Re: [Gimp-developer] GIMP's future internal image data format

[Martin Renold <martinxyz gmx ch>]

On Tue, Jan 31, 2012 at 06:25:14PM +0100, Bogdan Szczurek wrote:
> W dniu 12-01-31 15:48, gespertino gmail com pisze:
> >Here I have to quote my friend Troy Sobotka who's usually a jerk towards
> >GIMP :-p but makes a point:
> >http://troy-sobotka.blogspot.com/2011/01/bit-depth-and-confusion.html
> 
> As much as I agree with general idea I want to point out two things
> about image comparing 8 bit dithered and 15 bit outputs (the square
> regions). 

My point was to show the dithering noise that I got rid of by switching from
8bpc to 15bpc.  It wasn't really meant for scientific analysis ;-)

> First: they're not of the same dynamic range. The upper one is "brighter",
> "wider"

Well, I just opened both MyPaint versions and painted roughly the same
(large) number of circles with a faint brush.  The right box is a
magnification of the left box (which is a section of the full circle).

> and "not grayscale" like the bottom one.  It makes its "ugliness" more
> apparent.

Both images were made with a pure black brush painting over pure white.  The
color that you see comes from the dithering.  I used a different random
number to dither each color channel.  This actually improves the perceived
smoothness of the result.

> Second: why to use dithering anyway since internally you're using sRGB,
> monitor "should be" at least sRGB?… Do you have any other examples maybe?

Dithering was done per dab. When you stamp a dab, 8bpc over 8bpc, you get a
16bpc intermediate result.  When scaling this back to 8bpc I used dithering
instead of simple rounding.  If I had done rounding, I would have gotten
banding instead of noise.  Same problem, different manifestation.

-- 
Martin Renold