Re: [Paracomp-spec] RFC : Extension EXT_datatype_float - floating point datatypes for compositing

[Shree K. <shr...@hp...>]

Hi Randy,

Randall Frank wrote:
> Hello,
>    I have to ask why not 16bit ints, perhaps in W space?

That's a good idea too. 16 bit unsigned ints would be good
enough for alpha blending. Using 16 bits for just the W component
seems like an improvement over using 16 bits for all components
- but I'm not sure how this will affect the readback speed
from the GPU.

I'm keen to hear what folks from TU Budapest think about this.

16 bit int/unsigned int could be a separate extension too.

> The 16bit float is necessary, I agree, but expensive to
> implement while the int form is relatively cheap.  I know
> HDR folks may not like the linear space, but for extended
> blending, it works pretty well and is fast.

Are you already using 16 bit ints ?

With the hardware coming out, the 16 bit floats could be fast
too - but with GPUs compositing the images. I don't have anything
to prove this though.

Thanks
-- Shree
> 
> Shree Kumar wrote:
>> Motivation :
>>
>> Paracomp currently has support for only 8 bit unsigned color components.
>> This is not enough in many cases where better precision is needed.
>> Floating point data types are increasingly being used in rendering
>> algorithms. So it is useful to extend the API to support higher
>> precision floating point data types.
>>
>> Floating point values with 16 bit & 32 bit precision are supported on
>> GPUs. 16 bit values supports a lower range of values, but are adequate
>> for a good number of applications. They take lesser bandwidth and
>> storage space, and could offer higher performance for the applications
>> which don't need the extra precision.
>>
>> Rough extension description :
>>
>> These new values will be supported by PC_PIXEL_FORMAT:
>>    1. PC_PF_RGBA32F_EXT - 4 component RGBA, with each being a single
>> precision value (following the IEEE 754 standard)
>>    2. PC_PF_RGB32F_EXT - 3 component RGB, with each being a single
>> precision value (following the IEEE 754 standard)
>>    3. PC_PF_RGBA16F_EXT - 4 component RGBA, with each a half precision
>> floating point value. The floating point format is the same as for the
>> OpenGL extension ARB_half_float_pixel. This floating-point format is
>> very similar to the IEEE single-precision floating-point standard,
>> except that it has only 5 exponent bits and 10 mantissa bits.
>>    4. PC_PF_RGB16F_EXT - 3 RGBA, each being a half precision floating
>> point value.
>>
>> RGBA values will be valid for both DEPTH & ALPHA_SORT compositing. (It
>> will also be valid for the ADD operator, once that is included as an
>> extension)
>>
>> RGB values will be valid for DEPTH compositing, but not for ALPA_SORT.
>> It may also make sense to support RGB values for the ADD operator with
>> PC_ALPHA_BEHAVIOUR set to PC_ALPHA_SEPARATE. Please refer to the
>> extension proposal for ADD operator in a separate email.
>>
>> Questions/Comments are welcome. 32 bit floating point values are
>> proposed by Technical University of Budapest. The 16 bit floating point
>> values are included by me.
>>
>> Note :
>>
>> An implementation of these pixel formats is already available for trying
>> out at :
>>
>> http://paracomp.svn.sourceforge.net/viewvc/paracomp/branches/developer/shree/feature/composite_float/
>>
>> samples/intermediate/multithreaded.cpp demonstrates using these
>> datatypes on a single node.
>>
>> It uses the Half class from the OpenEXR software distribution
>> (www.openexr.com) to composite 16 bit floating point values. These
>> values are not native types on the CPU, so compositing is quite slow.
>> Compositing on the GPU seems to be a better bet in this case.
>>
>> For 32 bit floating point, CPU compositing is used. No optimization has
>> been done, so expect things to be slow in this case too.
>>
>> Regards
>> -- Shree
>>
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> 
> --
> rjf.
> 
> -------------------------------------------------------------
> | Quando omni flunkus moritati                              |
> | Randall Frank                  ran...@co... |
> -------------------------------------------------------------
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