trasparent image width antialiasing

I have a problem, Excused but I do not speak English


I have in input one image png with antialising
with uniform solid background

I would want

1) to eliminate the background
2) recreate the channel alpha in order not to lose the antialising on the 
text

It is possible?
you can give me of the information ?

I try this code but not work:

//
//load a, load b  and create test
//
a = myLoadPng("a.png");
b = myLoadPng("b.png");
test = gdImageCreateTrueColor(400,400);

//
//leave background
//
gdImageColorTransparent(a, 0xA33333);   //trasp for background image a
gdImageColorTransparent(b, 0xDCE322); //trasp for background image b
gdImageColorTransparent(test, 0x000000); //trasp for truecolor image


//
//merge two image with text antialising
//
gdImageAlphaBlending(test, true);
gdImageCopyMerge(test, a, 0, 0, 0, 0,400, 400, 100);
gdImageCopyMerge(test, b, 0, 0, 0, 0, 400, 400, 100);

gdImageColorTransparent(test, 0x000000);
mySavePng ("test.png", test);

the problem is that the result image
it has the edges antialiasing that they create an ugly effect



thanks,