[TYPO3] GIFBUILDER output png-24 with alpha channel

[Franz Josef]

Hi!

I want to generate 24 bit pngs (including alpha channel) with GIFBUILDER.

I need to render text on top of an existing 24 bit png, but I couldn't 
get it to work. No matter what I do, the result is a 24 bit png, but it 
has just one transparent color.

I set these options in install tool:

[GFX][gdlib_png] = 1
[GFX][png_truecolor] = 1

Thats the Test Typoscript:

temp.testAlpha = IMAGE
temp.testAlpha.file = GIFBUILDER
temp.testAlpha.file {
       transparentBackground = 1
       XY = 181,40
       5 = IMAGE
       5.file = fileadmin/img/background.png // has an alpha channel 
already
       10 = TEXT
       10.text.data = leveltitle: 2
       10.text.case = upper
       10.fontFile = fileadmin/fonts/gil.normal.ttf
       10.fontColor = #333333
       10.fontSize = 12
       10.niceText = 1
       10.offset = 15,25
}

I tried to use IM 6.1.8 and GM 1.1.6, both show the same result. GDLib 
version is 2.0.36.

TSRef says, that transparentBackground determines the color on the 0/0 
pixel and takes it as the transparent color, is there a way to keep the 
whole alpha channel for the output image?

I read about imagesavealpha() and imagealphablending(), which I tried to 
hack into class.t3lib_stdgraphic.php, but still it didnt work.

Searched everywhere for advice, but couldn't find a working solution! 
Can anybody help, please?

Thanks

Franz