[Public WebGL] gl.sizeInBytes

Vladimir Vukicevic [email protected]
Mon Jan 11 12:31:34 PST 2010


On 1/11/2010 11:48 AM, Kenneth Russell wrote:
> On Mon, Jan 11, 2010 at 10:37 AM, Chris Marrin <[email protected] 
> <mailto:[email protected]>> wrote:
>
>
>     On Jan 10, 2010, at 11:44 PM, Patrick Baggett wrote:
>
>     > It is hardly a matter of "does a GL implementation have 64-bit
>     GL_FLOATs", but more of "the WebGL spec explicitly states the size
>     of its types." -- the latter entirely shutting off the concept of
>     "implementation dependent" sizes.
>     >
>     > Vlad's right, even if gl.sizeInBytes(GL_FLOAT) did return 8
>     (double prec.) there would be no way to efficiently/portably
>     buffer the data.
>
>
>     So then maybe it would be better to replace these with constants
>     (ctx.FLOAT_SIZE, ctx.INT_SIZE, ctx.UNSIGNED_SHORT_SIZE), etc.?
>
>
> I think we should leave sizeInBytes as a function rather than defining 
> constants. On a hypothetical platform which defined GLfloat as a 
> double, the WebGL implementation would be responsible for making 
> WebGLFloatArray manage double-precision rather than single-precision 
> floating point numbers.

Hmm.. I don't think it should do this, see below.

> As we consider proposing broader use of these array-like types, we 
> will have to specify the exact size of the machine types they manage. 
> However, the mapping between e.g. WebGLFloatArray to e.g. FloatArray 
> vs. DoubleArray would need to be flexible.

We already have the exact size of the machine types specified for the 
WebGL Arrays; I think that this needs to remain the case, because 
otherwise we have the problem that people will just assume 4 bytes 
anyway because it's currently the probably-100% case and the world 
breaks if there is an 8-byte "GL_FLOAT" platform.  Otherwise, people 
have to use sizeInBytes constantly to get correct portable behaviour, 
and we've tried pretty hard to avoid requirements like that (e.g 
UniformIndex and friends)..

     - Vlad

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