Search in
Sort by:

Question Status:

Search help

  • Simple searches use one or more words. Separate the words with spaces (cat dog) to search cat,dog or both. Separate the words with plus signs (cat +dog) to search for items that may contain cat but must contain dog.
  • You can further refine your search on the search results page, where you can search by keywords, author, topic. These can be combined with each other. Examples
    • cat dog --matches anything with cat,dog or both
    • cat +dog --searches for cat +dog where dog is a mandatory term
    • cat -dog -- searches for cat excluding any result containing dog
    • [cats] —will restrict your search to results with topic named "cats"
    • [cats] [dogs] —will restrict your search to results with both topics, "cats", and "dogs"

Office Holiday

Epic Games' offices will be on holiday from June 22nd to July 7th. During this period support will be limited. Our offices will reopen on Monday, July 8th. 

What does "Correction for lumen units" mean?

When Use Inverse Squared Falloff is turned on, Intensity of point/spot light seems to be internally multiplied by 16.0.

 // Correction for lumen units
 DeferredLightUniformsValue.LightColor *= 16.0f;

What is the meaning of this 16.0f ?

This value results in white diffuse material lit by 1963 lm point light with distance of 100 cm returns color 1.0.

 1963 * 16 / 100^2 / PI  = 1

In other words, white diffuse material lit by 156 lx returns color 1.0.

 1963 / (4 * PI) = 156

Is this a magic number without any physical meaning? If so, why this value is chosen?

Product Version: UE 4.9
more ▼

asked Oct 29 '15 at 11:12 AM in Rendering

avatar image

1 1 2 2

(comments are locked)
10|2000 characters needed characters left
Viewable by all users

2 answers: sort voted first

f at the end means number type is "float", not "double" which is default fraction number type when you use ".x" (x being faction number) at the end

Other then that i guess it's to correct units used in rendering libraries to be closer in resemblance of real world unit of lumen

more ▼

answered Oct 29 '15 at 05:05 PM

avatar image

36.6k 931 169 1111

avatar image tenten Nov 01 '15 at 01:57 AM

Hi Shadowriver,

Yes, I also think 16 is to be closer in resemblance but I don't have any idea how this value is derived. Is this based on knowledge of camera or something?

(comments are locked)
10|2000 characters needed characters left
Viewable by all users

I know this is a somewhat old thread, but I would like to inform you about the rationale behind this x16 factor, as I just got a reply from Epic on this regard.

Basically, it is a compensation factor to make things look bright enough at default exposure settings.
In the case of point light falloff contribution, the expected 1/(4pi) distribution factor has been omitted.

For the interested reader, this is the question: https://udn.unrealengine.com/questions/280050/clearing-confusion-with-photometric-measurements.html

more ▼

answered Mar 11 '16 at 01:21 AM

avatar image


(comments are locked)
10|2000 characters needed characters left
Viewable by all users
Your answer
toggle preview:

Up to 5 attachments (including images) can be used with a maximum of 5.2 MB each and 5.2 MB total.

Follow this question

Once you sign in you will be able to subscribe for any updates here

Answers to this question