glTF light intensity requires unit conversion #91035

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Pete Chown · 2021-08-29T19:29:31+02:00

Pete Chown commented

2021-08-29 19:29:31 +02:00

System Information
Operating system: Ubuntu Linux 20.04
Graphics card: RTX 2080 Ti

Blender Version
Broken: 2.93.3
Worked: None as far as I know

Short description of error

When point lights are exported to a glTF file, it looks as though the intensity is supposed to be expressed in candela . In Blender, the intensity is expressed in watts; presumably it's the amount of illumination you are supposed to get from an incandescent bulb of that power. Unfortunately no unit conversion takes place during export, so for example a 100W bulb in Blender will be exported as a glTF light of intensity 100 candela.

Exact steps for others to reproduce the error

Open Blender with the default scene.
Select the light and observe that the power is 1000W.
Export as glTF. Select Format/glTF Embedded and Include/Punctual Lights.
Examine the glTF file in a text editor and observe that the intensity is 1000 under extensions/KHR_lights_punctual.

I hope this is helpful. If you need anything else please let me know, and thank you for your hard work on Blender!

**System Information** Operating system: Ubuntu Linux 20.04 Graphics card: RTX 2080 Ti **Blender Version** Broken: 2.93.3 Worked: None as far as I know **Short description of error** When point lights are exported to a glTF file, it looks as though the intensity is supposed to be [expressed in candela ](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_lights_punctual#light-shared-properties). In Blender, the intensity is expressed in watts; presumably it's the amount of illumination you are supposed to get from an incandescent bulb of that power. Unfortunately no unit conversion takes place during export, so for example a 100W bulb in Blender will be exported as a glTF light of intensity 100 candela. **Exact steps for others to reproduce the error** 1. Open Blender with the default scene. 1. Select the light and observe that the power is 1000W. 1. Export as glTF. Select Format/glTF Embedded and Include/Punctual Lights. 1. Examine the glTF file in a text editor and observe that the intensity is 1000 under extensions/KHR_lights_punctual. I hope this is helpful. If you need anything else please let me know, and thank you for your hard work on Blender!

Pete Chown commented

2021-08-29 19:29:31 +02:00

Added subscriber: @PeteX-2

Pratik Borhade commented

2021-08-30 05:13:14 +02:00

Added subscriber: @PratikPB2123

Pratik Borhade commented

2021-08-30 06:08:30 +02:00

Changed status from 'Needs Triage' to: 'Confirmed'

Scurest commented

2021-08-30 07:37:14 +02:00

Added subscriber: @scurest

Scurest commented

2021-08-30 07:37:14 +02:00

There is also a report on the addon's tracker: https://github.com/KhronosGroup/glTF-Blender-IO/issues/564

Mindinsomnia commented

2021-08-30 08:03:48 +02:00

Added subscriber: @Grady

Mindinsomnia commented

2021-08-30 08:03:48 +02:00

How is the conversion handled for non-point lights?
Sun lights, area lights, spot lights, what units should they be expressed in?

How is the conversion handled for non-point lights? Sun lights, area lights, spot lights, what units should they be expressed in?

Scurest commented

2021-08-30 08:43:11 +02:00

Point	candela (lm/steradian)
Spot	candela (lm/steradian)
Sun/Directional	lux (lm/m^2)
Area	doesn't exist in KHR_lights_punctual, the exporter just skips these

Don McCurdy commented

2022-10-13 05:02:09 +02:00

Added subscriber: @donmccurdy

Don McCurdy commented

2022-10-13 05:02:09 +02:00

Taking point lights as an example, I believe the conversion ought to go like this:

watts = blender_lamp.energy
LUMENS_PER_WATT = 683
PI = 3.14159
candela = watts * LUMENS_PER_WATT / (4 * PI)

For a 20W Point light, this produces 1087 cd. However, I've tested three different engines that all claim to do point lights in units of candela, and they're all consistently overexposed compared to the original Blender scene. In the test case I've got, 1cd produces a result much closer to the Blender original. But I don't have any way of knowing whether I'm just off by a constant factor here. I'm also a bit confused that the conversion table in the Blender documentation (see https://docs.blender.org/manual/en/dev/render/lights/light_object.html#power-of-lights) does not appear to use anything like the conversions above. It states that a candle is comparable to a 0.05 W point light in Blender. But the expression above yields .05 * 683 / (4*PI) = 2.71 cd. Any idea I'm missing here?

Taking point lights as an example, I believe the conversion ought to go like this: watts = blender_lamp.energy LUMENS_PER_WATT = 683 PI = 3.14159 candela = watts * LUMENS_PER_WATT / (4 * PI) For a 20W Point light, this produces 1087 cd. However, I've tested three different engines that all claim to do point lights in units of candela, and they're all consistently overexposed compared to the original Blender scene. In the test case I've got, 1cd produces a result much closer to the Blender original. But I don't have any way of knowing whether I'm just off by a constant factor here. I'm also a bit confused that the conversion table in the Blender documentation (see https://docs.blender.org/manual/en/dev/render/lights/light_object.html#power-of-lights) does not appear to use anything like the conversions above. It states that a candle is comparable to a 0.05 W point light in Blender. But the expression above yields .05 * 683 / (4*PI) = 2.71 cd. Any idea I'm missing here?

Will commented

2022-10-13 05:51:14 +02:00

Added subscriber: @WCN

Will commented

2022-10-13 05:51:14 +02:00

It states that a candle is comparable to a 0.05 W point light in Blender. But the expression above yields .05 * 683 / (4*PI) = 2.71 cd.

About this specifically, I think that's a red herring. "0.05" feels like the kind of number that's meant to give a ballpark estimate more than an exact definition. It might just be a big candle— Or a candle meant to show up better in renders that will inevitably be shown on low-dynamic-range monitors. Actually, if anything, I'd be inclined to interpret the fact that the conversion shifts the value by three (Base-10) orders of magnitude and still ends up at roughly one candela as a sign that the conversion ratio of 683 is appropriate for Blender.

May be worth pointing out too that the values in that table also all have vastly different conversion ratios from each other. The lowest ratio is 227, and the highest is 556. Even between lights of the same type there is significant variation. To me that feels like an artist at the Blender Institute/Foundation arrived at those exact values through trial and error and copied and pasted them from their personal notes as a rule-of-thumb guide when they were writing the documentation.

> It states that a candle is comparable to a 0.05 W point light in Blender. But the expression above yields .05 * 683 / (4*PI) = 2.71 cd. About this specifically, I think that's a red herring. "0.05" feels like the kind of number that's meant to give a ballpark estimate more than an exact definition. It might just be a big candle— Or a candle meant to show up better in renders that will inevitably be shown on low-dynamic-range monitors. Actually, if anything, I'd be inclined to interpret the fact that the conversion shifts the value by three (Base-10) orders of magnitude and still ends up at *roughly* one candela as a sign that the conversion ratio of 683 *is* appropriate for Blender. May be worth pointing out too that the values in that table also all have vastly different conversion ratios from each other. The lowest ratio is `227`, and the highest is `556`. Even between lights of the same type there is significant variation. To me that feels like an artist at the Blender Institute/Foundation arrived at those exact values through trial and error and copied and pasted them from their personal notes as a rule-of-thumb guide when they were writing the documentation.

Julien Duroure commented

2022-10-21 21:24:56 +02:00

Added subscriber: @JulienDuroure