[PD] more about float limitation (was: weird float/add limitation)

Cyrille Henry ch at chnry.net
Thu Jan 29 17:58:26 CET 2015


ok, claude was faster to answer, but since i already write my mail, i send it anyway...

pd internal resolution is float32.
(i.e, 23 bit, so a bit less than 17 millions, i.e more that 7 digit but less than 8 digits)
pd graphical representation is 6 digits

so, 4/3 =! 1.33333 but 4/3 == 1.33333333 (8 "3")
even if both are represented with the same number of 3...
this is a generic problem of computer float.

the only odd thing concerning pd is that number are also saved with 6 digit.
(so precision can be lost when a patch is saved)

try the attachment patch.
then save the patch, and open it back, and see that precision is lost.
(I have to modifies the patch as text file to have this behaviors, but you can also have the save precision when creating an object... until you save/load the patch)

you can also have a look on the top right of the patch: a weird effect of float precision...


Le 29/01/2015 17:17, Alexandre Torres Porres a écrit :
> Well, thanks everyone.
> And now for some related issues.
> Pd can only represent up to 6 significant digits, so they say. For example, in a message, you can have a number with up to 5 decimal places, like: -5.29314e+12
> but it does have a better internal resolution, if you compare 4 / 3 to 1.33333 you'll see 4 / 3 is higher ( try [expr 4./3 > 1.33333] and check).
> So, what's this internal resolution? And why can't you have the same resolution in a message?
> thanks
> 2015-01-28 16:06 GMT-02:00 Martin Peach <chakekatzil at gmail.com <mailto:chakekatzil at gmail.com>>:
>     On Wed, Jan 28, 2015 at 12:00 PM, Cyrille Henry <ch at chnry.net <mailto:ch at chnry.net>> wrote:
>         Le 28/01/2015 17:47, Alexandre Torres Porres a écrit :
>               > it's a limitation of 32 bit float
>             I thought so, but same happens when I use the new Pd Vanilla 64 bits...
>         this mean that it's compiled for 64 bit CPU, not that float are store on 64 bits
>     Also last time I checked, Pd saves floats by first printing them to 6 digit precision, so they have even less range than a 'float' type.
>     You could use an object made with pdlua to manipulate large floating-point numbers, as there is no(?) limit to the size of a float in lua.
>     Martin
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