Re: BGRA, ARGB, etc. suggestion

On Mon, Dec 21, 2015 at 11:18 AM, John Bowler <
[email protected]> wrote:


> [...] gamma of 36 is a reasonable value for original
> perceptual data stored with ridiculous accuracy and dynamic range.
>
> It's probably worth adding that the obvious encoding of perceptual
> brightness using a log scale, as suggested by at least one expert many
> years ago and as partially implemented in SLog encodings, requires
> only 12 bits.
>

Ridiculous accuracy and dynamic range can be achieved with the
pCAL chunk.

Quoting from the pCAL spec in the png extensions document:

    Using equation type 3, floating-point data can be reduced (with loss)
   to a set of integer samples such that the resolution of the stored data
   is roughly proportional to its magnitude. For example, floating-point
   data ranging from -10^31 to 10^31 (the usual range of 32-bit
   floating-point numbers) can be represented with:
   Equation type 3
   x0 = 0
   x1 = 65535
   p0 = 0.0
   p1 = 1.0e-30
   p2 = 280.0
   p3 = 32767.0

   The resolution near zero is about 10^-33, while the resolution near
   10^31 or -10^31 is about 10^28. Everywhere the resolution is about 0.4
   percent of the magnitude.

   Note that those floating-point parameters could be stored in the chunk
   more compactly as follows:
   p0 = 0
   p1 = 1e-30
   p2 = 280
   p3 = 32767

Glenn

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