Page 1 of 1
GLSL Math
Posted: Fri Aug 30, 2013 4:13 pm
by Ref
Could someone provide GLSL logic?
Why doesn't:
equal zero?
see line 17 of grid.lua shader.
Re: GLSL Math
Posted: Fri Aug 30, 2013 4:29 pm
by raidho36
Simple: foating point math has some degree of error, and it tends to accumulate along the computations.
Re: GLSL Math
Posted: Sat Aug 31, 2013 12:48 pm
by RedHot
Raidho is wrong, as usual.
On the serious note. Your function signature is :
Code: Select all
int mod(int a, int b) { return a - ((a / b) * b); }
GLSL acts like the C language.
In C,C++, Java (that's the languages I have observed with this behaviour) an int divided by int ALWAYS return an integer.
4/3 = 1 (not 1.(3) ). That has to do with type casting. If both arguments are integers none gets promoted to a float or double, hence the return value is an integer aswell.
Re: GLSL Math
Posted: Sat Aug 31, 2013 1:08 pm
by raidho36
Previous statement wasn't wrong, just not exactly related. When people are talking about GLSL, it is assumed that any numbers are floats, so did I.
It is true that integer division always results in integer result rounded down. You can cast values at any time by prefixing them with desired cast type (or use function-like syntax):
Code: Select all
float myfloat = (float)myint
float myfloat = float(myint)
float mod ( int a, int b ) { return (float)a - ( ( (float)a / (float)b ) * (float)b ); }
Last line is what you should probably use. You can omit some of the explicit typecasts, but that only would result in implicit typecast. Implicit things may not work the way you expect it, and it's side-effects could be evil, so make sure you avoid it.
Re: GLSL Math
Posted: Sat Aug 31, 2013 1:13 pm
by Nixola
Also, if I remember correctly some video cards just hate implicit casts and won't compile any shader that uses them
Re: GLSL Math
Posted: Sat Aug 31, 2013 1:21 pm
by vrld
Even more unrelated, GLSL already defines a modulo function,
see here.
Re: GLSL Math
Posted: Sat Aug 31, 2013 1:26 pm
by RedHot
Nixola wrote:Also, if I remember correctly some video cards just hate implicit casts and won't compile any shader that uses them
Quite the opposite. A lot of ATI drivers require implcit casting as they are often reluctant to cast on their own.
@Vrld
Just my thought
Re: GLSL Math
Posted: Sat Aug 31, 2013 1:46 pm
by Robin
RedHot wrote:A lot of ATI drivers require implcit casting as they are often reluctant to cast on their own.
Aren't you confusing "implicit" with "explicit" there?
Re: GLSL Math
Posted: Sat Aug 31, 2013 2:01 pm
by RedHot
Robin wrote:RedHot wrote:A lot of ATI drivers require implcit casting as they are often reluctant to cast on their own.
Aren't you confusing "implicit" with "explicit" there?
I most certainly am! My bad. Robin is of course right. Dunno why I confused the two.
Re: GLSL Math
Posted: Sat Aug 31, 2013 10:12 pm
by Ref
Thanks all for your responses.
GLSL still is a black art for me.
Any and all help greatly appreciated.
[quote="raidho36"]
Code: Select all
float mod ( int a, int b ) { return (float)a - ( ( (float)a / (float)b ) * (float)b ); }
quote]
Added this to grid.lua and got:
ERROR: 0:23: ')' : syntax error parse error
Further playing around and found that I didn't need any of this code to run the script.
Don't know what the original intent was. Just part of the magic I guess.