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schupf

HDR Blur

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Short question: When using a blur filter in HDR rendering the lights goes "over" the edges, for example the sun shines over the silhouette of a mountain. I want to google about the physics of this effect but I dont know the name of this "blurring of light over edges". Does anyone know how this effect is called in english?

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Well, I dont think "bloom" is the right word.

And I dont mean the computer graphics word. I mean the word for the effect in real world. So actually I rather need a native speaker instead of a cg expert;)

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Quote:
Original post by schupf
Well, I dont think "bloom" is the right word.

And I dont mean the computer graphics word. I mean the word for the effect in real world. So actually I rather need a native speaker instead of a cg expert;)



light doesn't do that in the real world... if you stare at the sun, the sun doesn't "bleed" over the mountain crest. your eye adapts and adjusts its perceptive range to a higher one.

Computer monitors don't have enough brightness range to show the difference of the sun and the mountain, the sun is simply too bright in relation to the mountaint to display it proportionately.. so we run a bloom effec on the sun to make it appear brighter than it really is.

http://en.wikipedia.org/wiki/High_dynamic_range_rendering#Light_bloom

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http://img504.imageshack.us/img504/3297/donald129jk2.jpg <- the streaks here are due to lens flare.. and the glowy bit is more a property of the atmosphere refracting the light around than it is of the light.

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Actually, bloom is a real effect, although games often exaggerate it. It's caused by diffraction in the lens that makes the incoming light spread out; if the light source is bright enough, nearby areas of the image become overexposed. It's why the power lines in that photo seem to disappear "behind" the sun. You don't usually notice it in real life because your pupils contract in the presence of bright light.

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One question about the implementation of such a blur effect:

First I render the scene to a texture, fetch the bright parts of the texture, blur them by applying a Gaussian filter and finally blend the blurred image to the initial image.

Is this true? Is the initial image the image before I fetched the bright parts (so the result of the rendering) or is it the rendered image minus the bright parts (so actually a darker image)?

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Quote:
Original post by schupf

First I render the scene to a texture, fetch the bright parts of the texture, blur them by applying a Gaussian filter and finally blend the blurred image to the initial image.



Yup, that's the basic process. Typically the initial render target is also downscaled once or twice, which so that a huge blur isn't needed to get desirable results. A few of the D3D9 samples in the DX SDK implement this, namely PostProcess, HDRLighting, and HDRPipeline.

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The effect does exist in the real world, and is called glare. It happens both with with photographic lenses, as well as in the human eye. In the latter, it is caused by bright light being scattered within the viscous parts of the eyeball. It also happens when an optically dense participating medium is involved, such as mist or fog.

Quote:

First I render the scene to a texture, fetch the bright parts of the texture, blur them by applying a Gaussian filter and finally blend the blurred image to the initial image.

More or less, yes.

Quote:

Is this true? Is the initial image the image before I fetched the bright parts (so the result of the rendering) or is it the rendered image minus the bright parts (so actually a darker image)?

The former.

Quote:

if you stare at the sun, the sun doesn't "bleed" over the mountain crest. your eye adapts and adjusts its perceptive range to a higher one.

If you stare at the sun, your retina burns away and you go blind... ;)

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