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Re: [Public WebGL] comparing typed arrays



On Wed, Jun 6, 2012 at 2:42 PM, Brandon Jones <tojiro@gmail.com> wrote:
I'd like to point out that while I think a memcmp on typed arrays would be a really good idea, that may not accurately cover the case that you're describing Fabrice (comparing matrices). Given the quirks of floating point calculations if the matrices you were comparing had been calculated mathematically rather than copied directly from a static source you could easily introduce minor variations into the matrix elements that would cause a memcmp to fail. A more realistic (though unfortunately slower) usage would be to compare each value with a small epsilon to account for floating point jitter.

Of course, that entirely depends on your use case. I'm just saying that while I fully support fast TypeArray comparisons I don't think it should be advocated for comparing vectors/matrices in general.

I agree with Brandon that you shouldn't use memcmp to compare floats, even for _exact_ comparisons (true in _javascript_ as it is elsewhere). IEEE754 includes a signed zero, that compare equal but have different bit representations.

In general, you want some kind of slop for floating point comparisons, but element-by-element epsilons for transformation matrices might be nonsensical. Better to do comparisons in a domain where transformations are easily manipulated. Things like offset vectors or quaternions would use absolute epsilons, scaling factors would use relative epsilons, etc.

-Won
 

--Brandon 


On Wed, Jun 6, 2012 at 11:15 AM, Fabrice Robinet <cmg473@motorola.com> wrote:
Hi Florian, 

On Wed, Jun 6, 2012 at 1:25 AM, Florian Bösch <pyalot@gmail.com> wrote:
On Wed, Jun 6, 2012 at 8:18 AM, Fabrice Robinet <cmg473@motorola.com> wrote:
Looking at the typed array spec I did not find a way to compare (efficiently) 2 arrays, 
Is that something that was considered and discarded for some reason ? Or did it just not made it into the spec yet ?
I don't know the reason why it's not in the spec. I would think it'd be useful  if the comparison operator between typed arrays/views would make a range check and if matching performed a memcmp.

Indeed, specifying a range for the compare and underneath relying on memcmp is *exactly* what I believe is missing.

I'll follow up by filing an issue about this.

Thanks,
Fabrice.