convert images

[Marc 'BlackJack' Rintsch]

On Sun, 27 Apr 2008 06:42:13 -0700, wilson wrote:


> i converted some P5 type .pgm images to .jpg using
> […]
> ie if oldimage.pgm has pixels
> [29 31 38 ..., 10  4 18]
> then the corresponding jpg image has
> [29 31 38 ..., 10  3 17]
> 
> why this difference? shouldn't they be identical?can someone pls
> explain this?

JPEG uses a lossy compression algorithm, i.e. the image loses quality and
therefore the pixel values are not exactly the same as before saving.  If
you want get the exact values back you have to use another image format. 
For RGB or RGBA data PNG is a good format.

Ciao,
	Marc 'BlackJack' Rintsch