Second Life Forums Archive - floating point errors?

Eep Quirk

Absolutely Relative

Join date: 15 Dec 2004

Posts: 1,211

03-08-2006 13:14

Why doesn't this increase?

CODE

float num;
default
{
    state_entry()
    {
        num = 99.0;
        while(1)
        {
            num = num + 0.000001;
            llSetText((string)num,<1,1,1>,1);
        }
    }
}

And why DOES this increase?

CODE

float num;
default
{
    state_entry()
    {
        num = 9.0;
        while(1)
        {
            num = num + 0.000001;
            llSetText((string)num,<1,1,1>,1);
        }
    }
}

Masakazu Kojima

ケロ

Join date: 23 Apr 2004

Posts: 232

03-08-2006 13:21

http://secondlife.com/badgeo/wakka.php?wakka=float

From: someone

A 32-bit float is divided into a 24-bit signed "mantissa" ... A 23-bit (1 bit lost for sign) mantissa gives a precision equivalent to approximately 7 decimal digits ... addition or subtraction of two numbers of vastly different magnitudes might yield unexpected results, as the mantissa can't hold all the significant digits.

Eep Quirk

Absolutely Relative

Join date: 15 Dec 2004

Posts: 1,211

03-08-2006 14:31

Ah, so THAT'S what that means. OK, so, then, I guess the next obvious question is when will LSL be getting 64-bit, or whatever, floats?

Seifert Surface

Mathematician

Join date: 14 Jun 2005

Posts: 912

03-08-2006 14:45

From: Eep Quirk

Ah, so THAT'S what that means. OK, so, then, I guess the next obvious question is when will LSL be getting 64-bit, or whatever, floats?

"Double"s I believe.

I doubt it's really worth it generally. Introducing doubles would double the storage and bandwidth associated to any kind of vector, which is a large chunk of the data being shuffled around. It would also make calculations slower.

With sims the size that they are, I don't really see much of a need for the precision given by doubles. If you're talking about a position vector, you need to be up very very high to start seeing distortions (those photos of people looking unwell at some millions of meters altitude). If sims at some point get very very large (say your sim is planet sized) then it starts making sense to be using doubles for positions.

_____________________

-Seifert Surface
2G!tGLf 2nLt9cG

floating point errors?
Eep Quirk Absolutely Relative Join date: 15 Dec 2004 Posts: 1,211	03-08-2006 13:14 Why doesn't this increase? CODE float num; default { state_entry() { num = 99.0; while(1) { num = num + 0.000001; llSetText((string)num,<1,1,1>,1); } } } And why DOES this increase? CODE float num; default { state_entry() { num = 9.0; while(1) { num = num + 0.000001; llSetText((string)num,<1,1,1>,1); } } }
Masakazu Kojima ケロ Join date: 23 Apr 2004 Posts: 232	03-08-2006 13:21 http://secondlife.com/badgeo/wakka.php?wakka=float From: someone A 32-bit float is divided into a 24-bit signed "mantissa" ... A 23-bit (1 bit lost for sign) mantissa gives a precision equivalent to approximately 7 decimal digits ... addition or subtraction of two numbers of vastly different magnitudes might yield unexpected results, as the mantissa can't hold all the significant digits.
Eep Quirk Absolutely Relative Join date: 15 Dec 2004 Posts: 1,211	03-08-2006 14:31 Ah, so THAT'S what that means. OK, so, then, I guess the next obvious question is when will LSL be getting 64-bit, or whatever, floats?
Seifert Surface Mathematician Join date: 14 Jun 2005 Posts: 912	03-08-2006 14:45 From: Eep Quirk Ah, so THAT'S what that means. OK, so, then, I guess the next obvious question is when will LSL be getting 64-bit, or whatever, floats? "Double"s I believe. I doubt it's really worth it generally. Introducing doubles would double the storage and bandwidth associated to any kind of vector, which is a large chunk of the data being shuffled around. It would also make calculations slower. With sims the size that they are, I don't really see much of a need for the precision given by doubles. If you're talking about a position vector, you need to be up very very high to start seeing distortions (those photos of people looking unwell at some millions of meters altitude). If sims at some point get very very large (say your sim is planet sized) then it starts making sense to be using doubles for positions. _____________________ -Seifert Surface 2G!tGLf 2nLt9cG

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floating point errors?