HDR and Bloom?

Sorry for such a noob question, but can someone explain to me what HDR and Bloom is? I get a feeling that they help with the visuals in a game but am unsure as to how, and how much more powerful a card must be to run them

 

Bloom is a dynamic light effect, where you see a light, and the light source is different and casts that particular light source on "something." Like a lampost at night. HDR, is a much more sophisticated technique, only available on videocards capable of running Shader Model 3.0 HDR stands for High Dynamic Range, and increases the contrast of an image by a greater ratio than a typical monitor by itself is capable of. An example of HDR would be staring at a dark corner of a street, then looking up in the sky, you are blinded and the screen is very white, and slowy, the image of clouds fade in and you can start to see objects in the sky. HDR is really neat as it makes the environment more realistic. Also, if you are done looking at that bright sky, once you look back down in the corner, it will appear rather dark, and then lighten up and you will be able to see the details of the corner better. HDR is used to simulate how an eye adjusts to different environments. Like going out in the pitch black, and eventually being able to see better. HDR is more taxing on a system, since the lighting changes according to your position, and bloom looks very nice as well, but it certainly isn't like HDR, bloom light doesn't adjust to your "eye," but bloom doesn't hog as many precious resources. Hope this helped ya.

 

HDR basically allows the program to adjust brightness, depending on what's on screen. When looking at something very bright (the sun), the game "moves" some shades to the higher spectrum, so darker areas lose detail (because fewer shades are left to represent them), while bright areas become more detailed. Similarly, if you look at a dark place, you'll be able to see a lot more detail, because more shades are used to represent dark colours.

The effect works over time too (in the real world). If you let your eyes adjust to a dark place, then look at something bright, then you're blinded. The same can be achieved with HDR (Basically, when your eyes are "adjusted" to the dark scene, all, or most shades are allocated to showing this dark spectrum. When something bright appears, only the very brightest shades are still "unused", so the bright object appear even brighter. And if parts of it were actually less bright, tough luck. There's not enough shades left to represent this, so they're all just smeared with the 100% bright colours, giving a nice blinding effect.

You can think of it as the renderer "zooming in" on a specific part of the brightness spectrum. Instead of reserving lots of colours for the brightest shades, which might not even be visible in the current scene, we "zoom in" on a subset of the spectrum, and pretend that the top part of this spectrum is white, and the bottom part is black (even if we're zoomed in on the bottom 10% of the brightness scale, which means our "white" is actually used for a pretty **** dark colour). Then we can use all the in-between shades to represent detail in this subset.

And if something bright then appears? Well, that's literally "off the scale", since our current scale only goes from 100% to 90% dark. So anything brighter than that seems, well, bright. Blindingly so.

Visually, there are two effects of this. One is, as M1CH43L said, to simulate blinding effects. The other is to actually reveal detail that wouldn't normally be possible. (For simplicity, imagine a grey-scale game. A monitor can do only 256 shades of grey, so the dark areas will basically just be one dark blob, while the light areas will just be a whiteish blob. By "zooming in" on one part of this scale, we can use all 256 shades to represent the dark areas, when nothing white is on the screen. Then we can suddenly see details that weren't there before)

Sorry for the long ramble, but people often get this wrong, and think it's "just" a fancyway of blinding the player.

Bloom? A very popular trick, but much simpler. Basically, it just gives bright objects a soft "glow". If you've played Guild Wars, you'll know exactly what I mean.
Otherwise, here's an example

 

thank you very much for your answers! It cleared up a lot of questions I had and I really hope my 7600 will be able to handle these things

 

Which game, Oblivion? In the interest of best frame rates, you will probably just leave both of them off If my GPU could handle native rez, with high frame rates with both HDR and Bloom, I would use them.

 

Hmm, these answers are pretty far out there.

Bloom: Escentialy its the glow created by surfaces that have a high intensity light being reflected off it. When you go out on a sunny day, you may notice some objects that are light colored end up reflecting alot of light your way, and the high intensity of the light creates a sort of glow around the object.

HDR: Jalf sort of had the idea, but trying to describe it as removing shades is a bit off as it is not doing anything to colors, just lighting. HDR emulates the human eye's pupil in that it adjust the brightness depending on the average level of light hitting the users eye. An old and common example of this is when you walk outside and its blinding for a few moments. Or do you have a video camera? Ever notice that when your taking shots indoors, any light from windows appears blinding white? Thats because the cameras shutter is compensating for the low light indoors by opening wide, however any bright light like that from outdoors becomes blinding.

HDR was an effect created to emulate the oversaturation of light that happens alot in real life, but not really possible on a monitor because it dosn't produce the amount of light needed.

Performance wise, Bloom is super easy on todays modern mid level and high end cards. HDR however should be left to high end (7800/x1800 and up).

 

Same thing really, as far as the graphics card is concerned. But basically, it actually *is* working with shades.
If you want to get technical, it first renders a scene using a colour scale that's far more precise than the limited number of shades a monitor can display. (again, for simplicity, let's assume a greyscale scene. The monitor can display 256 different shades of grey, but we might render it using 65,000 shades, or more)

Then, it determines how to map these colours to the limited spectrum available to the monitor. (With normal rendering, this would be done just by rounding the colour to the closest match on the monitor, so everything loses an equal amount of detail). Here, it can take into account the average light level, and temporal effects, like if the player is just coming out of a dark area.

If that's the case, it will use most of the monitor's available colours to display the darker shades in the scene, and allow the light areas to be "stretched thin", represented by only a handful of shades. (In the above example, it might decide that the 20,000 darkest shades (which are used a lot in our very dark scene), should be represented using the first 240 of the monitor's available shades. That leaves only 16 shades on the monitor to display the remaining 45,000 shades in our rendered scene. So obviously, the bright areas will be smeared out and oversaturated (Because many of our "internal" shades will have to be represented using the same shade on the monitor), while the dark areas will have a larger level of detail visible.

The exact way this tone mapping is defined varies from game to game. But it is *usually* set up to more or less simulate the human eye's pupil.

That's only part of it. It's not *just* to create oversaturation. Like I said in my post, it can do these effects, *and* it can be used to render detail that otherwise would simply be a grey blob.