View Full Version : Bright pixels appearing when use HDRI & Radiosity??

11 November 2003, 05:37 PM
Hmm I don't understand what is going on here?

Im using a HDR image as a background (with Image world) to light (using radiosity) my scene.

However when I render the image the scene turns out with lots of noise (bright pixels!?) dotted all over the object?

I've rendered the same scene using a conventional image (tga) format as a background and it turns out fine??

I'm probably doing something incredibly stupid... sorry in advance.

Any help much appreciated.


11 November 2003, 05:43 PM
try to blur the BG- image a bit in Imageeditor.
HDRIs have higher intensities in them than normal RGB- images.
The bright pixels ar caused by high intensitiy- noise.
A slight blur will remove the noise.

11 November 2003, 06:04 PM
E_Moelzer: Just tried your suggestion but Im getting the same thing?? Its as though Ive just applied noise over the rendered object, all pixelly!?


11 November 2003, 07:34 PM
I've just used what everyone should use before posting... the search function! :rolleyes:

Found some good info about all this noise stuff, hoorah for CGTalk. ;)

11 November 2003, 07:47 PM
First, be sure you have Shading Noise Reduction on.

Second, take a look at your HDR image; is there a lot of "action" (detail, high contrast) going on in it? The problem, is that radiosity involves a sampling process, and the noise is happening because of that... every once in a while, the sampling process hits a pixel that is orders of magnitude brighter than the rest of the image. If it's only 1/100th of the image, but a sampling process with 8 samples hits that pixel, it contributes 1/8th of the lighting, out of all proportion to its actual relevance. If that pixel gets hit on every 10th render pixel or so, you get anomalously bright spots. 24-bit files don't have that much range in comparison, so the sampling errors never get that big, and noise is much less, as you observed.

When you consider the sampling resolutions available in the Global Illumination panel, the highest one is 16x48 or so -- so who needs the added information? It only creates noise, eats memory and slows things down.

So, here's what I do: I take the HDR source, and downsample it in HDRshop (download here) ( to 256x128, then gaussian blur it to 4 pixels distance at least. The image should look *really* blurred. Then plug that into the background and go. You can blur it in Image Ed, but the smaller pics conserve memory and render faster. I just keep both versions on disk.

If you need the hires image for the seen background, map that onto a skydome and set it to cast no shadows & unseen by rays.

11 November 2003, 08:39 PM
A quick solution is to add a Full Precision Blurr in the image Editor in Layout. It's under the processing tab. Just use the default setings and check the result. Adjust if needed.


11 November 2003, 08:47 PM
If blur doesn't work... then in the image editor in LW... apply the HDR Exposure filter to your loaded HDRI image... (again in the image editor).

You have to decrease the Extreme bright next to Extreme dark info info in the HDRI. Basically... you up the white point to as high as 1.5 sometimes... (instead of 1)... and if further needed... increase the black point to 120. Those are both extreme settings so just start tweaking from there.

Works everytime. =)

11 November 2003, 10:38 PM
the bright pixels are caused by having too few samples and a contrasty hdri image.

11 November 2003, 10:58 PM
The noise is due to LW's renderer, and it's type of radiosity. I usually would workaround the problem by turning down the Brightness on the Image World panel.

11 November 2003, 11:48 PM
Thanks again guys, some really good advice.

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