convert images

Marc 'BlackJack' Rintsch bj_666 at gmx.net
Sun Apr 27 18:36:22 CEST 2008


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



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