On 06/12/2013 01:00 PM, Bruce Perens wrote:
> I didn't immediately find a full-featured half float library in OpenEXR,
> but there seems to be one at http://half.sourceforge.net/
On Wed, Jun 12, 2013 at 3:09 PM, Bruce Perens <[email protected]> wrote:
> Under the hood, this is implemented with single-precision float. I would
> expect functions for OpenEXR to be done the same. I'd expect it to offer
> more consistency for testing, but it might still have some issues.
>
Good find at Sourceforge. I haven't dug into either of them deeply enough
to compare the internals (I did compile the OpenEXR version as a library
for Blackfin). But judging by the description of OpenEXR, I think it is
truly 16-bit internally. http://www.openexr.com/about.html says, "ILM
decided to develop a new HDR file format with 16-bit floating-point color
component values. Since the IEEE-754 floating-point specification does not
define a 16-bit format, ILM created the "half" format. Half values have 1
sign bit, 5 exponent bits, and 10 mantissa bits...The half format supports
denormalized numbers, positive and negative infinities, and NaNs. It is
identical to the half data type in NVIDIA's Cg graphics language, allowing
a developer to process values from an OpenEXR image directly on current
NVIDIA GPUs such as the GeForce FX family."
I think the half class is part of IlmBase which can be downloaded here:
http://www.openexr.com/downloads.html. Bruce, I will email you the rest of
my notes about it; they are are kind of large to attach here. If anyone
else is interested in them, just email me directly.
Note that I wasn't suggesting that a 16-bit float library would necessarily
be the right tool to use for verifying codec2; I was thinking that a very
high precision software float library would probably be most appropriate
for that, as it would tend to give the same output even if the order of
operations (and any resulting rounding/truncation) changed. I haven't
looked for such a high-precision library.
Steve
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