Re: flattening: two ways, two different results

Am 31.08.23 um 15:17 schrieb Bob Friesenhahn:
> On Thu, 31 Aug 2023, frechefuchs via Graphicsmagick-help wrote:
>>
>>> gm convert logo: PNG32:- | gm convert - -operator matte assign 50% -verbose info:-
>>> /tmp/gmx7zyAz PNG 654x418+0+0 DirectClass 8-bit 52.2Ki 0.010u 0m:0.046921s (5.6Mi pixels/s)
>>> Image: /tmp/gmx7zyAz
>>>   Format: PNG (Portable Network Graphics)
>>>   Geometry: 654x418
>>>   Class: DirectClass
>>>   Type: true color with transparency
>>>   Depth: 9 bits-per-pixel component        <== here
>>>   Channel Depths:
>>>     Red:      8 bits
>>>     Green:    8 bits
>>>     Blue:     8 bits
>>>     Opacity:  9 bits                       <== here
>>
>> Shouldn't that be 8 bits?
> 
> A Q16 build could produce a number which does not fit perfectly in 8 
> bits. 32768 does not scale down perfectly to 8 bits.  That said, I 
> have suspected for a while that my depth estimation algorithm needs 
> improvement.

Ah, makes sense!  What I thought was that the 1 bit transparency from
the 'logo: PNG32:-' output somehow got mixed up with the new full 8 bit
transparency channel depth during conversion.  Changing the argument to
'assign' to different values I can now see random transparency channel
bit depths.  Sorry for the noise!


> If you are going to pipe PNG to PNG then using this syntax will likely 
> help.  At least with modern GM, it avoids using a temporary file:
> 
>    gm convert logo: PNG32:- | gm convert PNG:- -operator ...

Nice.  Though, thanks to the -type TrueColorMatte option, in my use-case
I don't need to pipe PNG to PNG any more.  The 'logo: PNG24:-' was just
used here to simulate my original 24 bit PNG input to GM.

But while we're at it, while converting logo: to PNG24: and PNG32:
works, the conversion fails for PNG48: (and PNG64:).

> gm convert logo: PNG48:- | gm convert - info:-
> gm convert: No encode delegate for this image format (PNG48) [No such file or directory].
> gm convert: Unable to open file (/tmp/gmI3cJGb) [No such file or directory].

Is that to be expected once again?

Thanks!
fux

> gm version
> GraphicsMagick 1.3.35 2020-02-23 Q16 http://www.GraphicsMagick.org/