Cycles: Light sampling threshold inadvertently clamps negative lamps #50975

New Issue

Jason Clarke · 2017-03-17T05:41:41+01:00

Jason Clarke commented

2017-03-17 05:41:41 +01:00

System Information
Windows 10, Nvidia GTX 980

Blender Version
Broken: 2.78b

Short description of error

Not sure this is truly a bug, but: It's possible to make a negative lamp in Cycles by giving a light a negative value for strength. However, when using light sampling threshold, the negative power will fall below the threshold and sampling will constantly get canceled for the negative light

Exact steps for others to reproduce the error
Add a lamp with a negative strength, note the difference in effect between light sampling threshold of 0 (disabled), vs any larger value (enabled).

A test blend with a negative lamp is located here: negative_light.blend

**System Information** Windows 10, Nvidia GTX 980 **Blender Version** Broken: 2.78b **Short description of error** Not sure this is truly a bug, but: It's possible to make a negative lamp in Cycles by giving a light a negative value for strength. However, when using light sampling threshold, the negative power will fall below the threshold and sampling will constantly get canceled for the negative light **Exact steps for others to reproduce the error** Add a lamp with a negative strength, note the difference in effect between light sampling threshold of 0 (disabled), vs any larger value (enabled). A test blend with a negative lamp is located here: [negative_light.blend](https://archive.blender.org/developer/F511479/negative_light.blend)

Jason Clarke commented

2017-03-17 05:41:41 +01:00

Changed status to: 'Open'

Jason Clarke commented

2017-03-17 05:41:41 +01:00

Added subscriber: @JasonClarke

Sergey Sharybin commented

2017-03-20 12:12:15 +01:00

Added subscribers: @LukasStockner, @brecht, @Sergey

Sergey Sharybin self-assigned this 2017-03-20 12:12:15 +01:00

Sergey Sharybin commented

2017-03-20 12:12:15 +01:00

Don't really think it's a bug. Negative colors and strength is going to be always problematic. What is the reason you use negative strength on lamps?

@brecht, @LukasStockner, shouldn't we disable possibility of setting negative light strength?

Don't really think it's a bug. Negative colors and strength is going to be always problematic. What is the reason you use negative strength on lamps? @brecht, @LukasStockner, shouldn't we disable possibility of setting negative light strength?

Lukas Stockner commented

2017-03-20 13:29:59 +01:00

I don't think that we should explicitly disable negative lights - after all, they don't really cause any problems for regular use and there's no real chance of using them by accident, so I don't see the harm in keeping them as an "undocumented feature" as long as we don't officially support them.

As for the light threshold - I guess adding a fabsf() wouldn't noticably change performance, but of course some benchmarks are needed to check that.

I don't think that we should explicitly disable negative lights - after all, they don't really cause any problems for regular use and there's no real chance of using them by accident, so I don't see the harm in keeping them as an "undocumented feature" as long as we don't officially support them. As for the light threshold - I guess adding a fabsf() wouldn't noticably change performance, but of course some benchmarks are needed to check that.

Brecht Van Lommel commented

2017-03-20 13:52:53 +01:00

So far we've supported negative light strengths, though in practice I would suggest to never use it. So if we want remove it at some point that's fine with me, but maybe better for 2.8.

Adding the fabsf() here should be fine, I wouldn't even bother to benchmark it, clearing the sign bit is super fast.

So far we've supported negative light strengths, though in practice I would suggest to never use it. So if we want remove it at some point that's fine with me, but maybe better for 2.8. Adding the `fabsf()` here should be fine, I wouldn't even bother to benchmark it, clearing the sign bit is super fast.

Sergey Sharybin commented

2017-03-20 14:48:22 +01:00

@brecht, this is just weird to forbid negative color but allow negative light.. Would be better if both are allowed or both are disallowed. But i'm fine keeping things as-is and just use fabs() for now.

blender-admin commented

2017-03-20 14:48:37 +01:00

This issue was referenced by blender/cycles@6aade41f46

This issue was referenced by blender/cycles@6aade41f4693d6b1b35143a59955d355f4865de7

blender-admin commented

2017-03-20 14:48:37 +01:00

This issue was referenced by a201b99c5a

This issue was referenced by a201b99c5aaa7945bd0702c42987a60e3d70ee46

Sergey Sharybin commented

2017-03-20 14:49:03 +01:00

Changed status from 'Open' to: 'Resolved'

Sergey Sharybin closed this issue

2017-03-20 14:49:03 +01:00

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Cycles: Light sampling threshold inadvertently clamps negative lamps #50975