HDRI with super-bright pixels causing long render time?

[jasonhuang1115]

Hi,

I am test rendering a simple scene with a pure diffuse sphere and pure reflective chrome sphere. Lighting comes from a HDRI where light sources having bright pixel values (RGB float value of hundreds or even thousands).

The rendering almost finished all up in about 8 seconds but hanged for about 20 seconds at the last tile of where the CG chrome sphere is reflecting the bright light sources. If the pixel values in the HDRI gets higher (like thousands for the sun), that last tile will hang even longer like minutes.

What is causing that internally? Is this normal? I have tried clamping the output at level 5, but in vain. Is there other setting that can help bring down the render time?

Thanks.
Jason

[pechart]

Do you use an domelight or just a texture in the environment? I think you should maybe try the domelight, because it gets sampled differently.

[jasonhuang1115]

Do you use an domelight or just a texture in the environment? I think you should maybe try the domelight, because it gets sampled differently.

@pechart, I tried, with Adaptive DMC sampler, both domelight and a HDRI as environment override and both end up with about the same render time (a bit faster with domelight). But in both scenarios, I can observe that the rendering hanged for quite a white for the last couple tiles where reflections of the light sources are. The test scenes are simple and I didn't run into this while rendering with MR, so I am just wondering if this is normal in Vray as it samples the super high pixel values in the HDRI and if there is a specific setting to alleviate this.

[shadevfx]

Try changing the cutoff threshold. This evaluates how many times it keeps sampling a reflection even if the area is just "white" keep upping it till the reflection stops looking normal and then go back a hair. Can really speed things up.

[Morbid Angel]

this can occur in some cases. To solve this you need to enable color clamping in color mapping section and set your limit to be around 2.5. This will clip off all unnecessary float ranges beyond user specified value eliminating the bright pixels (hopefully)
Some people confuse this option with making images 8 bit like. If you give it a range above 1, then it will still retain float value, its just that by default it has infinite range, may cause undesired result.

[jasonhuang1115]

Try changing the cutoff threshold. This evaluates how many times it keeps sampling a reflection even if the area is just "white" keep upping it till the reflection stops looking normal and then go back a hair. Can really speed things up.

Sorry for my newbie question. Can you be more specific on which cutoff threshold you are referring to?

this can occur in some cases. To solve this you need to enable color clamping in color mapping section and set your limit to be around 2.5. This will clip off all unnecessary float ranges beyond user specified value eliminating the bright pixels (hopefully)
Some people confuse this option with making images 8 bit like. If you give it a range above 1, then it will still retain float value, its just that by default it has infinite range, may cause undesired result.

I set the Clamp level from 5 to 2.5, but still a simple chrome sphere can take 2 min and half to finish (Adaptive DMC, Min subdivs: 1, Max subdivs: 100, Threshold: 0.1). But if I switch the sample to Adaptive subdivision, the render time is around 30 second. (Min rate: -1, Max rate: 1, Threshold: 1.5).

[shadevfx]

The cutoff threshold is in the shader attributes.

[jasonhuang1115]

The cutoff threshold is in the shader attributes.

I see. Thanks.

[Morbid Angel]

I would never set it to be 1/100. Keep in mind that adaptive subdivision and adaptive dmc are different samplers, using different thresholds and cannot be compared. -1/1 in adaptive subdivision will return 1 sample per 4 pixels, and max of 4 samples per 1 pixel, where as adaptive dmc will return 1 min and 10000 samples.

[rainingwaffles]

Do you have "Subpixel Mapping" on ("Render Settings -> VRay -> Color Mapping -> Subpixel Mapping)? This should help your render times quite a bit and in most cases it won't affect the final image. My understanding is that this allows the "clamp level" to happen at the *subpixel level* rather than evaluating an entire pixel before clamping. You will see a warning message when you're rendering which says --

"warning: Subpixel color mapping is on: rendered result may have incorrect brightness."

There are certain cases where Subpixel Mapping will make a motion-blurred, streaked highlight darker than with it off, but I feel the trade off in speed is usually significant when you're using lots of high dynamic range sources for lighting.

Blake