packing greyscale values

[John Machin]

On Feb 11, 9:11 pm, Peter Otten <__pete... at web.de> wrote:
> 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)
>
> Judging from
>
> >>> hex(0x100000000 - 4473925)
> '0xffbbbbbb'
> >>> 0xbb
>
> 187
>
> -4273925 indeed seems to denote a gray value, but the lighter 187 rather
> than 127. Are you sure the lists you give belong to the same image?
>
> Peter

Let Y = Java-derived numbers (187, etc)
and X = PIL-derived numbers (127, etc)
Y approx = 0.75 * X + 90
What the underlying cause of this relationship is, I have not a clue.

Cheers,
John