Le 23/02/2012 15:59, Warp nous fit lire :
> crowbait <cro### [at] mailgoonejp> wrote:
>> This is screenshot of render window.
>> http://blogimg.goo.ne.jp/user_image/69/3c/83e31ff151644d9a6a6c3f89583d54e9.png
> 
>> And this is the output file.
>> http://blogimg.goo.ne.jp/user_image/57/ce/dcf2d63a98a4e0f8fd0c682d3d90b290.png
> 
>   I see no difference.
> 

Internal data from png shows:
Render image is saved as Grey PNG, with a palette of 32 entries, with a
depth of 8 bits per channel (3: rgb).
Rendered on Windows with 3.7RC4;

I do not know the windows specific code, but the general code of povray
would push for 16 bits per channel in case of Gray output. That does not
seems to be the case in your render. (hint: try it as option from
command-line box ?)

Note on palette: it's compact (excepted for black, the 31 other entries
cover 26 to 56 values.
What you might be observing is a side effect of computation: The png
writing get the float value (0 to 1 usual range) into an integer (0 to
255 for 8 bits) via a gamma curve, whereas the preview/display get the
pixel colour probably in a different way (from the same float anyway).

You might want to (either or both):
 * Use deeper PNG (+FN16 is the max)
 * experiment with dithering (+TH or +THfs or +THB2 or +THB3 or +THB4 or
+THD1 or +THD2 )
 * try the wrath of gods by playing with FILE_GAMMA setting.

From your picture view, the png file you got is correct and cannot
perform better due to the compact values used with +FN8 and the gamma
you specified for file (or its default of 2.2);

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