Shouldn't that be (for SRC OVER DST blending): (256-a_source) * a_destination/ 256 * destination + a_source/256*source
where a_destination is the alpha of the destination pixel? Chris =========================== Chris Bore BORES Signal Processing www.bores.com -----Original Message----- From: directfb-users-boun...@directfb.org [mailto:directfb-users-boun...@directfb.org] On Behalf Of Jérémy Morel Sent: 21 October 2009 13:30 To: Niels Roest Cc: directfb-users@directfb.org Subject: Re: [directfb-users] Fade out effect The formula for blending is : (256-a_source) / 256 * destination + a_source/256*source where: * a_source is the alpha component of the source, * destination is one of the color component of the destination * source is the same color component, but for the source Let's consider for example the red component (it works with any component) Our source is 01ff ff ff, ie white with a minimal alpha component, and its red component is therefore 255 Our destination can be any colour, let's assume its red component is 50. each time I apply my filter, the color of the destination becomes : (256 - 1)/256 * destination + 1/256*255 This is an sequence defined by u_{n+1} = 255/256 * u_{n} + 255/256, which generic term is: u_n = (255/256)^n * (50 - 255) + 255 which will tend to 255. Therefore it should work. However, a simple verification shows that 1000 iterations will only get you to 251, which means that the residue shape is perfectly normal. I'm sorry for this mathematical post, but it explains the phenomena. Thanks for the blending formula, I had no idea how it was done. -- Jérémy _______________________________________________ directfb-users mailing list directfb-users@directfb.org http://mail.directfb.org/cgi-bin/mailman/listinfo/directfb-users _______________________________________________ directfb-users mailing list directfb-users@directfb.org http://mail.directfb.org/cgi-bin/mailman/listinfo/directfb-users
