the limitations of camera equipment. HDR photography has the following advantages when compared to normal photography:
HDR photography captures higher dynamic range
HDR photography can produce noise free photos “
RM
