Re: Interaction of pCAL and gAMA

On Thu, Jan 21, 2016 at 6:43 PM, John Bowler <
[email protected]> wrote:

> On Thu, Jan 21, 2016 at 3:18 PM, Glenn Randers-Pehrson <[email protected]>
> wrote:
>
>> The PNG extensions document, which specifies the pCAL chunk, does not
>> address the interaction of pCAL and gAMA.  Since the purpose of the pCAL
>> chunk is to relate PNG data to physical values, I think that gamma==1
>> should
>> be assumed, if there's no gAMA chunk.  I'd say that if both are present,
>> it
>> means that the pCAL conversion should be made, then when displaying
>> that, assume that whatever the gAMA chunk says should be used with
>> the already-converted samples.  But since the relationship isn't
>> specified,
>> most anything could happen, for example, a viewer could only recognize
>> the gAMA chunk and ignore the pCAL chunk, and the display would not
>> likely be what the author intended.
>>
>
> Surely that's the wrong way round.
>

Yes, I got it backwards.

Here's an interpretation that maybe makes more sense:  Ignore gAMA and use
pCAL
to recover the physical data.  Ignore pCAL and use gAMA to view the image,
but
don't expect the image to be pretty.  If there's pCAL but no gAMA, assume
sRGB
when displaying the image.  One could use pCAL to construct a legend to
display
along with the image.

Glenn

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