Cycles lamp "strength" ignores scene's "unit scale", producing inconsistent renders in scenes with consistent scaling #53517

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Chase Moskal · 2017-12-08T22:17:50+01:00

Chase Moskal commented

2017-12-08 22:17:50 +01:00

System Information
Windows 7, GTX 580

Blender Version
Broken: 2.79 5bd8ac9
Worked: ---

Short description of error
Lamp emission 'strength' values are not correctly factoring the scene's Unit Scale, leading to inconsistent renders for scenes of the same dimensions (by the blender ruler).

In a blender scene configured for Centimeters unit scale, an area lamp of size 1 meter, requires 1 000 000 (one million) emission strength in order to reasonably light a 6m scene. A strength value of 10 is expected instead.

Mesh emitters are not affected by this bug — in the same "Centimeters" scene described above, a mesh emitter of the same 1 meter size needs only 10 emission strength.

If the scene is remastered to use Meters unit scale (such that the blender ruler still makes the same measurements) — the mesh emitter still works consistently with 10 strength, and the lamp then agrees is also happy with 10 strength — this illustrates the inconsistency between mesh-emitters and lamps with respect to unit scale.

Exact steps for others to reproduce the error

stage-crazy-lamps-2.blend
the above scene demonstrates strength one-million is required when using centimeters scale
also see the stack overflow post which spurred the discovery of this bug

**System Information** Windows 7, GTX 580 **Blender Version** Broken: 2.79 5bd8ac9 Worked: --- **Short description of error** Lamp emission 'strength' values are not correctly factoring the scene's Unit Scale, leading to inconsistent renders for scenes of the same dimensions (by the blender ruler). In a blender scene configured for `Centimeters` unit scale, an area lamp of size `1 meter`, requires `1 000 000` (one million) emission strength in order to reasonably light a 6m scene. A strength value of `10` is expected instead. Mesh emitters are not affected by this bug — in the same "Centimeters" scene described above, a mesh emitter of the same `1 meter` size needs only `10` emission strength. If the scene is remastered to use `Meters` unit scale (such that the blender ruler still makes the same measurements) — the mesh emitter still works consistently with 10 strength, and the lamp then agrees is also happy with 10 strength — this illustrates the inconsistency between mesh-emitters and lamps with respect to unit scale. **Exact steps for others to reproduce the error** - [stage-crazy-lamps-2.blend ](https://drive.google.com/open?id=1l_gT-6_DrfSYaHDPYFvzTlxh5SJqlLo0) - the above scene demonstrates strength one-million is required when using centimeters scale - also see the [stack overflow post ](https://blender.stackexchange.com/questions/96115/why-do-my-lamps-require-an-insane-strength-value-of-1-million) which spurred the discovery of this bug

Chase Moskal commented

2017-12-08 22:17:50 +01:00

Added subscriber: @chase-moskal

Brecht Van Lommel commented

2017-12-24 16:06:06 +01:00

Added subscriber: @brecht

Brecht Van Lommel commented

2017-12-24 16:06:06 +01:00

Changed status from 'Open' to: 'Archived'

Brecht Van Lommel closed this issue

2017-12-24 16:06:06 +01:00

Brecht Van Lommel self-assigned this 2017-12-24 16:06:06 +01:00

Brecht Van Lommel commented

2017-12-24 16:06:06 +01:00

Lamps are in Watts, the size of the lamp intentionally does not affect the intensity, only the softness of the shadows. Mesh lights are in Watts/m^2 and are affected by the mesh scale.

In the future we may add some options to controls this behavior, but it's not a bug.

Lamps are in Watts, the size of the lamp intentionally does not affect the intensity, only the softness of the shadows. Mesh lights are in Watts/m^2 and are affected by the mesh scale. In the future we may add some options to controls this behavior, but it's not a bug.

mathieu menuet commented

2017-12-24 16:50:06 +01:00

Added subscriber: @bliblubli

mathieu menuet commented

2017-12-24 16:50:06 +01:00

@brecht adding this information in the tooltip or as a unit like for the distance values would clarify such discrepancies.

John Boyle commented

2018-09-10 06:51:05 +02:00

Added subscriber: @novelistparty

John Boyle commented

2018-09-10 06:51:05 +02:00

@brecht When I change my units to cm, the light from a point lamp becomes dim in the render. Why is that?

Brecht Van Lommel commented

2018-09-10 15:56:47 +02:00

Changing unit settings in the render does affect the render, unless perhaps you are using an addon or importer that takes them into account. It only affects the display and editing of values in the UI.

John Boyle commented

2018-09-12 22:38:08 +02:00

@brecht After I change units, I expect that I could re-add the default scene objects and that the render would be the same as the unchanged default scene. That isn't what happens. The most noticeable difference is that the Point Lamp strength has to be increased a lot; I would expect it's strength to be scaled to match it's larger size.

Things (I've found) that don't automatically scale to match the default blender-units scene when I set units to Centimeters:

a Point Lamp's strength
the View Clip distance. Before the unit change, it is 0.1 to 1000 (blender units). After the unit change it becomes 1mm to 10m. That doesn't match 1 blender unit = 1 meter.

If I'm not understanding something about why these things behave this way, could you point me at some documentation or blog posts about how this works in Blender. I can't find anything definitive about this and it seems really common to immediately change a project to Centimeters. I'm hoping for a more self-consistent experience with Blender. Thanks!

@brecht After I change units, I expect that I could re-add the default scene objects and that the render would be the same as the unchanged default scene. That isn't what happens. The most noticeable difference is that the Point Lamp strength has to be increased a lot; I would expect it's strength to be scaled to match it's larger size. Things (I've found) that don't automatically scale to match the default blender-units scene when I set units to Centimeters: - a Point Lamp's strength - the View Clip distance. Before the unit change, it is 0.1 to 1000 (blender units). After the unit change it becomes 1mm to 10m. That doesn't match 1 blender unit = 1 meter. If I'm not understanding something about why these things behave this way, could you point me at some documentation or blog posts about how this works in Blender. I can't find anything definitive about this and it seems really common to immediately change a project to Centimeters. I'm hoping for a more self-consistent experience with Blender. Thanks!

vurolele commented

2020-05-23 15:19:05 +02:00

(Migrated from localhost:3001)

Added subscriber: @vurolele

vurolele commented

2020-05-23 15:19:05 +02:00

(Migrated from localhost:3001)

This comment was removed by @vurolele

*This comment was removed by @vurolele*

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Cycles lamp "strength" ignores scene's "unit scale", producing inconsistent renders in scenes with consistent scaling #53517