HDR and Bloom

Bloom is a non-physically-correct hack to simulate light scattering effects inside optic devices (camera, eye) by adding some blur. It has, in general, nothing to do with HDR, although it is often used in combination. In its essence, Bloom is adding a blurred copy of the image, only including bright pixels above some threshold.

HDR is not a post-process effect, but has to do with the data representation. HDR means that the difference between "maximum dark" and "maximum bright" is much greater than the "usual" one.
A good camera can capture a difference upwards of 1:10000 in a scene. The human eye is able to register a difference of about 10⁵ in a scene and it can adapt to a total range of about 10¹⁰.

Normally, HDR will mean that either 16 bit integers, half-floats, or floats are used to represent intensities, or some logarithmic scheme. As you allow for a wider range of intensities you will normally want more shades in between too, or else image quality won't be good. Note that this is not strictly necessary for "HDR", though.

Since a "normal" monitor can typically represent only 1:200 to 1:1000, HDR rendering usually includes a post processing step called "tone mapping", and Bloom is often used to beef it up a bit. HDR as such, is not a post-process, though.

They usually go hand-in-hand because...

A) Whether or not you do HDR and how you do it will typically affect how you handle bloom

and

B) 99% of gamers who know what HDR is think that HDR == bloom :-P

You do not have to do both, it's certainly possible to do HDR rendering + tone mapping and never add any bloom. Unless you specifically add in the steps for bloom, you won't get it "automatically" with HDR or anything like that.

Also just to be pedantic for a moment...tone mapping doesn't have to be done as a post process. [smile]