I have a bone to pick with game developers and graphics artists out there using the term HDR incorrectly. I understand the notion that a word can mean something different under two different contexts. For HDR, however, the idea and term are rooted in how a scene is lit. Whether you are using it to describe a scene in a game or a photograph the term means the same thing. The H in HDR stands for High, not Low.

I am sick of seeing articles that explain how a developer can equalize tones in a scene by increasing or decreasing the exposure for each rendered object only to end up with the addition of glowing and blurring the scene. Bloom has nothing to do with HDR, it is an entirely different concept used to simulate the effect of an object appearing overly shiny. This is commonly done by downsizing the scene, blurring it and then pushing each pixel towards dark or bright. This effect works great if you are attempting to render a lightsaber, but not if you are attempting to use HDR.

The point of HDR, or High Dynamic Range, is to equally expose different parts of a scene. For example let’s say you had a dark room with a desk. On the desk is a lamp that is lighting the top of the desk. To the right of the desk is a chair that is dimly lit by the ambient light. Using HDR in this case would allow an artist (game developer, photographer, etc) to brighten the chair while darkening the desk (or leaving it alone). You can think of HDR as corrective lighting surgery for an image of a scene.

Even Wikipedia contradicts itself on this topic:

One of the primary features of HDR is that details in a scene with a large contrast ratio are preserved. Without HDR (for the purpose of this article, low dynamic range or LDR) areas that are too dark are clipped to black and areas that are too bright are clipped to white. These are represented by the hardware as a floating point value of 0.0 and 1.0 for pure black and pure white, respectively. Graphics processor company nVIDIA summarizes one of HDRR’s features in three points[4]:

The Wiki page is stating that HDR scenes provide a mechanism for preserving the details over the entire scene regardless of current exposure. It goes on to say that without HDR, you get over and under exposure problems. The contradiction occurs in the sample image (below) which depicts HDR incorrectly. On the side that is supposedly HDR, the scene contains hot spots of over exposure, a component of a LDR (low dynamic range) scene. Fortunately the right side of the image isn’t HDR either but simply the left side without the bloom applied.

It doesn’t stop there: one of the most commonly used examples is a person coming out of a tunnel. We all know the effect as we come out into the sun from being in the dark. Because our eyes adjusted to the relative darkness of the tunnel they are over compensating when they come out of the tunnel and thus it seems bright for a fraction of time. The problem is that people are attempting to use this as an example of HDR. Rather the desired effect is done through blooming (over exposing) the entire scene for a fraction of a second and then turning the exposure back down.

I am open to discussion (you will have to register to comment) to whether or not HDR means something different in terms of gaming but I will tell you right now that the term must be based on some idea. It isn’t valid for us to just use a term that has a meaning and change it because we need a word for that meaning. I am a firm believer that the term is rooted in how a scene is lit, as demonstrated by this other Wikipedia entry which actually uses 3D graphics to get the point across.