 # SSS Scale factor

Hey there. Got a question about the “Scale” factor in the SSS settings. This is what I read in the Blender manual:

If you want to work out what scale value to use in your scene, just use the formula: (size in blender units)/(real world size in millimetres)= scale

What I am wondering is this: What is the “size” of a model in blender units? What is the “size” of a model in real life? Objects both in Blender and in the real world have three dimensions of size which can’t be expressed as a single number (“Scale” value).

How do I determine the “size” of my models?

Thanks. –Robo

Hi,

Ben Simonds has a good explanation of sss size:

Scale: This is the big one. This is really important to get right as it will determine how far light scatters through the skin, and is the difference between looking like wax and getting nice skin. A scale of 1.0 means that one blender unit equals one millimeter – this is obviously way too large for most scenes. The average head is around 20 to 25cm tall, which would mean that if you scaled your head to exactly one blender unit high, that a value of 0.004 would give correct results. In general you may want to try a bit of tweaking to get the right results. Adjust accordingly for the size of your model according to this formula: size in blender units/real world size in millimeters = scale value.

In this tut, Ben mentions height, one dimension of size. What about the others?

If I divide the height of the Blender object by real world height, I will get one number. If I divide the width of the Blender object by real world width, I’ll get another. But there must only be one number that is the “correct” value for the Scale factor.

What am I missing?

Then the Blender object is a distorted model of the real world one. You can make one Blender unit be whatever you want, but you should make the same choice in all three dimensions: don’t measure height in feet and width in metres!

But there must only be one number that is the “correct” value for the Scale factor.

For a given choice of Blender unit, there is one correct SSS scale factor. The most common choice is 1BU = 1 metre, for which the correct SSS scale is 0.001. More generally, the correct SSS scale is 1 mm converted to BUs. So if you are using 1BU = 1 foot, then the SSS scale should be about 0.0033; for 1 BU = 1 cm it should be 0.1.

Best wishes,
Matthew

I usually just mess with the size slider until it looks good for my purposes… 1 Like