+static double find_aa_density(const RgbImage *ri, Point p, long background,
+ long foreground, int fg_extra) {
+ Rgb here= ri_rgb(ri, p.x, p.y);
+
+ double alpha[3], alpha_mean=0;
+ int i;
+ for (i=0; i<3; i++) {
+ unsigned char here_chan= here >> (i*8);
+ unsigned char bg_chan= background >> (i*8);
+ unsigned char fg_chan= foreground >> (i*8);
+ double alpha_chan=
+ ((double)here_chan - (double)bg_chan) /
+ ((fg_chan + fg_extra) - (double)bg_chan);
+ alpha[i]= alpha_chan;
+ alpha_mean += alpha_chan * (1/3.0);
+ }
+
+ double thresh= 1.5/AAMAXVAL;
+ double alpha_min= alpha_mean - thresh;
+ double alpha_max= alpha_mean + thresh;
+ for (i=0; i<3; i++)
+ MUST( alpha_min <= alpha[i] && alpha[i] <= alpha_max,
+ MRGB(here);MRGB(background);MRGB(foreground);MI(fg_extra);
+ MF(alpha_min); MI(i);MF(alpha[i]);MF(alpha_max) );
+
+ MUST( 0 <= alpha_mean &&
+ (fg_extra ? alpha_mean < 0.999 : alpha_mean >= 1.0),
+ MRGB(here);MRGB(background);MRGB(foreground);MI(fg_extra);
+ MF(alpha_mean); MF(alpha[0]);MF(alpha[1]);MF(alpha[2]); );
+
+ return alpha_mean;
+}
+