android - Filter alpha with ColorMatrixColorFilter -

i want filter color colormatrixcolorfilter. have code working but... doesn't work alpha.

static private colormatrixcolorfilter getfilter(int backgroundcolor, int foregroundcolor) {     float rb = (float) (color.red(backgroundcolor)) / 255;     float gb = (float) (color.green(backgroundcolor)) / 255;     float bb = (float) (color.blue(backgroundcolor)) / 255;      float rf = (float) (color.red(foregroundcolor)) / 255;     float gf = (float) (color.green(foregroundcolor)) / 255;     float bf = (float) (color.blue(foregroundcolor)) / 255;      float[] colortransform = { rb - rf, 0, 0, 0, rf * 255, 0, gb - gf, 0, 0, gf * 255, 0, 0, bb - bf, 0, bf * 255, 0, 0, 0, 1, 0 };      colormatrix colormatrix = new colormatrix(colortransform);      return new colormatrixcolorfilter(colormatrix);  } 

how can alpha ? because here, alpha "1".

thanks.