[Mesa-dev] SNORM conversion equation woes

[Chris Forbes]

What are other implementations doing?

IIRC, GL3-class NV hardware passes the current piglit tests.

On Sat, Jul 27, 2013 at 9:20 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> Hello all,
>
> I've been looking at https://bugs.freedesktop.org/show_bug.cgi?id=59150 and
> could use your opinion.
>
> OpenGL defines two equations for converting from SNORM to floating point
> data.  These are:
>
> f = (2c + 1)/(2^b - 1)                (equation 2.2)
> f = max{c/2^(b-1) - 1), -1.0}         (equation 2.3)
>
> Equation 2.2 cannot represent 0.0f, while equation 2.3 can.
> Equation 2.3 has two representations for -1.
>
> The spec language for which to use is a bit wishy-washy...GL 3.2:
> "In general, [equation 2.2] is used for signed normalized fixed-point
>  parameters in GL commands, such as vertex attribute values."
> "In general, [equation 2.3] is used for signed normalized fixed-point
>  texture or floating point values."
>
> ARB_vertex_type_2_10_10_10_rev clearly specifies equation 2.2.
>
> *However*, OpenGL 4.2 and ES 3.0 both remove equation 2.2, instead saying to
> use equation 2.3 in all cases.
>
> Intel hardware always uses equation 2.3, which provides a dilemma: should we
> just use the native hardware rules, which are fast, or do equation 2.2 via
> shader workarounds?
>
> The maximum error/difference between the two equations is:
>
> 16-bit -> 0.00003
> 10-bit -> 0.00195
>  8-bit -> 0.00784
>  2-bit -> 0.66666
>
> (Data and a script from Keith are available from:)
>  http://people.freedesktop.org/~kwg/snorm/
>
> Furthermore, since GL 4.2 is supposed to be backwards compatible,
> implementations can silently promote contexts to 4.2, likely giving them the
> new SNORM rules.  Which means applications can't really rely on the exact
> equation used...
>
> Always using 2.3 seems preferable since it can actually represent 0.
>
> I am tempted to rip out the code in vbo_attrib_tmp.h that implements
> equation 2.2 and always use 2.3.  That would also require changing Piglit's
> draw-vertices-2101010 test to accept either equation.
>
> What do you all think?
>
> Thanks for your time!
>   --Ken
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