Re: [png-mng-misc] pHYs chunk is 16 [was PNG is 16]

[Glenn Randers-P. <gl...@gm...>]

On Sat, Mar 12, 2011 at 3:35 PM, Glenn Randers-Pehrson
<gl...@gm...> wrote:

>
> One of the unresolved bugs in ImageMagick is
>   1. start with a TIF that defines 300dpi
>   2. convert it to PNG.  Now it has 11811 pixels/meter
>       So far, so good.
>   3. Convert the PNG to another PNG.
>       Somehow it remembered the orginal figure 300
>       in a text chunk containing Exif data or 8bim
>       data or something, but it did not remember
>       the new units.
>       So now it has 30000 pixels/meter.  Oops.
>   4. Convert the second PNG to a third PNG.
>       It still has 300000 pixels/meter, so I guess

                          30,000

>       it could be worse.

Incidentally GraphicsMagick is also wrong
but differently and only insignificantly but
a bit weirdly.

The TIF has 300dpi
The first PNG has 11811 pixels/meter
The second PNG has 11810 pixels/meter
The third PNG has 11809 pixels/meter.

As far as I can tell the error stops increasing after that
(I only ran a couple more).