packing greyscale values

[Diez B. Roggisch]

jimgardener wrote:

> hi
> i am getting value of a pixel of a greyscale image using PIL's
> image.getdata and need to pack it as an int as below
> 
> ie greyvalues
> 127 , 56 , 135 , 99 , 102 , 101 , 146 , 112 , 155 , 154 , 112 , 169
> 
> should become
> -4473925, -8289919, -4144960, -5789785, -5592406, -5658199 ,-3684409,
> -5131855, -3289651,-3355444, -5131855, -2763307
> 
> (i got these values using java's BufferedImage class..)I would like to
> know if there is a way to do the conversion in python.(.i need 'these
> 'for some calculations)

I have no idea how the bytes from PIL are possibly mapped to the wicked
numbers Java seems to generate (although there might be some systematics to
it). But judging from the fact that 

112->-5131855

appears twice, I presume as last resort you could create an explicit mapping
of 0..255 to the values Java generates. To get these, create an image, 256
pixels wide, one high, with grey-values from #000000 to #ffffff. Then build
up the mapping. Not elegant, but works.

Diez