#27 16bit Grayscale PNG on MAC OSX looks wrong.

[David Burnett]

I've written some software that produces 16bit
grayscale PNG files. However when I load them into
filmgimp they look wrong. At first I suspected my
software however the data going into the compression
routines looked correct so I downloaded the PNG test
suite at

http://www.schaik.com/pngsuite/pngsuite.html

and loaded the 16bit grayscale test image basn4a16.png
and the outcome was similar to my PNG's.

I've attached a screenshot below, the top square shows
the reference GIF from the test suite, the bottom is
FilmGimps view of the test PNG.

After a small play with the source of my program it
looks like FilmGimp is performing a byte reversal
somewhere. If a create a 32767 grey pixel as 0x7fff I
get a white pixel, if I swap the bytes around to 0xff7f
I get mid grey. PNG data is network endian.

[Sam Richards]

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Just to confirm that this worked fine on intel boxes. Could
somebody on a Mac or SGI take a look at it?

[Sam Richards]

Logged In: YES
user_id=211347

Just to confirm that this worked fine on intel boxes. Could
somebody on a Mac or SGI take a look at it?

[Sam Richards]

Logged In: YES
user_id=211347

Just to confirm that this worked fine on intel boxes. Could
somebody on a Mac or SGI take a look at it?

[David Burnett]

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I'm not an expert in GIMP internals but shouldn't

if(info->bit_depth == 16)
png_set_swap(pp);

in filmgimp/plug-ins/png/png.c

have an endian test for those platforms that like their
data big-endian style.

[Sam Richards]

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Applying the suggested fix, somebody will have to test the
upcoming 0.16 release to see if its fixed.

[David Burnett]

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I've just compiled fresh from CVS on Mac OSX and it works
just fine on the PNG suite and my own PNGs.

Good Stuff.